U1

DPA426R

1

4

T1

5

2

C5

0.22 µF

VR1

SMBJ

150 A

C6

68 µF

10 V

C7 1 nF

1.5 kV

R14

10 Ω

U2

PC357

NT

R3

11 kΩ

1%

R1

619 kΩ

1%

R4

1.0 Ω

C25

1 µF

50 V

D3

BAV19WS

D4

BAV19WS

C8

1 µF

Q2

Si4486

Q1

Si4486

C21

2.2 nF

VR21

15 V

VR20

15 V

C1-C4

0.22 µF

100 V

C9

150 pF

200 V

D2

ESD1

D1

ESD1

DL

SX

F

C

CONTROL

CONTROL

PI-3550-062403

DPA-Switch

RTN

12 V, 5 A

U2

D3

BAV19WS

U3

LMV431

AIM5X

R9

220 Ω

R11

10 kΩ

1%

R10

38 kΩ

1%

R12

5.1 Ω

C23

100 µF

16 V

C24

100 µF

16 V

D20

12 CWQ

10 FN

R7

10 kΩ

C17

1 µF

C15

10 µF

10 V

C13

100 nF

R6

150 Ω

L4

40 µH

L3

2.2 mH

40 mA

36-75 VDC

6,7

10,9

4,5

9,8

L1

1 µH

2.5 A

L2

220 µH

C20

1 nF

R21

10 Ω

R24

10 kΩ

R22

10 kΩ

R23

10 Ω

R20

0.5 Ω

1 W

C22

1 nF

www.powerint.com

Design Idea DI-52

DPA-Switch

60 W DC-DC Converter

July 2003

DI-52

Application

Telecom

DPA426R

12 V

Power Output

60 W

Input Voltage

Output Voltage

Topology

Device

36-75 VDC

Forward Sync. Rect.

Figure 1. DPA426R - 60 W, 12 V, 5 A, DC-DC Converter.

.

Design Highlights

• Low component count

• High efficiency: 91.5% at 36 VDC using synchronous

rectification

• Capacitor coupled synchronous rectification allows higher

output voltages without overstressing MOSFET gates

• No current sense resistor or current transformer required

• Output overload, open loop and thermal protection

• 300 kHz switching frequency to allow sufficient

transformer reset time

• 3.55 x 2.1 x 0.6 inch (approx. 13.4 W/cubic inch)

Operation

DPA-Switch greatly simplifies the design compared to a discrete

implementation. The capacitor coupled synchronous rectifier

drive used in this design is useful for higher voltage outputs, still

allowing passive MOSFET drive without gate overvoltage,

which would result from direct resistor drive.

Resistor R1 programs the under/over voltages and linearly

reduces the maximum duty cycle with input voltage to

prevent core saturation during load transients. Components

D1, D2, C9, and L2 implement a resonant clamp circuit to

catch and re-circulate the transformer leakage energy during

normal operation, with Zener VR1 providing absolute

clamping for transient conditions.

Capacitor C21 charges the gate of Q2, the forward synchronous

rectifier MOSFET. Resistor R21 limits gate oscillation and

R22 provides gate pull down. Zener diode VR20 limits the

Q2 gate voltage during conduction and also reverse charges

(resets) C21 during the Q2 off time.

A similar drive technique is used for the catch synchronous

rectifier MOSFET Q1 (with C22, R23, R24, and VR21).

MOSFET Q1 is driven by the transformer (T1) reset voltage

and operates only when Q2 is off. Diode D20 provides a

®

™