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Design Idea DI-24

DPA-Switch

30 W DC-DC Converter

September 2002

DI-24

®

Application

DC-DC Converter

DPA424R

5 V

Power Output

30 W

Input Voltage

Output Voltage

Topology

Device

36-75 VDC

Forward

Design Highlights

• Extremely low component count

• High efficiency – 85% using Schottky rectifiers

• No current sense resistor or current transformer required

• Output overload, open loop and thermal protection

• Accurate input under/over voltage meets ETSI standards

• Operates to zero load

• 400 kHz operation minimizes size of magnetics

• Available for prototyping in DAK-21

Operation

DPA-Switch greatly simplifies the design compared to a discrete

implementation. Resistor R1 programs the input under/over

voltages to 33 V and 86 V, respectively, and linearly reduces the

maximum duty cycle with input voltage to prevent core saturation

during load transients. These thresholds have tolerances that

guarantee the converter is operational at 36 V, without the cost

of additional line sense components.

Figure 1. DPA-Switch 30 W DC-DC Converter.

Resistor R3 programs the internal DPA424R current limit to

85% of nominal, just above the level needed at full load,

limiting overload power. This feature also allows a larger

DPA-Switch to be selected, without requiring any other

circuit changes. A larger DPA-Switch reduces conduction

losses, raising efficiency.

Capacitors C8 and C9 provide transformer core reset; C8 also

limits the leakage inductance spike on the DRAIN. Resistor

R5 in series with C9 damps ringing. Zener diode VR1

provides a hard voltage clamp to limit DRAIN voltage, but is

only active during transients and overload conditions.

The bias supply for U1 is provided from an auxiliary winding

on output inductor L2. This gives higher efficiency than a

transformer winding, since it provides a fixed voltage

independent of input voltage. Pre-load R13 maintains the

bias voltage

≥8 V at zero load.

On the secondary, a soft finish network, C13, D3 and R7,

eliminates output turn-on overshoot. The remaining

components provide output voltage regulation and loop

compensation.

U1

DPA424R

D2

MBRB2545CT

D1

BAV19WS

C3

1 µF

100 V

1

7,8

1

7,8

5,6

4

3

C8

47 pF

200 V

C5

220 nF

VR1

SMBJ

150

C6

68 µF

10 V

C7 1 nF

1.5 kV

R14

10 Ω

U2

R5

1.0 Ω

C9

2.2 nF

50 V

R3

8.25 kΩ

1%

R1

619 kΩ

1%

R4

1.0 Ω

C10

100 µF

10 V

C11

100 µF

10 V

C12

1 µF

10 V

-VIN

+VIN

36-75 VDC

L1

1 µH 2.5 A

L2

8 µH 6 A

C4

4.7 µF

20 V

R13

160 Ω

C1

1 µF

100 V

C2

1 µF

100 V

DL

SX

F

C

CONTROL

CONTROL

PI-2993-090602

DPA-Switch

T1

RTN

5 V, 6 A

U2

D3

BAV19WS

U3

LM431AIM3

R9

220 Ω

R11

10.0 kΩ

1%

R10

10.0 kΩ

1%

R12

5.1 Ω

R7

10 kΩ

C14

1 µF

C13

10 µF

10 V

C16

100 nF

R6

150 Ω

5,6

2

®