ST AN2951 APPLICATION NOTE

AN2951

Application note

3 kW fixed-off-time (FOT) power factor correction

Introduction

In recent years, regulation in terms of electrical efficiency, electromagnetic noise, and reliability of the electronic apparatus, in all the application fields, are pushing to use power factor correction in almost all applications; from low power range to high power range, and from industrial to domestic applications.

For this reason the design of a new PFC architecture is required to satisfy these new priorities. The PFC must work from a few watts to thousands of watts and also use cheap devices and materials. The design described in this paper is able to cover most of the common domestic and industrial applications.

A few examples of the application fields are: air conditioning, inverters (for fans and pumps), welding machinery, and industrial battery chargers.

Figure 1. STEVAL-ISF001V1, 3 kW FOT PFC board-top view

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Contents	AN2951

Contents

1	PFC working mode general description . . . . . . . . . . . . . . . . . . . . .	. . . . 5
2	Practical implementation of an FOT-controlled PFC preregulator	. . . . 6
3	Fixed-off-time using the L6562 . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . 7
4	Working mode along line period in a FOT PFC . . . . . . . . . . . . . . .	. . . . 9
5	Practical 3 kW FOT PFC design . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . 12
6	Power dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . 16
7	Schematic and circuit description . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . 18
8	Experimental results and scope acquisition . . . . . . . . . . . . . . . . .	. . . 22
9	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . 26

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AN2951	List of tables

List of tables

Table 1. Technical specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Table 2. Measurements for Vinac = 185 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Table 3. Measurements for Vinac = 230 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Table 4. Measurements for Vinac = 265 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Table 5. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

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List of figures	AN2951

List of figures

Figure 1. STEVAL-ISF001V1, 3 kW FOT PFC board-top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. Line, inductor, switch and diode current on TM and CCM control . . . . . . . . . . . . . . . . . . . . 5 Figure 3. Block diagram of a fixed-off-time architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 4. FOT control with the L6562 and relevant timing waveforms. . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 5. Working mode along line semiperiod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 6. Inductor, switch and diode currents in a CCM FOT-controlled PFC stage . . . . . . . . . . . . . 10 Figure 7. 3 kW FOT PFC demonstration board (STEVAL-ISF001V1) . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 8. 3 kW PFC STEVAL-ISF001V1 schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 9. MAGNETICA technical sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Figure 10. Scope acquisition. Working conditions: Vinac=220 AC; Pout=500 W . . . . . . . . . . . . . . . . 24 Figure 11. Scope acquisition: startup with a connected load of 1200 W . . . . . . . . . . . . . . . . . . . . . . . 25 Figure 12. EMI tests after the design and assembling of the filters . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

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AN2951	PFC working mode general description

1 PFC working mode general description

Essentially two methods of controlling power factor corrector (PFC) preregulators are well known and used. Fixed frequency PWM average current control and fixed-on-time variable frequency control.

The first method is a relatively complex control that requires a controller (e.g. L4981) which is usually sophisticated and expensive. With this method the current on the inductor is continuous and oscillates around the sinusoidal semiperiod value with a predetermined and fixed ripple.

In the second method the current on the inductor is discontinuous, reaching zero and starting to increase again on each switching period. It is cheaper but cannot be used for high power range applications due to the large peak current it causes during its action according to the load.

Other than these two methods, a third may be approached. Instead of maintaining the ontime fixed, such as TM PFC, the Toff is kept constant and the Ton is free to be changed in order to modulate the power drained from the source according to the load. This is called the “fixed-off-time modulation method”. Obviously, it is also a variable frequency control with some advantages, in particular the spread of noise energy conducted on the net that gives a lower noise power density in respect to a fixed frequency modulation, which simplifies the design and realization of the main filter required to match the EMC regulation.

The interesting point is that this kind of modulation can be obtained using a simple and cheap controller designed for TM (transition mode), e.g. the L6562 or L6563 by STMicroelectronics.

This method, with a complete theoretical explanation, is well described and depicted in the AN1792 application note by STMicroelectronics; “Design of fixed-off-time controller PFC preregulators with L6562”. We suggest first reading the antecedent AN1792 application note before continuing to read. Starting from this point we refer to the AN1792 to study in depth the design of a 3 kW FOT PFC preregulator.

Figure 2. Line, inductor, switch and diode current on TM and CCM control

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Practical implementation of an FOT-controlled PFC preregulator	AN2951

2Practical implementation of an FOT-controlled PFC preregulator

Figure 3. Block diagram of a fixed-off-time architecture

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The power conversion topology is classically based on a boost converter modulated to drain a current that follows the shape of the main AC voltage; in order to have a current as sinusoidal as possible, and in phase with the mains, assuring a high power factor and low THD (total harmonic distortion).

The architecture is based on a fixed-off-time generator, a multiplier, a PWM generator, and a voltage amplifier with frequency compensation to implement the output voltage control against the load variations.

To modulate the current on the inductor a current reading is required. The information regarding the peak current can be retrieved by reading, at the end of the Ton period, the voltage drop on a sense resistor in series with the power switch.

The input at the multiplier (Vmult) is responsible of the “shaping” action dictated by the input AC voltage.

The amplitude of the sinusoidal shaping action that directly imposes the reference to the PWM generator is given by the voltage loop control through the error amplifier and the frequency compensation.

In practice, the PWM block fixes the Ton time. As soon as the current on the power switch reaches the value fixed by the output multiplier level (Vcsref), the output of the comparator resets the flip-flop and turns off the power switch.

At the same time the output of the comparator triggers the timer and the counting of the offtime starts. At the end of the fixed-off-time period the timer block sets the flip-flop and the power switch is again switched on.

These functions can be implemented by using a TM controller such as the L6562 or L6563.

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AN2951	Fixed-off-time using the L6562

3 Fixed-off-time using the L6562

The circuit to implement FOT control with the TM L6562 driver is shown in Figure 4.

On the L6563 driver the VZCD pin is available which allows, working in TM, to detect the zero crossing of the current on the main inductor. Here we show how it is possible to use this pin to force the driver to work in a fixed-off-time mode.

During Ton of the power switch, pin 7 (GD gate driver) is high and the D diode is direct biased. Under this condition, because the output GD is at Vcc (usually 15v), the voltage on VZCD is internally clamped to VZCDclamp=5.7 V.

During Toff, the GD pin is low, the D diode is reverse-biased and the C capacity can be discharged by the R resistor following the exponential law:

Equation 1

−t

VZCD = VZCDclampeRC

Until the voltage on the VZCD pin reaches the triggering threshold (about 1.4 V) the power switch is switched on. Using this passive net the Toff can be set by design. Solving Equation 1 in respect to the time the Toff can be calculated:

Equation 2

Toff = RCln VZCDclamp ≈ 1.4RC

VZCDtrigger

As suggested with AN1792, it is better to select a capacitor first, and calculate the needed resistor to set the desired Toff.

The passive net composed by CS and Rs assures that, as soon as the VGD goes high the C

capacitor is charged at VZCDclamp as quickly as possible without exceeding the current capability of the VZCD pin.

Equation 3

VGD −VZCDclamp −VF				<RS <R	VGD −VZCDclamp −VF
I	+	VZCDclamp			VZCDclamp
|ZCD		R

Cs acts as a speed-up capacitor needed to instantaneously charge C in case of very short Ton when working at high AC input voltage and light load.

The Cs capacitor should be chosen as:

Equation 4

	Cs	< C		VZCDclamp
			VGD	−VZCDclamp	−VF

The FOT control allows a CCM operation and high power capability but with the circuit complexity and driver performance generally sufficient on a TM controller.

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Fixed-off-time using the L6562	AN2951

This is possible because a characteristic of an FOT control is the possibility to use a simple peak current control instead of a more difficult average mode control as required on a classic CCM control.

For further details and implications of FOT control, please refer to the AN1792 application note.

Figure 4. FOT control with the L6562 and relevant timing waveforms

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AN2951	Working mode along line period in a FOT PFC

4 Working mode along line period in a FOT PFC

Figure 5. Working mode along line semiperiod

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In Figure 5 a full description of the main characteristics and the evolution of some key parameters of an FOT, along the semiperiod of the input AC source, is shown. The dotted line represents the envelope of the peak inductor current. As clearly noted, the shape is sinusoidal as required, in order to have a high power factor. This is a consequence of the Ton modulation carried out by following the output of the multiplier Vcsref (see Figure 3).The dash-dot line represents the inductor average current. A distortion on the average instantaneous boost current value can be noted around the zero crossing of the line voltage. Though this aspect is not negligible, it is inevitable. It is due to the working mode (discontinuous) in this region. A trade off between operating frequency and line current distortion has also to be taken into account. To limit the line current distortion at high line voltage a suitably large Toff must be selected; also fixing a limited current ripple on the boost inductor, a bigger inductor size may be needed. Returning to Figure 5, the continuous line represents the inductor current ripple along the sinusoidal semiperiod (0 -π).

Some important considerations can be taken into account regarding this matter. Two kinds of working modes can be recognized during the semiperiod; DCM and CCM. For low input voltage (near zero crossing of the mains) the converter works in discontinuous mode; this means that the current reaches zero for a certain time during the switching period. The behavior along the semiperiod is symmetrical in respect to the centre of the semiperiod (π/2). Two points can be identified where the current ripple on the boost inductor is equal to the peak value. Those two points dictate the transition between the discontinuous and continuous mode, therefore we call the respective angle; the transition angle. If θ is the transition angle:

Equation 5
	θ		and		π−θ			π
DCM	0 < t < ---						< t <
	ω				ω			ω
Equation 6
	CCM	θ	< t <	π−θ
		ω		ω

The amplitude of the DCM portion, and so the angle, are not fixed but depends on input voltage and load power. As the power is reduced, the DCM region increases.

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