Design Idea DI-29
®
DPA-Switch
25 W Flyback DC-DC Converter
Application
DC-DC Converter
Device
DPA425R
Power Output
25 W
Input Voltage
36 - 75 VDC
Output Voltage
7V
®
Topology
Flyback
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
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
DSS
. 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-on overshoot. The remaining components provide output
voltage regulation and loop compensation.
R5
6.8
Ω
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.
C7 1 nF
1.5 kV
+
VIN
36-75 VDC
L1
1
µH
2.5 A
1
R14
10
Ω
T1
7,8
C9
2.2 nF R13
50 V 6.8
Ω
C12
C11
C10
100
µF
100
µF
100
µF
10 V 10 V
10 V
L2
C14
C13
300 nH
100
µF
100
µF
C15
6A
10 V 10 V 1
µF
7 V, 3.57 A
FL
5,6
D2 MBRB2545CT
D1 BAV19WS
RTN
2
4
3
C23
4.7
µF
20 V
U2
R7 U2
10 kΩ
R10
18.2 kΩ
1%
R1
619 kΩ
1%
C1
1
µF
100 V
C2
1
µF
100 V
C3
1
µF
100 V
DPA-Switch
U1
DPA425R
D
L
CONTROL
D3
BAV19WS
R6
150
Ω
C16
100 nF
R12
5.1
Ω
C17
1
µF
R9
220
Ω
C
S
X
F
VR1
SMBJ
130
-VIN
R3
15 kΩ C5
1% 220 nF
R4
1.0
Ω
C6
68
µF
10 V
C18
10
µF
10 V
U3
LM431AIM3
R11
10.0 kΩ
1%
PI-3012-120902
Figure 1. DPA-Switch Flyback DC-DC Converter Schematic.
DI-29
www.powerint.com
December 2002
POWERINT [ POWER INTEGRATIONS, INC. ]
