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Design Idea DI-53

DPA-Switch

50 W DC-DC Dual Output Converter

July 2003

DI-53

Application

Telecom

DPA425R

5 V & 3.3 V

Power Output

50 W

Input Voltage

Output Voltage

Topology

Device

36-75 VDC

Forward Sync. Rect.

Figure 1. DPA425R - 50 W, 5 V, 6 A and 3.3 V, 6 A DC-DC Converter.

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®

™

Design Highlights

• High efficiency: 90% at 36 VDC using synchronous

rectification

• Dual output with tight cross-regulation

±4% from zero to

full load on both outputs

• Output overload, open loop and thermal protection

• 300 kHz switching frequency to allow sufficient

transformer reset time with sync rectification

Operation

DPA-Switch greatly simplifies the design compared to a discrete

implementation. This design uses a coupled output inductor and

synchronous rectification to achieve excellent cross-regulation

and high efficiency.

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

recirculate 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 of the 5 V output. 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 C21 during the Q2 off time. The same drive

technique is used for the forward synchronous rectifier

MOSFET (Q4) of the 3.3 V output (with C22, R24, R25, and

VR21).

MOSFETs Q1 and Q3 are driven via resistors R23 and R26

from the transformer (T1) reset voltage and operate only

when Q2 and Q4 are off. Diodes D20 and D21 provide a

conduction path for the output inductor (L4) current when the

transformer reset is complete.

A winding on the coupled inductor L4, along with diode D4

and capacitor C9, provide the DPA-Switch bias voltage.

U1

DPA425R

D20

D4

D21

1

4,5

2,3

10

C5

0.22 µF

C9

4.7 µF

16 V

C7

1 nF

1.5 kV

C9

150 pF

200 V

VR1

SMBJ

150 A

C6

68 µF

10 V

U2

PC267

N1T

R3

15 kΩ

1%

R1

619 kΩ

1%

R4

1.0 Ω

L1

1 µH

2.5 A

C1-C3

1 µF

100 V

DL

SX

F

C

CONTROL

CONTROL

PI-3561-072203

DPA-Switch

RTN

5 V, 6 A

(8 A pk)

3.3 V, 6 A

(8 A pk)

U2

D3

BAV19WS

U3

LM431

AIM3X

R9

220 Ω

R11

10.0 kΩ

1%

R12

15.9 kΩ

1%

R10

8.66 kΩ

1%

R12

5.1 Ω

R7

10 kΩ

C23-C26

100 µF

10 V

C21-C22

4700 pF

C20

3.3 nF

C27

1 µF

C28

1 µF

C14

1 µF

C15

10 µF

10 V

C16

100 nF

R6

150 Ω

R21

10 Ω

36-75 VDC

8,9

T1

6,7

L2

220 µH

L4

3.5 µH

D1

ES1D

D2

ES1D

R20

3.3 Ω

R23

10 Ω

R26

10 Ω

R24

10 Ω

R25

10 kΩ

R22

10 kΩ

VR21

15 V

VR20

15 V

Q1

Q3

Q4

Q2

9,10

7,8

1,2

3,4

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