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Design Idea DI-40

Converter with Synchronous Rectification

May 2003

DI-40

Application

DC-DC Converter

DPA424R

2.5 V

Power Output

20 W

Input Voltage

Output Voltage

Topology

Device

36-75 VDC

Forward

Figure 1. DPA-Switch 20 W DC to DC Converter.

.

Design Highlights

• Extremely low component count

• High efficiency - 86% using synchronous rectification

• No current sense resistor or current transformer required

• Output overload, open loop and thermal protection

• Accurate input under/overvoltage meets ETSI standards

• 300 kHz switching frequency - optimizes efficiency when

simple self-driven synchronous rectification is used

Operation

DPA-Switch greatly simplifies the design compared to a discrete

implementation. Resistor R1 programs the input UV/OV

thresholds. The tight tolerance of the UV/OV thresholds limits

the range of gate drive voltages applied to MOSFETs Q1 and

Q2, eliminating the need for gate voltage clamp circuitry. The

self-driven synchronous rectification configuration is therefore

very simple, with R13 filtering voltage spikes at the gate of Q2,

and D3 preventing the body diode of Q1 from conducting.

Capacitor C8 and the gate capacitance of Q1 reset T1 during

DPA-Switch off-time. Zener VR1 provides a hard voltage

clamp to limit DRAIN voltage under output transient and

overload conditions.

Since the output voltage is low, the U2 LED is supplied with

a higher voltage derived from a winding on output choke L2.

The DPA-Switch bias supply is derived from a forward

winding on transformer T1. Flyback windings are not

recommended for this purpose, since the bias capacitor C4

would create high capacitive loading during DPA-Switch

off- time, preventing the transformer from efficiently resetting.

Key Design Points

• For

nominal

under-voltage

set

point

V

R1=(V

• Locate C5, C6, and R4 close to U1 CONTROL pin, with

ground connected to SOURCE pin.

•Minimize primary and secondary layout loop area to

reduce parasitic inductance.

®



C5

0.22 µF

VR1

SMBJ150

C6

68 µF

10 V

D3

12CWQ03FM

C4

1 µF

R3

15 kΩ

1%

R4

1.0 Ω

C9, C10

100 µF

10 V

C11

1 µF

10 V

36 - 75 VDC

L1

1 µH 2.5 A

L2

3.3 µH

8 A

L3

2.2 mH

40 mA

PI-3264-051303

DPA-Switch

2.5 V, 8 A

R13

10 Ω

U1

DPA424R

1

4

6, 7

2

3

C7

1 nF

1.5 kV

R1

619 kΩ

T1

R5

1.0 Ω

D

S

C

CONTROL

L

F

X

U2

PC357

N1T

9, 10

C8

2.2 nF

C12

4.7 µF

20 V

D2

BAV19WS

7, 8

2

1

5, 6

RTN

C1, C2

1 µF

100 V

Q1

SI4842DY

Q2

SI4842DY

D1

BAV19WS

R14

10 Ω

V

IN

+

V

IN

U2

D3

BAV19WS

U3

LMV431

AIM5X

R9

220 Ω

R11

10.0 kΩ

1%

R10

10.2 kΩ

1%

R12

5.1 Ω

R7

10 kΩ

C14

1 µF

C15

10 µF

10 V

C13

100 nF

R6

150 Ω

C3

1 µF

100 V

D4

BAV19WS