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 1/13

www.st.com

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 V

or 220 Vac. The operation

ac

is quite straightforward alternating the turn-on of each transistor.

Figure 1. SMPS half bridge simplified schematic

3

220VAC

110VAC

D3D4D1

D2

Q1

1

C3

2

3

1

Q2

C1

C2

1 5

T1

4 8

D5

6

D6

C4

+

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

220VAC

110VAC

D3

D4

D1

D2

Q1

1

L1

2

3

1

Q2

2

C1

1 5

T1

4 8

C2

D5

6

D6

C4

+

Ta bl e 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 V

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

Conn. 1 C22 0.47 µ R16 10 KΩ

ac

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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