ST AN2644 Application note

AN2644

Application note

An introduction to LLC resonant half-bridge converter

Introduction

Although in existence for many years, only recently has the LLC resonant converter, in particular in its half-bridge implementation, gained in the popularity it certainly deserves. In many applications, such as flat panel TVs, 85+ ATX PCs or small form factor PCs, where the requirements on efficiency and power density of their SMPS are getting tougher and tougher, the LLC resonant half-bridge with its many benefits and very few drawbacks is an excellent solution. One of the major difficulties that engineers are facing with this topology is the lack of information concerning the way it operates. The purpose of this application note is to provide insight into the topology and help familiarize the reader with it, therefore, the approach is essentially descriptive.

Figure 1. LLC resonant topology schematics and waveforms

September 2008

Rev 2

1/64

www.st.com

Contents	AN2644

Contents

1	Classification of resonant converters . . . . . . . . . . . . . . . . . . . . . . . . . .		. 5
2	The LLC resonant half-bridge converter . . . . . . . . . . . . . . . . . . . . . . . .		. 7
	2.1	General overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	2.2	The switching mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
	2.3	Fundamental operating modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20

2.3.1 Operation at resonance (f = fR1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

2.3.2 Operation above resonance (f > fR1) . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

CCMA operation at heavy load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26 DCMA at medium load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28 DCMAB at light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

2.3.3 Operation below resonance (fR2 < f < fR1, R>Rcrit) . . . . . . . . . . . . . . . . 30

DCMAB at medium-light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30 DCMB at heavy load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

2.3.4 Capacitive-mode operation below resonance (fR2 < f < fR1, R<Rcrit) . . . 34

Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

2.4 No-load operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.5 Overload and short circuit operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 2.6 Converter's startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 2.7 Analysis of power losses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 2.8 Small-signal behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

2.8.1 Operation above resonance (f > fR1) . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 2.8.2 Operation below resonance (fR2 < f < fR1, R>Rcrit) . . . . . . . . . . . . . . . . 44 2.8.3 Operation at resonance (f = fR1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	46
4	References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	47
Appendix A Power MOSFET driving energy in ZVS operation . . . . . . . . . . . . . .		48

2/64

AN2644	Contents

Appendix B Resonant transitions of half-bridge midpoint. . . . . . . . . . . . . . . . . 50

Appendix C Power MOSFET effective Coss and half-bridge midpoint's transition times . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Appendix D Power MOSFETs switching losses at turn-off. . . . . . . . . . . . . . . . . 59

Appendix E Input current in LLC resonant half-bridge with split resonant capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

3/64

List of figures	AN2644

List of figures

Figure 1.	LLC resonant topology schematics and waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 1
Figure 2.	General block diagram of a resonant inverter, the core of resonant converters. . . . . . . . .	. 5
Figure 3.	LLC resonant half-bridge schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 6
Figure 4.	LLC resonant half-bridge with split resonant capacitor. . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 8
Figure 5.	Equivalent schematic of a real transformer (left, tapped secondary; right, single
	secondary) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 9
Figure 6.	Example of high-leakage magnetic structures (cross-section) . . . . . . . . . . . . . . . . . . . . . .	. 9
Figure 7.	Equivalent schematic of a transformer including parasitic capacitance . . . . . . . . . . . . . . .	12
Figure 8.	Power MOSFET totem-pole network driving a resonant tank circuit in a half-bridge
	converter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
Figure 9.	Detail of Q1 ON-OFF and Q2 OFF-ON transitions with soft-switching for Q2 . . . . . . . . . .	14
Figure 10.	Q2 gate voltage at turn-on: with soft-switching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
Figure 11.	Q2 gate voltage at turn-on: with hard-switching (no ZVS) . . . . . . . . . . . . . . . . . . . . . . . . .	14
Figure 12.	Q1 ON-OFF and Q2 OFF-ON transitions with hard switching for Q2 and recovery for
	DQ1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
Figure 13.	Bridge leg transitions in the neighborhood of inductive-capacitive regions boundary . . . .	17
Figure 14.	Bridge leg transitions under no-load conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
Figure 15.	Reference LLC converter for the analysis of the fundamental operating modes . . . . . . . .	21
Figure 16.	Operation at resonance (f = fR1): main waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	22
Figure 17.	Operation above resonance (f > fR1): main waveforms in CCMA operation at heavy load 25
Figure 18.	Operation above resonance (f > fR1): main waveforms in DCMA operation at medium
	load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	27
Figure 19.	Operation above resonance (f > fR1): main waveforms in DCMAB operation at light
	load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	30
Figure 20.	Operation below resonance (fR2 < f < fR1 , R>Rcrit): main waveforms in DCMAB
	operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	31
Figure 21.	Operation below resonance (fR2 < f < fR1, R>Rcrit): main waveforms in DCMB2 operation 32
Figure 22.	Capacitive mode operation below resonance (fR2 < f < fR1, R<Rcrit): main waveforms . . .	35
Figure 23.	No-load operation (cutoff): main waveforms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	36
Figure 24.	Circuit's equivalent schematic under: no-load conditions . . . . . . . . . . . . . . . . . . . . . . . . . .	38
Figure 25.	Circuit's equivalent schematic under: short-circuit conditions. . . . . . . . . . . . . . . . . . . . . . .	38
Figure 26.	Converter's startup: main waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	40
Figure 27.	High-side driving with bootstrap approach and bootstrap capacitor charge path . . . . . . . .	41
Figure 28.	Frequency compensation with an isolated type 3 amplifier (3 poles + 2 zeros) . . . . . . . .	45
Figure 29.	Comparison of gate-charge characteristics with and without ZVS . . . . . . . . . . . . . . . . . . .	48
Figure 30.	Equivalent circuit to analyze the transitions of the half-bridge midpoint node when Q2
	turns off. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	50
Figure 31.	Voltage of HB node vs time (see Equation 32). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	53
Figure 32.	Resonant current vs time (see Equation 34) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	53
Figure 33.	Capacitance associated to the half-bridge midpoint node . . . . . . . . . . . . . . . . . . . . . . . . .	54
Figure 34.	Coss capacitances vs. half-bridge midpoint's voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	55
Figure 35.	Simplified schematic to analyze transition times of the node HB when Q2 turns off . . . . .	56
Figure 36.	Schematization of Q2 turn-off transient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	57
Figure 37.	Input current components in a split-capacitor LLC resonant half-bridge. . . . . . . . . . . . . . .	61
Figure 38.	Timing diagram showing currents flow in the converter of Figure 37 . . . . . . . . . . . . . . . . .	61

4/64

AN2644	Classification of resonant converters

1 Classification of resonant converters

Resonant conversion is a topic that is at least thirty years old and where much effort has been spent in research in universities and industry because of its attractive features: smooth waveforms, high efficiency and high power density. Yet the use of this technique in off-line powered equipment has been confined for a long time to niche applications: high-voltage power supplies or audio systems, to name a few. Quite recently, emerging applications such as flat panel TVs on one hand, and the introduction of new regulations, both voluntary and mandatory, concerning an efficient use of energy on the other hand, are pushing power designers to find more and more efficient AC-DC conversion systems. This has revamped and broadened the interest in resonant conversion. Generally speaking, resonant converters are switching converters that include a tank circuit actively participating in determining input-to-output power flow. The family of resonant converters is extremely vast and it is not an easy task to provide a comprehensive picture. To help find one's way, it is possible to refer to a property shared by most, if not all, of the members of the family. They are based on a "resonant inverter", i.e. a system that converts a DC voltage into a sinusoidal voltage (more generally, into a low harmonic content ac voltage), and provides ac power to a load. To do so, a switch network typically produces a square-wave voltage that is applied to a resonant tank tuned to the fundamental component of the square wave. In this way, the tank will respond primarily to this component and negligibly to the higher order harmonics, so that its voltage and/or current, as well as those of the load, will be essentially sinusoidal or piecewise sinusoidal. As shown in Figure 2, a resonant DC-DC converter able to provide DC power to a load can be obtained by rectifying and filtering the ac output of a resonant inverter.

Figure 2. General block diagram of a resonant inverter, the core of resonant converters

		Vindc	Switch	Resonant	Voutac
			network	tank circuit
			Resonant Inverter
		Resonant Inverter
	Vindc	Switch	Resonant	Rectifier	Low-pass	Voutdc
		network	tank circuit		filter

Resonant Converter

Different types of DC-AC inverters can be built, depending on the type of switch network and on the characteristics of the resonant tank, i.e. the number of its reactive elements and their configuration [1].

As to switch networks, we will limit our attention to those that drive the resonant tank symmetrically in both voltage and time, and act as a voltage source, namely the half-bridge and the full-bridge switch networks. Borrowing the terminology from power amplifiers,

5/64

Classification of resonant converters	AN2644

switching inverters driven by this kind of switch network are considered part of the group called "class D resonant inverters".

As to resonant tanks, with two reactive elements (one L and one C) there are a total of eight different possible configurations, but only four of them are practically usable with a voltage source input. Two of them generate the well-known series resonant converter and parallel resonant converter considered in [2] and thoroughly treated in literature.

With three reactive elements the number of different tank circuit configurations is thirty-six, but only fifteen can be used in practice with a voltage source input. One of these, commonly called LCC because it uses one inductor and two capacitors, with the load connected in parallel to one C, generates the LCC resonant inverter commonly used in electronic lamp ballast for gas-discharge lamps. Its dual configuration, using two inductors and one capacitor, with the load connected in parallel to one L, generates the LLC inverter.

As previously stated, for any resonant inverter there is one associated DC-DC resonant converter, obtained by rectification and filtering of the inverter output. Predictably, the abovementioned class of inverters will originate the "class D resonant converters". Considering off-line applications, in most cases the rectifier block will be coupled to the resonant inverter through a transformer to guarantee the isolation required by safety regulations. To maximize the usage of the energy handled by the inverter, the rectifier block can be configured as either a full-wave rectifier, which needs a center tap arrangement of transformer's secondary winding, or a bridge rectifier, in which case tapping is not needed. The first option is preferable with a low voltage / high current output; the second option with a high voltage / low current output. As to the low-pass filter, depending on the configuration of the tank circuit, it will be made by capacitors only or by an L-C type smoothing filter. The so-called "series-parallel" converter described in [2], typically used in high-voltage power supply, is derived from the previously mentioned LCC resonant inverter. Its dual configuration, the LLC inverter, generates the homonymous converter, addressed in [3], [4] and [5], that will be the subject of the following discussion. In particular we will consider the half-bridge implementation, illustrated in Figure 3, but the extension to the full-bridge version is quite straightforward.

Figure 3. LLC resonant half-bridge schematic

Vin

	Q1
Half-bridge Driver	Cr	Ls
	Q2
	LLC tank circuit

Preferably integrated into a single magnetic structure

	Center-tapped output with full-wave
		rectification
	(low voltage and high current)
a:1:1
		D1	R
Lp			Vout
		D2
	Single-ended output with bridge
		rectification
	(high voltage and low current)
a:1		D1	R
	D4
		D3	Vout
	D2

6/64

AN2644	The LLC resonant half-bridge converter

In resonant inverters (and converters too) power flow can be controlled by the switch network either by changing the frequency of the square wave voltage, or its duty cycle, or both, or by special control schemes such as phase-shift control. In this context we will focus on power flow control by frequency modulation, that is, by changing the frequency of the square wave closer to or further from the tank circuit's resonant frequency while keeping its duty cycle fixed.

2 The LLC resonant half-bridge converter

2.1General overview

According to another way of designating resonant converters, the LLC resonant half-bridge belongs to the family of multiresonant converters. Actually, since the resonant tank includes three reactive elements (Cr, Ls and Lp, shown in Figure 3), there are two resonant frequencies associated to this circuit. One is related to the condition of the secondary winding(s) conducting, where the inductance Lp disappears because dynamically shorted out by the low-pass filter and the load (there is a constant voltage a Vout across it):

Equation 1

	fR1	=	1
			π -------------------Ls Cr
		2

the other resonant frequency is relevant to the condition of the secondary winding(s) open, where the tank circuit turns from LLC to LC because Ls and Lp can be unified in a single inductor:

Equation 2

	fR2	=	1
			π ---------------------------------( Ls + Lp)Cr
		2

It will be of course fR1 > fR2. Normally, fR1 is referred to as the resonance frequency of the LLC resonant tank, while fR2 is sometimes called the second (or lower) resonance frequency. The separation between fR1 and fR2 depends on the ratio of Lp to Ls. The larger this ratio is, the further the two frequencies will be and vice versa. The value of Lp/Ls (typically > 1) is an important design parameter.

It is possible to show that for frequencies f > fR1 the input impedance of the loaded resonant tank is inductive and that for frequencies f < fR2 the input impedance is capacitive. In the frequency region fR2 < f < fR1 the impedance can be either inductive or capacitive depending

on the load resistance R. A critical value Rcrit exists such that if R < Rcrit then the impedance will be capacitive, inductive for R> Rcrit. For a given tank circuit the value of Rcrit depends on f. More precisely, in [6] it is shown that for any tank circuit configuration (then for

the LLC in particular):

Equation 3

Rcrit = Zo0 Zo∞

where Zo0 and Zo∞ are the resonant tank output impedances with the source input shortcircuited and open-circuited, respectively.

7/64

The LLC resonant half-bridge converter	AN2644

For certain reasons that will be clarified in the following sections, the LLC resonant converter is normally operated in the region where the input impedance of the resonant tank has inductive nature, i.e. it increases with frequency. This implies that power flow can be controlled by changing the operating frequency of the converter in such a way that a reduced power demand from the load produces a frequency rise, while an increased power demand causes a frequency reduction.

The Half-bridge Driver switches the two power MOSFETs Q1 and Q2 on and off in phase opposition symmetrically, that is, for exactly the same time. This is commonly referred to as "50% duty cycle" operation even if the conduction time of either power MOSFET is slightly shorter than 50% of the switching period. In fact, a small deadtime is inserted between the turn-off of either switch and the turn-on of the complementary one. The role of this deadtime is essential for the operation of the converter. It goes beyond ensuring that Q1 and Q2 will never cross-conduct and will be clarified in the next sections as well. For the moment it will be neglected, and the voltage applied to the resonant tank will be a square-wave with 50% duty cycle that swings all the way from 0 to Vin. Before going any further, however, it is important to make one concept clear.

Figure 4. LLC resonant half-bridge with split resonant capacitor

Vin

Driver

Half-bridge

Q1

Cr / 2

Ls

a:1:1

Q2

Lp

Cr / 2

A few paragraphs above, the impedance of the tank circuit was mentioned. Impedance is a concept related to linear circuits under sinusoidal excitation, whereas in this case the excitation voltage is a square wave.

However, as a consequence of the selective nature of resonant tanks, most power processing properties of resonant converters are associated with the fundamental component of the Fourier expansion of voltages and currents in the circuit. This applies in particular to the input square wave and is the foundation of the First Harmonic Approximation (FHA) modeling methodology presented in [2] as a general approach and used in [4] and [5] for the LLC resonant converter specifically. This approach justifies the usage of the concept of impedance as well as those coming from complex ac circuit analysis.

Coming back to the input square wave excitation, it has a DC component equal to Vin/2. In the LLC resonant tank the resonant capacitor Cr is in series to the voltage source and under steady state conditions the average voltage across inductors must be zero. As a result, the DC component Vin/2 of the input voltage must be found across Cr which consequently plays the double role of resonant capacitor and DC blocking capacitor.

It is possible to see, especially at higher power levels, a slightly modified version of the LLC resonant half-bridge converter, where the resonant capacitor is split as illustrated in

Figure 4. This configuration can be useful to reduce the current stress in each capacitor

8/64

AN2644	The LLC resonant half-bridge converter

and, in certain conditions, the initial imbalance of the V·s applied to the transformer at start up (see "Converter's start-up" section). Additionally, it makes the input current to the converter look like that of a full-bridge converter, as shown in Appendix E, with a resulting reduction in both the input differential mode noise and the stress of the input capacitor. Obviously, the currents through Q1 and Q2 will be unchanged. It is easy to recognize that the two Cr/2 capacitors are dynamically in parallel, so that the total resonant tank's capacitance is again Cr.

The system appears quite bulky, with its three magnetic components. However, the LLC resonant topology lends itself well to magnetic integration. With this technique inductors and transformers are combined into a single physical device to reduce component count, usually with little or no penalty to the converter's characteristics, sometimes even enhancing its operation. To understand how magnetic integration can be done, it is worth looking at the well-known equivalent schematics of a real transformer in Figure 5 and comparing them to the inductive component set of Figure 3.

Lp occupies the same place as the magnetizing inductance LM, Ls the same place as the primary leakage inductance LL1. Then, assuming that we are going to use a ferrite core plus bobbin assembly, Lp can be used as the magnetizing inductance of the transformer with the addition of an air gap into the magnetic circuit and leakage inductance can be used to make Ls.

Figure 5. Equivalent schematic of a real transformer (left, tapped secondary; right, single secondary)

i1(t)	LL1	LL2	i2(t)	i1(t)	LL1	n : 1	LL2	i2(t)
	iM(t)		v2(t)		iM(t)
v1(t)	LM			v1(t)	LM			v2(t)
			v2(t)
		LL2
	ideal					ideal

To do so, however, a leaky magnetic structure is needed, which is contrary to the traditional transformer design practice that aims at minimizing leakage inductance. The usual concentric winding arrangement is not recommended here, although higher leakage inductance values can be achieved by increasing the space between the windings.

Figure 6. Example of high-leakage magnetic structures (cross-section)

Primary

Primary

winding

winding

Secondary	Secondary
winding	winding
Windings on separate legs of an EE core	Side-by-side windings (EE or pot core)

9/64

The LLC resonant half-bridge converter	AN2644

It is difficult, however to obtain reproducible values, because they depend on parameters (such as winding surface irregularities or spacer thickness) difficult to control. Other fashions are recommended, such as placing the windings on separate core legs (using E or U cores) or side by side on the same leg which is possible with both E and pot cores and shown in Figure 6. They permit reproducible leakage inductance values, because related to the geometry and the mechanical tolerances of the bobbin, which are quite well controlled. In addition, these structures possess geometric symmetry, so they lead to magnetic devices with an excellent magnetic symmetry.

However, in the real transformer model of Figure 5 there is the secondary leakage inductance LL2 that is not considered in the model of Figure 3. The presence of LL2 is not a problem from the modeling point of view because the transformer's equivalent schematic can be manipulated so that LL2 disappears (it is transferred to the primary side and incorporated in LL1). This is exactly what has been done in the transformer model shown in Figure 3. Then, it is important to underline that Ls and Lp are not real physical inductances (LL1, LM and LL2 are), their numerical values are different (Ls ≠ LL1, Lp ≠ LM), and, finally, the turn ratio a is not the physical turn ratio n=N1/N2. Ls and Lp can be given a physical interpretation. Ls is the inductance of the primary winding measured with the secondary winding(s) shorted, while Lp is the difference between the inductance of the primary winding measured with the secondary windings open and Ls.

However, LL2 is not free from side effects. For a given impressed voltage, LL2 decreases the voltage available on the secondary winding, which carries a current i2, by the drop LL2·di2/dt. This is an effect that is taken into account by the above mentioned manipulation of the transformer model. In addition, in multioutput converters, where there is leakage inductance associated to each output winding, cross-regulation between the various outputs will be adversely affected because of their decoupling effect.

Finally, there is one more adverse effect to consider in the center-tapped output configuration. With reference to Figure 3 and 5, when one half-winding is conducting, the voltage v2(t) externally applied to that half-winding is Vout+VF (VF is the rectifier forward drop of the conducting diode). With no secondary leakage inductance, this voltage will be found across the secondary winding of the ideal transformer and then coupled one-to-one to the nonconducting half-winding. Consequently, the reverse voltage applied to the reversebiased rectifier will be 2·Vout+VF. If now we introduce the leakage inductance LL2, the drop

LL2·di2/dt adds up to Vout+VF. and is reflected to the other half-winding as well. As a result, the reverse voltage applied to the nonconducting rectifier will be increased by LL2·di2/dt.

Note that in case of single-winding secondary with bridge rectification, the voltage applied to

reverse-biased diodes of the bridge is only Vout+VF and is not affected by LL2. The reason is that the negative voltage of the secondary winding is fixed at -VF externally and is not

determined by internal coupling like in the case of tapped secondary.

It is worth pointing out that the 50% duty cycle operation of the LLC half-bridge equalizes the stress of secondary rectifiers both in terms of reverse voltage - as just seen - and forward conduction current. In fact, each rectifier carries half the total output current under all operating conditions. Then, if compared to similar PWM converters (such as the ZVS Asymmetrical Half-bridge, or the Forward converter), in the center-tapped output configuration the equal reverse voltage typically allows the use of lower blocking voltage rating diodes. This is especially true when using common cathode diodes housed in a single package. A lower blocking voltage means also a lower forward drop for the same current rating, and then lower losses.

It must be said, however, that in the LLC resonant converter the output current form factor is worse, so the output capacitor bank is stressed more. Although the stress level is

10/64

AN2644	The LLC resonant half-bridge converter

considerably lower than that in a flyback converter, as shown in Table 1, this is one of the few real drawbacks of the topology.

Table 1. Output stress for LLC resonant half-bridge vs. PWM topologies @ 50% duty cycle

Output current form factors	Forward - ZVS AHB	LLC resonant HB			Flyback (CCM-DCM boundary)
Peak-to-DC ratio	≈1.05÷1.15		π			4
			--	= 1.57
			≈ 2
Rms-to-DC ratio	≈1		π		8	≈ 1.63
			≈ ---------- = 1.11		--
			2	2	3
AC-to-DC ratio	≈0.03÷0.09	≈	π2		8	1 ≈ 1.29
			----- – 1 = 0.48		-- –
			8		3

Additional details concerning power losses will be discussed in Analysis of power losses on page 41.

To complete the general picture on the LLC resonant converter, there is another aspect that needs to be addressed concerning parasitic components which affect the behavior of the circuit.

The first parasitic element to consider is the capacitance of the midpoint of the half-bridge structure, the node common to the source of the high-side power MOSFET and to the drain of the low-side power MOSFET. Its effect is that the transitions of the half-bridge midpoint will require some energy and take a finite time to complete. This is linked to the previously mentioned deadtime inserted between the turn-off of either switch and the turn-on of the complementary one, and will be discussed in more detail in Section 2.2.

The second parasitic element to consider is the distributed capacitance of transformer's windings. This capacitance, which exists for both the primary and the secondary windings, in combination with windings' inductance, originates what is commonly designated as the transformer "self-resonance". In addition to this capacitance one needs to consider also the junction capacitance of the secondary rectifiers, which adds up to that of the secondary windings and lowers the resulting self-resonance frequency (loaded self-resonance).

The effect of all this parasitic capacitance can be modeled with a single capacitor CP connected in parallel to LM as illustrated in Figure 7. The resonant tank, as a consequence, turns from LLC to LLCC. This 4th- order tank circuit features a third resonance frequency at the transformer's loaded self-resonance (fLSR > fR1). When the operating frequency is considerably lower than fLSR the effect of CP is negligible. However, at frequencies greater than fR1 and if the load impedance is high enough, its effect starts making itself felt, eventually resulting in reversing the transferable power vs. frequency relationship as frequency approaches fLSR. Power now increases with the switching frequency, feedback becomes positive and the converter loses control of the output voltage. The onset of this "feedback reversal" in closed-loop operation is revealed by a sudden frequency jump to its maximum value as the load falls below a critical value (i.e. the frequency exceeds a critical value) and a simultaneous output voltage rise.

In some way, either appropriately choosing the operating frequency range (<< fLSR) or increasing fLSR, the converter must work away from feedback reversal. This usually sets the practical upper limit to a converter's operating frequency range.

11/64

The LLC resonant half-bridge converter	AN2644

Figure 7. Equivalent schematic of a transformer including parasitic capacitance

2.2The switching mechanism

Still another way of classifying resonant converters would include the LLC resonant halfbridge in the family of "resonant-transition" converters. This nomenclature refers to the fact that in this class of converters power switches are driven in such a way that a resonant tank circuit is stimulated to create a zero-voltage condition for them to turn-on.

To understand how this can be achieved in the LLC half-bridge, it is instructive to consider the circuits illustrated in Figure 8 where the switches Q1 and Q2 that generate the square wave input voltage to the resonant tank are power MOSFETs. Their body diodes DQ1, DQ2 are pointed out because they play an important role. In circuit a), the drain-to-source parasitic capacitances Coss1, Coss2 are pointed out as well. In fact, as far as voltage changes of the node HB are concerned, the parasitic capacitances Cgd and Cds are effectively in parallel, then Cgd+Cds=Coss has to be considered.

Additionally, other contributors to the parasitic capacitance of the node HB (e.g. that formed between the case of the power MOSFETs and the heat sink, the intrawinding capacitance of

the resonant inductor, etc.) are lumped together in the capacitor CStray. Note that Coss1, like Coss2, is connected between the node HB and a node having a fixed voltage (Vin for Coss1, ground for Coss2). Then, as far as voltage changes of the node HB are concerned, Coss1 is effectively connected in parallel to Coss2 and CStray. It is convenient to lump all of them together in a single capacitor CHB from the node HB to ground, as shown in the circuit b):

Equation 4

CHB = COSS1 + COSS2 + CStray

which we will refer to in the following discussion. Note also that Coss1 and Coss2 are nonlinear capacitors, i.e. their value is a function of the drain-to-source voltage. It is intended

that their time-related equivalent value will be considered (see Appendix A).

12/64

AN2644	The LLC resonant half-bridge converter

Figure 8. Power MOSFET totem-pole network driving a resonant tank circuit in a half-bridge converter

Vin

HB Driver

Q1

	Coss1
DQ1	IR
Node
HB
Q2
	Coss2
DQ2	CStray

a)

Resonant Vin

Tank &

Load

HB Driver

Q1

	DQ1	Resonant
Node	IR
HB		Tank &
Q2		Load
	CHB
	DQ2
	b)

As previously stated, there is no overlap between the conduction of Q1 and Q2. Additionally, a deadtime TD between the transitions from one state to the other of either switch, where they both are open, is intentionally inserted. It is intended that when Q1 is closed and Q2 is open, the voltage applied to the resonant tank circuit is positive. Similarly we will define as negative the voltage applied to the resonant tank circuit when Q1 is open and Q2 is closed. Consistently with two-port circuits sign convention, the input current to the resonant tank, IR, will be positive if entering the circuit, negative otherwise.

Let us assume Q1 closed and Q2 open. It is then IR = I(Q1). Despite that the voltage applied to the circuit is positive (VHB = Vin), IR can flow in either direction since we are in presence of reactive elements. Let us suppose that IR is entering the tank circuit (positive current) in the instant t0 when Q1 opens, and refer to the timing diagram of Figure 9.

The current through Q1 falls quickly and becomes zero at t = t1. Q2 is still open and IR must keep on flowing almost unchanged because of the inductance of the resonant tank that acts as a current flywheel. The electrical charge necessary to sustain IR will come initially from CHB, initially charged at Vin, which will be now discharged. Provided IR(t1) is large enough, the voltage of the node HB will then fall at a certain rate until t = t2, when its voltage becomes negative and the body diode of Q2, DQ2, becomes forward biased, thus clamping the voltage at a diode forward drop VF below ground. IR will go on flowing through DQ2 for

the remaining part of the deadtime TD until t = t3, when Q2 turns on and its RDS(on) shunts DQ2. When this occurs, the voltage across Q2 is -VF, a value negligible as compared to the

input voltage Vin. In the end, this is what is called zero-voltage switching (ZVS): the turn-on transition of Q2 is done with negligible dissipation due to voltage-current overlap and with CHB already discharged, there will be no significant capacitive loss either. Note, however, that there will be nonnegligible power dissipation associated to Q1's turn-off because there will be some voltage-current overlap during the time interval t0 - t1.

13/64

The LLC resonant half-bridge converter	AN2644

Figure 9. Detail of Q1 ON-OFF and Q2 OFF-ON transitions with soft-switching for Q2

Dead-time

	Q1 ON			Q1 OFF			Q1 OFF
	Q2 OFF			Q2 OFF			Q2 ON
	Low turn-off losses					Q2 is switched on with essentially
VHB						zero drain-to-source voltage: ZVS!
				Tank circuit’s current			Magnetizing current equals
IR	I(Q1) I(CHB)			is positive			tank circuit’s current
	Q1’s current	t0	t1	t2	t3	HB node’s parasitic capa-
	falls to zero					citance discharge current

t0 – t1	Q1 current fall time;
	voltage-current overlap for Q1
t0 – t2	Node HB transition time
t2 – t3	Q2’ body diode conduction time

Vc = Resonant capacitor voltage

VHB = Node HB voltage

IR = Tank circuit’s current

I(Lp) = Lp (magnetizing) current

I(Q1) = MOSFET Q1 current

I(CHB) = CHB current

There is an additional positive side effect in turning on Q2 with zero drain-to-source voltage. It is the absence of the Miller effect, normally present in power MOSFETs at turn-on when hard-switched. In fact, as the drain-to-source voltage is already zero when the gate is supplied, the drain-to-gate capacitance Cgd cannot "steal" the charge provided to the gate. The so-called "Miller plateau", the flat portion in the gate voltage waveform, as well as the associated gate charge, is missing here and less driving energy is therefore required. Note that this property provides a method to check if the converter is running with soft-switching or not by looking at the gate waveform of Q2 (which is more convenient because it is sourcegrounded), as shown in Figure 10 and 11.

Figure 10. Q2 gate voltage at turn-on: with	Figure 11. Q2 gate voltage at turn-on: with
soft-switching	hard-switching (no ZVS)

Current injection through

Cgd due to hard-switching

Miller effect

HB node is falling down here; current due to Cgd

With similar reasoning it is possible to understand that the same ZVS mechanism occurs to Q1 when it turns on if IR is flowing out of the resonant tank circuit (negative current).

In the end we can conclude that, if the tank current at the instant of half-bridge transitions has the same sign as the impressed voltage, both switches will be "soft-switched" at turn-on, i.e. turned on with zero voltage across them (ZVS). It is intuitive that this sign coincidence

14/64

AN2644	The LLC resonant half-bridge converter

occurs if the tank current lags the impressed voltage (e.g. it is still positive while voltage has already gone to zero), which is a condition typical of inductors. In other words, ZVS occurs if the resonant tank input impedance is inductive. The frequency range where tank current lags the impressed voltage is therefore called the "inductive region".

It is worth emphasizing the essential role of DQ1 and DQ2 in ensuring continuity to current flow and clamping the voltage swing of the node HB at Vin+VF and -VF respectively (the LLC resonant half-bridge belongs to the family of ZVS clamped-voltage topologies). Power MOSFETs, with their inherent body diodes are therefore the best suited power switches to be used in this converter topology. Other types of switches, such as BJT or IGBT, would need the addition of external diodes.

Going back to the state when Q1 is closed and Q2 open, let us now assume that at the instant t0 when Q1 opens, current is flowing out of the resonant tank towards the input source, i.e. it is negative. This operation is shown in the timing diagrams of Figure 12.

With Q1 now open the current will go on flowing through DQ1 throughout the deadtime, and will eventually be diverted through Q2 only when Q2 closes at t = t1, the end of the deadtime. As far as Q1 is concerned, then, there will be no loss associated to turn-off because the voltage across it does not change significantly (it is essentially the same situation seen at turn-on when the converter works in the inductive region).

Q2, instead, will experience now a totally different situation. As DQ1 is conducting during the deadtime, the voltage across Q2 at t = t1 equals Vin+VF so that there will be not only a considerable voltage-current overlap but the energy of CHB will be dissipated inside its

RDS(on) as well. In this respect, it is a "hard-switching" condition identical to what normally happens in PWM-controlled converters at turn-on. The associated power dissipation

½CHBVin2f may be considerably higher than that normally dissipated under "soft-switching" conditions and this may easily lead to Q1 overheating, since heat sinking is not usually sized to handle this abnormal condition.

In addition to that, at t = t1 the body diode of Q1, DQ1, is conducting current and its voltage is abruptly reversed by the node HB being forced to ground by Q2. Hence, DQ1 will keep its low impedance and there will be a condition equivalent to a shoot-through between Q1 and Q2 until it recovers (at t = t2).

It is well-known the power MOSFET's body diodes do not have brilliant reverse recovery characteristics. Hence DQ1 will undergo a reverse current spike large in amplitude (it can be much larger than the forward current it was carrying at t=t1) and relatively long in duration (in the hundred ns) that will go through Q2 as well. This spike, in fact, cannot flow through the resonant tank because Ls does not allow for abrupt current changes.

This is a potentially destructive condition not only because of the associated power dissipation that adds up to the others previously considered, but also due to the current and voltage of DQ1 which are simultaneously high during part of its recovery. In fact, there will be an extremely high dv/dt (many tens of V/ns!) experienced by Q1 as DQ1 recovers and the voltage of the node HB goes to zero. This dv/dt may exceed Q1 rating and lead to an immediate failure because of the second breakdown of the parasitic bipolar transistor intrinsic in power MOSFET structure. Finally, it is also possible that Q1 is parasitically turned on if the current injected through its Cgd and flowing through the gate driver's pull down, which is holding the gate of Q1 low, is large enough to raise the gate voltage close to the turn-on threshold (see the spike after turn-off in the graph of Figure 11). This would cause a lethal shoot-through condition for the half-bridge leg.

An additional drawback of this operation is the large and energetic negative voltage spikes induced by the recovery of DQ1 because of the unavoidable parasitic inductance of the PCB

15/64

The LLC resonant half-bridge converter	AN2644

subject to its di/dt, which may damage any control IC coupled to the half-bridge leg, not to mention the big EMI generation.

Similarly, it is possible to show that the same series of adverse events will happen to Q1 and Q2, with exchanged roles, when Q2 is turned off if IR is flowing into the resonant tank circuit (positive current).

The obvious conclusion is that, if the tank current and the impressed voltage at the instant of half-bridge transitions have opposite signs, both switches will be hard-switched and the reverse recovery of their body diodes will be invoked, with all the resulting negative effects. It is intuitive that this sign opposition occurs if the tank current leads the impressed voltage, which is typical of capacitors and then occurs if the resonant tank input impedance is capacitive. This kind of operation is often termed "capacitive mode" and the frequency range where tank current leads the impressed voltage is called the "capacitive region".

Figure 12. Q1 ON-OFF and Q2 OFF-ON transitions with hard switching for Q2 and recovery for DQ1

Dead-time

		Q1 ON		Q1 OFF	Q1 OFF
		Q2 OFF		Q2 OFF	Q2 ON
	Q1 has soft turn-off				Q2 is hard-switched
VHB
	Node HB voltage				Resonant capacitor voltage
					High dv/dt
					Q1’s body diode
IR	I(Q1)	I(Q2)			is recovered
	Tank circuit’s current		t0	t1 t2	Current is circulating
		is negative			through Q1’s body diode

t0 – t1	Q2’ body diode conduction time;
	coinciding with dead-time
t1 – t2	Q1’s body diode recovery time

Vc = Resonant capacitor voltage

VHB = Node HB voltage

IR = Tank circuit’s current

I(Lp) = Lp (magnetizing) current

I(Q1) = MOSFET Q1 current

Then, the converter must be operated in the region where the input impedance is inductive (the inductive region), that is, for frequencies f > fR1 or in the range fR2 < f < fR1 provided the

load resistance R is such that R> Rcrit. This is a necessary condition in order for Q1 and Q2 to achieve ZVS, which is evidently a crucial point for the good operation of the LLC resonant

half-bridge.

To summarize, ZVS brings the following benefits:

1.low switching losses: either high efficiency can be achieved if the half-bridge is operated at a not too high switching frequency (for example < 100 kHz) or high switching frequency operation is possible with a still acceptably high efficiency (definitely out of reach with a hard-switched converter);

2.reduction of the energy needed to drive Q1 and Q2, thanks to the absence of Miller effect at turn-on. Not only is turn-on speed unimportant because there is no voltagecurrent overlap but also gate charge is reduced, then a small source capability is required from the gate drivers.

3.low noise and EMI generation, which minimizes filtering requirements and makes this converter extremely attractive in noise-sensitive applications.

4.all of the above-mentioned adverse effects of capacitive mode, which not only impair efficiency but also jeopardize the converter, are prevented.

16/64

AN2644	The LLC resonant half-bridge converter

Note, however, that working in the inductive region is not a sufficient condition in order for ZVS to occur.

In the above discussion, it has been said that the voltage of the node HB could swing from Vin to zero "provided IR is large enough". Of course the same holds if we consider node HB's swing from zero up to Vin. What actually happens when Q1 turns off with positive IR current is that the associated inductive energy level of the resonant tank circuit is maintained at the expense of the energy contained in the capacitance CHB. If the inductive energy ( IR2) is greater than that owned by CHB ( Vin2) CHB will be completely depleted and the voltage of the node HB will be able to reach -VF, injecting DQ2 and allowing Q2 to turn-on with essentially zero drain-to-source voltage. Similarly, when Q2 turns off with negative current, part or all of the associated inductive energy will be transferred to CHB. If the available inductive energy is greater than that needed to charge CHB up to Vin+VF, the node HB will be allowed to swing all the way up until DQ1 is injected, thus clamping the voltage, and Q1 will be able to turn-on with essentially zero drain-to-source voltage.

Seen from a different perspective, the inductive part of the tank circuit resonates with CHB, and this is the origin of the term "resonant transition" used for designating resonant converters having this property. This "parasitic" tank circuit active during transitions is formed by CHB with the series inductance Ls if during the half-bridge transition there is current circulating on the secondary side (so that Lp is shorted out) or with the total inductance Ls + Lp if there is no current conduction on the secondary side.

Figure 13. Bridge leg transitions in the neighborhood of inductive-capacitive regions boundary

	Q1 ON	Q1 OFF	Q1 OFF					Q1 ON	Q1 OFF	Q1 OFF
	Q2 OFF	Q2 OFF	Q2 ON					Q2 OFF	Q2 OFF	Q2 ON
VHB			Q1’s body diode conduction				VHB			Q1’s body diode conduction
			Q2 is hard switched			VHB = Node HB				Q2 is hard switched
			Q1’s body diode is recovered			voltage				Q1’s body diode is recovered
IR	I(Lp)		IR = 0				IR	I(Lp)		IR = 0
						IR = Tank circuit’s
						current
						I(Lp) = Lp (magnetizing)
			a)			current				b)
		Q1 ON		Q1 OFF	Q1 OFF			Q1 ON	Q1 OFF	Q1 OFF
		Q2 OFF		Q2 OFF	Q2 ON			Q2 OFF	Q2 OFF	Q2 ON
VHB							VHB
					Q2 is hard switched	VHB = Node HB				Q2 is soft switched
						voltage
IR	I(Lp)				IR = 0	IR = Tank circuit’s	IR	I(Lp)		IR = 0
						current
						I(Lp) = Lp (magnetizing)
						current
			c)							d)

The above mentioned energy balance considerations, however, are not still sufficient to guarantee ZVS under all operating conditions. There is an additional element that needs to be considered, the duration of the deadtime TD.

The first obvious consideration is that the duration of the deadtime represents an upper limit to the time the node HB takes to swing from one rail to the other: in order for the mosfet that is about to turn on to achieve ZVS (i.e. to be turned on with zero drain-to-source voltage), the transition has to be completed within TD as depicted in Figure 9. However, the way the

17/64

The LLC resonant half-bridge converter	AN2644

deadtime and ZVS are related is actually more complex and depends on converter's operating conditions.

It is instructive to see this in Figure 13, which shows typical node HB waveforms occurring when working in the inductive region but too close to the capacitive region, so that ZVS is not achieved. They refer to the Q1 →OFF, Q2 → ON transition; those related to the opposite transition are obviously turned upside down.

●Case a) is very close to the boundary between inductive and capacitive regions. Tank current reverses just after Q1 is switched off, a portion of node HB ringing appears as a small "dip", then the tank current becomes negative enough to let the body diode of Q1 start conducting. When Q2 turns on there are capacitive losses and the recovery of the Q1's body diode with all the related issues.

●Case b) is slightly more in the inductive region but still IR crosses zero within the deadtime. The node HB ringing becomes larger and the body diode of Q1 still conducts for a short time and its recovery is invoked as Q2 turns on.

●Case c) Is even more in the inductive region but still not sufficiently away from the capacitive-inductive boundary. The ringing of the node HB is large enough to reach

zero but IR reverses within the deadtime and the voltage goes up again. At the end of the deadtime the voltage does not reach Vin, hence the body diode of Q1 does not conduct and Q2, when turned on, will experience only capacitive losses.

●Case d) Is further in the inductive region and IR crosses zero nearly at the end of the deadtime. Q2 is now almost soft-switched with no losses. This can be considered as the boundary of the operating region where ZVS can be achieved with the given duration of TD.

Note that the resonant tank's current during node HB ringing is lower than the one flowing through Lp. This means that their difference is flowing into the transformer and, consequently, that one of the secondary half-windings is conducting. Therefore, CHB is resonating with Ls only.

This analysis shows that there is a "border belt" in the inductive region, close to the

boundary with the capacitive region (fR2 < f < fR1, R = Rcrit) and that as converter's operation is moved away from the capacitive-inductive boundary and pushed more deeply in the

inductive region there is a progressive behavior change from hard-switching to softswitching. In the cases a and b the inductive energy in the resonant tank is too small to let the node HB even swing "rail-to-rail"; moving away from the boundary, as shown in case c, the energy is higher and allows a rail-to-rail swing, but it is not large enough to keep the node HB "hooked" to the rail throughout the deadtime TD. If the converter is operated in this border belt, Q1 and Q2 will be hard-switched at turn-on and, in cases such as case a and case b, the body diode of the just turned off power MOSFET is injected and then recovered as the other power MOSFET turns on.

Case b and, especially, case c highlight that it is possible to look at the deadtime TD also from another standpoint: looking at those waveforms, one might conclude that the current IR at the beginning of the deadtime is too low or, conversely, that the deadtime is too long. In case c, for example, if the dead-time had been approximately half the value actually shown, Q2 would have been soft-switched at turn-on. Of course, the more appropriate interpretation depends on whether TD is fixed or not.

These cases are related to heavy load conditions.

Figure 14 shows a case typical of no-load conditions, where ZVS is not achieved because of a too slow transition of the node HB so that it does not swing completely within the deadtime TD. In this case the situation seems less stressful than operating in the capacitive region.

18/64

AN2644	The LLC resonant half-bridge converter

There is no body diode conduction and, consequently, no recovery. Q1 will be almost softswitched at turn-off, while Q2 will have capacitive losses at turn-on. It is true that the turn-on voltage is lower than Vin, thus the associated energy of CHB is lower, but at no-load the operating frequency is usually considerably higher than in the capacitive region, then these power losses may easily overheat Q1 and Q2. Finally, note in Figure 14 that I(Lp) is exactly superimposed on IR, then the secondary side of the transformer is open and CHB is resonating with the total inductance Ls+Lp.

Figure 14. Bridge leg transitions under no-load conditions

Q1 ON	Q1 OFF	Q1 OFF
Q2 OFF	Q2 OFF	Q2 ON
VHB
		Q2 is hard switched	VHB = Node HB voltage

IR I(Lp)

IR = Tank circuit’s current

I(Lp) = Lp (magnetizing) current

From what we have seen we can conclude that the conditions in order for the half-bridge switches to achieve ZVS are:

1.Under heavy load conditions, as one switch turns off, the tank current must have the same sign as the impressed voltage and be large enough so that both the rail-to-rail transition of the node HB is completed and the current itself does not reverse before the end of the deadtime, when the other switch turns on.

2.With no-load, the tank current at the moment one switch turns off (which has definitely the same sign as the impressed voltage) must be large enough to complete the node HB transition within the deadtime, before the other switch turns on.

Both conditions can be translated into specifying a minimum current value IRmin that needs to be switched when either power MOSFET turns off. In general, different IRmin values are needed to ensure ZVS at heavy load and at no-load. One can simply pick the greater one to ensure ZVS under any operating condition by design. On the other hand, this minimum required amount of current is to the detriment of efficiency.

At light or no-load a significant current must be kept circulating in the tank circuit, just to maintain ZVS, in spite of the current delivered to the load that is close to zero or zero. Using ac-analysis terminology, a certain amount of reactive energy is required even with no active energy.

Finally, also at heavy load the value of IRmin to be specified is the result of a trade-off. In fact, its value is directly related to the turn-off losses of both Q1 and Q2. The higher the switched

current is, the larger the switching loss due to voltage-current overlap will be.

The discussion on the switching mechanism has been focused on the primary-side switches, and the conditions in order for them to achieve soft-switching (ZVS at turn-on, precisely) have been found. One important merit of the LLC resonant converter is that also the rectifiers on the secondary side are soft-switched. They feature zero-current switching (ZCS) at both turn-on and turn-off. In fact, at turn-on the initial current is always zero and ramps up with a relatively low di/dt, so that forward recovery does not come into play. At turn-off they become reverse biased when their forward current is already zero, so that their

19/64

The LLC resonant half-bridge converter	AN2644

reverse recovery is not invoked. This topic will be addressed in Section 2.3, where it will be shown that this property is inherent in the topology, hence it occurs regardless of converter's design or operating conditions.

2.3Fundamental operating modes

The LLC resonant half-bridge converter features a considerable number of different operating modes, which stem from its multiresonant nature. Essentially, the term "multiresonant" means that the configuration of the resonant tank may change within a single switching cycle. We have seen that there are two resonant frequencies, one (the higher) associated to either of the secondary rectifiers conducting, the lower one associated to both rectifiers non-conducting. Then, depending on the input-to-output voltage ratio, the output load and the characteristics of the resonant tank circuit, the secondary rectifiers can be always conducting (with the exception of a single point in time), which is referred to as CCM (Continuous Conduction Mode) like in PWM converters, or there can be finite time intervals during which neither of the secondary rectifiers is conducting. This will obviously be called a DCM (Discontinuous Conduction Mode) operating mode.

Different kinds of CCM and DCM operating modes exist, although not all of them can be seen in a given converter, some are not even recommended, like those associated with capacitive mode operation. However, in all CCM modes the parallel inductance Lp is always shunted by the load resistance reflected back to the primary side, so that it never participates in resonance, rather it acts as an additional load to the remaining LC resonant circuit. Similarly, in all DCM modes, there will be some finite time intervals where Lp, being no longer shunted from the secondary side, becomes part of resonance.

In the following we will consider four fundamental operating modes and use the nomenclature defined in [3]:

1.Operation at resonance, when the converter works exactly at f = fR1;

2.Above-resonance operation, when the converter works at a frequency f > fR1. Moving away from resonance, we will consider three sub-modes:

a)CCMA operation at heavy load;

b)DCMA operation at medium load;

c)DCMAB operation at light load;

3.Below-resonance operation, when the converter works at a frequency fR2 < f < fR1 with a load resistor R > Rcrit. Moving away from resonance, we will consider two sub-modes:

a)DCMAB operation at medium-light load;

b)DCMB operation at heavy load;

4.Below-resonance operation, when the converter works at a frequency fR2 < f < fR1 with a load resistor R < Rcrit (capacitive mode), corresponding to the CCMB operating mode defined in [3];

In addition, two extreme operating conditions will be considered:

1.No-load operation (cutoff)

2.Output short-circuit operation

It is interesting to point out that, unlike PWM converters where DCM operation is invariably associated to light load operation and CCM to heavy load operation, in the LLC resonant converter this combination does not hold.

20/64