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
DI-29
Key Design Points
• For nominal under-voltage set point V
UV
:
R1 = (V
UV
-2.35 V)/50µA. V
OV
= (R1×135
µA)+2.5
V.
• Zener VR1 voltage is 130 V to safely limit the DRAIN
voltage below V
DSS
of 200 V.
• Opto U2 should have a CTR of between 100% and 200%
for optimum loop stability.
• Set resonance of L2 and C13 + C14 to beyond loop
crossover frequency (typically 5% to 10% of switching
frequency).
• Good layout practices should be followed:
- Locate C5, C6 and R4 close to U1, with grounds
returned to the SOURCE pin.
- Primary return should be connected to the
DPA-Switch
tab, not the SOURCE pin.
- Minimize the primary and secondary loop areas to
reduce parasitic leakage inductance.
Core Material
Bobbin
Winding Details
Winding Order
(pin numbers)
Inductance
Primary Resonant
Frequency
Transformer Parameters
PR1408
Siemens N87
2
Gap for 340 nH/T
P1408 8 pin (B&B B-096 or equivalent)
Primary: 6T + 6T, 2 x 27 AWG
Secondary: 3T, 4 x 25 AWG
Bias: 6T, 32 AWG
Primary (4-FL), tape, Bias (2-3), tape,
Secondary (5,6-7,8), tape,
Primary (FL-1), tape
Primary: 49
µH ±10%,
Leakage: 1
µH
(max)
3.8 MHz (minimum)
Table 1. Transformer Construction Information.
PI-3033-081402
100
80
Efficiency (%)
60
40
36 VDC
48 VDC
60 VDC
72 VDC
20
0
0
5
10
15
20
25
Output Current (W)
Figure 2. Efficiency vs. Output Power.
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Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability.
Power Integrations does not assume any liability arising from the use of any device or circuit described herein, nor does it convey any
license under its patent rights or the rights of others. The products and applications illustrated herein (including circuits external to the
products and transformer construction) may be covered by one or more U.S. and foreign patents or potentially by pending U.S. and foreign
patent applications assigned to Power Integrations. A complete list of Power Integrations’ patents may be found at www.powerint.com.
The PI Logo,
TOPSwitch
,
TinySwitch
,
LinkSwitch
and
EcoSmart
are registered trademarks of Power Integrations, Inc.
PI Expert
is a trademark of Power Integrations Inc. ©Copyright 2002, Power Integrations, Inc.
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