®
Design Idea DI-24
DPA-Switch®
30 W DC-DC Converter
Topology
Device
Power Output
Input Voltage
Output Voltage
Application
Forward
DC-DC Converter
30 W
36-75 VDC
DPA424R
5 V
Design Highlights
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.
• 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
CapacitorsC8andC9providetransformercorereset;C8also
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.
• 400 kHz operation minimizes size of magnetics
• Available for prototyping in DAK-21
Operation
DPA-Switchgreatlysimplifiesthedesigncomparedtoadiscrete
implementation. Resistor R1 programs the input under/over
voltagesto33Vand86V,respectively,andlinearlyreducesthe
maximumdutycyclewithinputvoltagetopreventcoresaturation
during load transients. These thresholds have tolerances that
guarantee the converter is operational at 36 V, without the cost
of additional line sense components.
ThebiassupplyforU1isprovidedfromanauxiliarywinding
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.
R14
10 Ω
C7 1 nF
1.5 kV
+V
36-75 VDC
L1
IN
1 µH 2.5 A
C9
T1
R5
1.0 Ω
2.2 nF
50 V
1
L2
C12
1 µF
10 V
C10
100 µF
10 V
C11
100 µF
10 V
8 µH 6 A
R13
160 Ω
5 V, 6 A
7,8
5,6
C8
47 pF
200 V
R1
619 kΩ
1%
7,8
5,6
3
4
2
1
D2
MBRB2545CT
D1
BAV19WS
RTN
C4
4.7 µF
20 V
U2
R7
10 kΩ
C1
C2
1 µF
100 V
C3
R10
1 µF
100 V
1 µF
10.0 kΩ
DPA-Switch
100 V
1%
U1
DPA424R
C16
D3
BAV19WS
U2
100 nF
D
S
L
R6
150 Ω
R12
5.1 Ω
CONTROL
C
R9
220 Ω
C13
R4
X
F
C14
1 µF
10 µF
R3
1.0 Ω
10 V
VR1
8.25 kΩ
C5
220 nF
U3
SMBJ
150
1%
C6
68 µF
10 V
R11
LM431AIM3
10.0 kΩ
-V
IN
1%
PI-2993-090602
Figure 1. DPA-Switch 30 W DC-DC Converter.
September 2002
DI-24
www.powerint.com
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