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Design Idea DI-29 (R) 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
(R)
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 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-on overshoot. The remaining components provide output voltage regulation and loop compensation.
R5 6.8
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 H 2.5 A
1
R14 10
T1 7,8
C9 2.2 nF R13 50 V 6.8 C12 C11 C10 100 F 100 F 100 F 10 V 10 V 10 V
L2 C14 C13 300 nH 100 F 100 F C15 6A 10 V 1 F 10 V
7 V, 3.57 A
FL
5,6
D2 MBRB2545CT D1 BAV19WS
RTN
2 4 3
C23 4.7 F 20 V U2 R7 U2 10 k R10 18.2 k 1%
R1 619 k 1% C1 1 F 100 V C2 1 F 100 V C3 1 F 100 V
DPA-Switch
U1 DPA425R
D L
CONTROL
D3 BAV19WS R6 150
C16 100 nF R12 5.1 C17 1 F R9 220
C
S VR1 SMBJ 130
X
F
R3 15 k C5 1% 220 nF
R4 1.0 C6 68 F 10 V
C18 10 F 10 V U3 LM431AIM3
-VIN
R11 10.0 k 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 VUV: R1 = (VUV-2.35 V)/50A. VOV = (R1x135 A)+2.5 V. * Zener VR1 voltage is 130 V to safely limit the DRAIN voltage below VDSS 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 H 10%, Leakage: 1 H (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. For the latest updates, visit our Web site: www.powerint.com 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. (c)Copyright 2002, Power Integrations, Inc.
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B 12/02
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