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Design Idea DI-29

DPA-Switch

25 W Flyback DC-DC Converter

December 2002

DI-29

®

Application

DC-DC Converter

DPA425R

7 V

Power Output

25 W

Input Voltage

Output Voltage

Topology

Device

36 - 75 VDC

Flyback

Figure 1. DPA-Switch Flyback DC-DC Converter Schematic.

Design Highlights

• Extremely low component count

• High efficiency – 85% using Schottky rectifiers

• No current sense resistor or transformer required

• Accurate input under/over voltage meets ETSI standards

• Operates to zero load with no pre-load required

• Output overload, open loop and thermally protected

• 400 kHz operation minimizes size of magnetics

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. Including tolerances

these thresholds guarantee the converter is operational between

36 V and 75 V, without the cost of additional line sense

components.

Resistor R3 programs the internal current limit of the

DPA425R to 50% of nominal. The larger DPA-Switch

selection reduces conduction losses, raising efficiency without

circuit changes or increased overload power.

Zener VR1 clamps leakage inductance spikes, keeping the

DRAIN voltage below BV

provided from an auxiliary flyback transformer winding.

On the secondary, a snubber across D2 (C9, R5 and R13)

limits the secondary leakage inductance spikes generated by

diode reverse recovery. Inductor L2, C13 and C14 form a

post-filter to reduce high frequency output switching ripple.

A soft-finish network, C18, D3 and R7, eliminates output

turn-on overshoot. The remaining components provide output

voltage regulation and loop compensation.

U1

DPA425R

D2 MBRB2545CT

D1 BAV19WS

C3

1 µF

100 V

1

7,8

5,6

FL

4

3

2

C5

220 nF

VR1

SMBJ

130

C6

68 µF

10 V

C7 1 nF

1.5 kV

R14

10 Ω

U2

R13

6.8 Ω

R5

6.8 Ω

C9

2.2 nF

50 V

R3

15 kΩ

1%

R1

619 kΩ

1%

R4

1.0 Ω

C13

100 µF

10 V

C14

100 µF

10 V

C15

1 µF

L1

1 µH 2.5 A

C23

4.7 µF

20 V

C1

1 µF

100 V

C2

1 µF

100 V

DL

SX

F

C

CONTROL

CONTROL

PI-3012-120902

DPA-Switch

T1

RTN

7 V, 3.57 A

U2

D3

BAV19WS

U3

LM431AIM3

R9

220 Ω

R11

10.0 kΩ

1%

R10

18.2 kΩ

1%

R12

5.1 Ω

C10

100 µF

10 V

C11

100 µF

10 V

C12

100 µF

10 V

R7

10 kΩ

C17

1 µF

C18

10 µF

10 V

C16

100 nF

R6

150 Ω

L2

300 nH

6 A

36-75 VDC

®