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Design Idea DI-31

Converter with Synchronous Rectification

December 2002

DI-31

™

Application

DC-DC Converter

DPA426R

5 V

Power Output

70 W

Input Voltage

Output Voltage

Topology

Device

36-75 VDC

Forward

Design Highlights

• Extremely low component count

• High efficiency, 90% 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 using

simple self-driven synchronous rectification

Operation

The 70 W converter shown in Figure 1 benefits from many of

the DPA-Switch integrated features. In particular, no external

current sense components are required. In a discrete

implementation, an expensive current transformer and a number

of additional components would increase the cost of this

converter significantly.

R1 programs the input UV/OV thresholds. The tight tolerance

of the UV/OV thresholds limits the range of gate drive voltages

Figure 1. DPA426 70 W, 5 V, 14 A DC-DC Converter.

®

applied to MOSFETs Q1 to Q6, eliminating the need for gate

voltage clamp circuitry. The self-driven synchronous

rectification configuration is therefore very simple, with R4

to R6 filtering voltage spikes at the gates of Q1, Q2 and Q3

and D4 preventing the body diodes of Q4, Q5 and Q6 from

conducting.

Capacitor C9, diodes D1-D2, and inductor L2 form a resonant

snubber that recycles leakage and magnetizing energy stored

in T1, and also helps to reset T1. Zener diode VR1 provides

a hard voltage limit and only conducts during output transient

and overload conditions. Capacitor C12 and Resistor R7

damp secondary switching spikes and help to reset T1.

Key Design Points

• For nominal undervoltage set point V

R1 = (V

• Locate C10, C11, and R3 close to the U1 CONTROL pin,

with ground connections returned to SOURCE pin.

36-75 VDC

C1, C2

1 µF

100 V

C3, C4,

C5, C6

1 µF 100 V

(x4)

C13, C14,

C15, C16, C17

100 µF 10 V

(x5)

U1

DPA426R

1

5

4

9,10

6,7

3

2

C9

150 pF

200 V

C10

220 nF

C11

68 µF

10 V

C7 1 nF

1.5 kV

R2

6.8 kΩ

1 %

R1

619 kΩ

R3

1.0 Ω

DL

SX

F

C

CONTROL

CONTROL

U2

PC357N1T

DPA-Switch

T1

D1

ESD1

D2

ESD1

L2

220 µH

C12 2.2 nF, 50 V

BAV19WS

D3

VR1

SMBJ

150

D4

42CTQ30S

PI-3375-121602

R4-R6

10 Ω

Q1-Q3

SI4842DY

Q4-Q6

SI4842DY

L1

1 µH

2.5 A

L3

3.8 µH

20 A

U2

C18, C19

100 µF

10 V

C20

1 µF

10 V

D6

BAV19WS

U3

LM431AIM3

C23

10 µF

10 V

R14

220 Ω

R16

10.0 kΩ

1%

R15

10.0 kΩ

1%

R11

150 Ω

C22

100 nF

R10

5.1 Ω

C21 1 µF

R13

10 kΩ

RTN

5 V, 14 A

L4

100 nH

20 A

D5

BAV19WS

L5

2.2 mH

40 mA

C8

1 µF

25 V

R7

1 Ω