ST AN2530 APPLICATION NOTE

AN2530

Application note

Solution for 150 W half bridge resonant DC-DC converter

Introduction

This application note describes a 150 W Half bridge resonant DC-DC converter. This type of SMPS is highly attractive due to its high achievable efficiency, very low noise and compact size.

Resonant converters are among the least common SMPS topologies. There are several reasons why they are not often used, but we will not discuss these reasons in this application note. However, it is worth noting that the resonant topologies have undeniable advantages over the "hard switching" topologies. The very high achievable efficiency of over 90% and up to 95% is very common, as well as their low generated noise due to ZVS (zero voltage switching) and resonant energy transfer.

Other related advantages derived from these converters are their compact size due to their need for smaller power switches (Power MOSFETs usually), smaller transformers, and less generated heat (the lower losses are a part of this). Less heat means a smaller heat sink and a longer life for power components.

If the necessary care is taken in the design phase, the results are very good and the typical issues normally associated with these topologies are avoided.

ST's L6598 half bridge driver has been chosen for this design. Please refer to the L6598 datasheet for full specifications and capabilities, or to other documentation, application notes and books where it is used, in order to have the best picture of this design. All references are provided in Figure 7.

This application note concentrates only on the power aspects, because as already mentioned, there are excellent guides for the driver (aside from the datasheet) as well as application notes for SMPS in general, magnetics, topologies, etc.

October 2007

Rev 1

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Contents	AN2530

Contents

1	Functional overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 3
2	Operational frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
3	Transformer and resonant components . . . . . . . . . . . . . . . . . . . . . . . . .	7
	3.1 Transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
4	Converter's protection schemes, overcurrent, overvoltage . . . . . . . . .	7
5	Full load, normal operation waveforms . . . . . . . . . . . . . . . . . . . . . . . . .	9
6	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
7	References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
8	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12

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AN2530	Functional overview

1 Functional overview

The simplest way of describing the functioning of a resonant converter is to compare it with a non-resonant type. Typically a "normal" half bridge transformer is connected to the principal DC bus through a capacitive divider network that creates a "false" ground to feed one of the transformer's ends (Figure 1). In this way, the transformer is fed with a voltage that swings (from the transformer's point of view) from zero to negative, negative to zero, zero to positive, then back to zero (therefore repeating the cycle).

The mains DC bus is connected as noted in Figure 1 for 110 Vac or 220 Vac. The operation is quite straightforward alternating the turn-on of each transistor.

Figure 1. SMPS half bridge simplified schematic

	3
D3	Q1	C1
	D1
	1
		C3			D5
	2	1	T1	5
220VAC				6	+	C4
110VAC					D6
		4		8
	3
		C2
D4	D2
	1
	Q2
	2

The resonant variation for this type of converter places an "external" inductor to cause a resonance between the capacitive divider network and the external inductor (Figure 2), which sums up to the already present leakage inductance of the main transformer.

These components are the ones that require most of the care for this variation of the converter. Nevertheless, remember that every aspect of the design stage has an impact on the overall behavior of the converter.

Figure 2. Resonant SMPS half bridge simplified schematic

	3
D3	Q1
	D1
	1
	L1
	2
220VAC
110VAC
	3
D4	D2
	1
	Q2
	2

C1
			D5
1	T1	5
		6	+ C4
			D6
4		8
C2

Table 1 gives the BOM (Bill of materials) for this converter. Most of the capacitors do not have an operating voltage, as they operate in low voltage. As for the driver, any voltage

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Functional overview	AN2530

greater than or equal to 16 V is acceptable. The construction details of L1 and Tr1 are discussed later.

Table 1.		Bill of materials (BOM)
Qty.		Ref.	Part	Qty.	Ref.	Part	Qty.	Ref.	Part
1		AC	220 Vac Conn.	1	C22	0.47 µ		R16	10 KΩ
2		Cac1	1 nF/400 V	2	C26b	4700 pF / 2 KV		R18	10 KΩ
		Cac2	1 nF/400 V		C26a	4700 pF / 2 KV		R23	10 KΩ
1		C1	220 µF / 400 V	1	DC	24VDC	1	R10	20 KΩ
1		C4	1 µ	1	D1	W08G	1	R11	100 KΩ
1		C8	47 µF	1	D2	STPS20H100CT	2	R13	15
3		C10	100 nF	1	D3	1N4148		R15	15
		C12	100 nF	1	D4	18 V	1	R17	39 KΩ
		C20	100 nF	1	L1	51 µ	4	R19d	1
1		C11	1 nF	2	Q2	STP8NM60N		R19c	1
2		C15	220 p		Q1	STP8NM60N		R19b	1
		C13	220 p	1	R2	150 KΩ/2 W		R19a	1
1		C14	0.22 µ	1	R3	10	1	R21	3.6 KΩ
1		C16	100 n	1	R4	150 KΩ	2	R25	1 KΩ
3		C17	33 n	1	R5	7.5 KΩ		R22	1 KΩ
		C18	33 n	2	R8	27 KΩ	1	R24	1.2 KΩ
		C23	33 n		R6	27 KΩ	1	Tr1	Transformer
3		C19c	470 µ	1	R7	6.8 KΩ	1	U1	L6598
		C19b	470 µ	6	R9	10 KΩ	1	U2	PC817
		C19a	470 µ		R12	10 KΩ	1	U3	TL431
1		C21	82 n		R14	10 KΩ

Refer to Figure 3 for the full electrical schematic of this converter.

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