®
Design Idea DI-29
®
DPA-Switch
25 W Flyback DC-DC Converter
Topology
Flyback
Device
Power Output
Input Voltage
Output Voltage
Application
DC-DC Converter
25 W
36 - 75 VDC
DPA425R
7 V
Design Highlights
Resistor R3 programs the internal current limit of the
DPA425R to 50% of nominal. The larger DPA-Switch
selectionreducesconductionlosses,raisingefficiencywithout
circuit changes or increased overload power.
• 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
Zener VR1 clamps leakage inductance spikes, keeping the
DRAIN voltage below BVDSS. The bias supply for U1 is
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-onovershoot.Theremainingcomponentsprovideoutput
voltage regulation and loop compensation.
Operation
DPA-Switchgreatlysimplifiesthedesigncomparedtoadiscrete
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.
R5
6.8 Ω
C9
2.2 nF
50 V
R13
6.8 Ω
C7 1 nF
R14
1.5 kV
10 Ω
L2
300 nH
6 A
C10
100 µF 100 µF 100 µF
10 V 10 V 10 V
C11
C12
C13
C14
+ V
L1
36-75 VDC 1 µH 2.5 A
IN
C15
100 µF 100 µF
1 µF 7 V, 3.57 A
10 V 10 V
T1
7,8
1
D2 MBRB2545CT
D1 BAV19WS
RTN
5,6
FL
2
3
C23
4.7 µF
20 V
4
R1
619 kΩ
1%
R10
18.2 kΩ
1%
U2
R7
10 kΩ
U2
C1
1 µF
100 V
C2
1 µF
100 V
C3
1 µF
100 V
DPA-Switch
C16
100 nF
U1
D3
BAV19WS
DPA425R
R6
R12
D
S
L
150 Ω
5.1 Ω
CONTROL
C
C17
R9
1 µF
220 Ω
C18
10 µF
10 V
R4
X
F
R3
1.0 Ω
VR1
15 kΩ
C5
220 nF
SMBJ
130
1%
U3
C6
68 µF
10 V
R11
LM431AIM3
10.0 kΩ
-V
IN
1%
PI-3012-120902
Figure 1. DPA-Switch Flyback DC-DC Converter Schematic.
December 2002
DI-29
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