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Design Idea DI-40 DPA-Switch 2.5 V, 20 W DC-DC
Converter with Synchronous Rectification
Application DC-DC Converter Device DPA424R Power Output 20 W Input Voltage 36-75 VDC Output Voltage 2.5 V
(R)
Topology Forward
Design Highlights
* * * * * * Extremely low component count High efficiency - 86% using synchronous rectification No current sense resistor or current transformer required Output overload, open loop and thermal protection Accurate input under/overvoltage meets ETSI standards 300 kHz switching frequency - optimizes efficiency when simple self-driven synchronous rectification is used
Capacitor C8 and the gate capacitance of Q1 reset T1 during DPA-Switch off-time. Zener VR1 provides a hard voltage clamp to limit DRAIN voltage under output transient and overload conditions. Since the output voltage is low, the U2 LED is supplied with a higher voltage derived from a winding on output choke L2. The DPA-Switch bias supply is derived from a forward winding on transformer T1. Flyback windings are not recommended for this purpose, since the bias capacitor C4 would create high capacitive loading during DPA-Switch off- time, preventing the transformer from efficiently resetting.
Operation
DPA-Switch greatly simplifies the design compared to a discrete implementation. Resistor R1 programs the input UV/OV thresholds. The tight tolerance of the UV/OV thresholds limits the range of gate drive voltages applied to MOSFETs Q1 and Q2, eliminating the need for gate voltage clamp circuitry. The self-driven synchronous rectification configuration is therefore very simple, with R13 filtering voltage spikes at the gate of Q2, and D3 preventing the body diode of Q1 from conducting.
Key Design Points
* For nominal under-voltage set point V UV : R1=(VUV- 2.35 V)/50 A. VOV = (R1 x 135 A)+2.5 V. * Locate C5, C6, and R4 close to U1 CONTROL pin, with ground connected to SOURCE pin. * Minimize primary and secondary layout loop area to reduce parasitic inductance.
L2 D3 3.3 H 12CWQ03FM 8 A
C7 1 nF 1.5 kV
R14 10 2.5 V, 8 A
+ VIN
36 - 75 VDC L1 1 H 2.5 A 1
T1
9, 10
7, 8
R13 10
R1 619 k Q1 SI4842DY 6, 7 Q2 SI4842DY R5 C8 2.2 nF 1.0
1
5, 6 2 D2 BAV19WS C9, C10 100 F 10 V C11 1 F 10 V RTN
C1, C2 1 F 100 V
C3 1 F 100 V
4 2
C12 4.7 F 20 V
D1 BAV19WS D4 BAV19WS
3
D L
L3 2.2 mH 40 mA C4 1 F
DPA-Switch
C
CONTROL
U1 DPA424R
U2 PC357 N1T
U2 R7 10 k R10 10.2 k 1% C13 100 nF R12 5.1
VR1 SMBJ150
S
X
F
R4 1.0 C5 0.22 F
R3 15 k 1% VIN
D3 BAV19WS R6 150 C15 10 F 10 V U3 LMV431 AIM5X
C6 68 F 10 V
R9 220
C14 1 F R11 10.0 k 1%
PI-3264-051303
Figure 1. DPA-Switch 20 W DC to DC Converter. .
DI-40
www.powerint.com
May 2003
DI-40
* Optocoupler U2 should have controlled CTR of 100-200% for optimum loop stability. * Size transformer reset components C8 and R5, and the Q1 gate capacitance to assure transformer reset at minimum operating voltage without exceeding 170 V drain voltage at high line. * Set Zener VR1 clamp voltage to 150 V to guarantee both transformer reset and limit DRAIN voltage below BVDSS. * Scale primary side forward bias winding to provide 12 V to 15 V at minimum input voltage, nominal load. * Secondary choke winding provides 5 V at nominal load. * Main primary power return should be connected to the DPA-Switch tab, not to the SOURCE pin. * Consult AN-31 for additional design tips.
PI-3378-091702
TRANSFORMER PARAMETERS
Core Material Bobbin PR1408 EPCOS N87 material ungapped P1408 8 pin (B&B B-096 or equivalent) Bias: 6T, 32 AWG Primary: 7T + 7T, 2 x 28 AWG Secondary: 2T, 4 x 26 AWG Bias: (3-2), Tape Primary: (4-FL), Tape Secondary: (6, 7-9, 10), Tape Primary: (FL-10), Tape 392 H 25% 3 MHz (minimum) 1 H (maximum)
Turns
Winding Order (Pin Numbers)
Primary Inductance Primary Resonant Frequency Leakage Inductance
95 90 85
Table 1. Transformer Construction Information.
Efficiency (%)
80 75 70 65 60
Winding Details Bias: 10T, 32 AWG Main: 4T, 4 x 26 AWG Bias: (1-2), Tape Main: (7,8-5,6), Tape 3.3 H 10%
OUTPUT INDUCTOR PARAMETERS
VIN = 36 VDC VIN = 48 VDC VIN = 60 VDC VIN = 72 VDC
Core Bobbin EPCOS N87 material Gap for AL of 206 nH/T2 P1408 8 pin (B&B B-096 or equivalent)
0
5
10
15
20
Winding Order (Pin Numbers) Inductance Pins 5,6-7,8
Output Power (W)
Figure 2. Efficiency vs. Output Power.
Table 2. Output Inductor Construction Information. 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 and DPA-Switch are trademarks of Power Integrations, Inc. (c)Copyright 2003, Power Integrations, Inc.
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C 5/03
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