DI-57
The bias winding is rectified and filtered by D1 and C23 and
provides operating power to the DPA-Switch. The regulation
is fed back from the secondary through the opto-transistor of
U2.
TRANSFORMER PARAMETERS
Core Material
Ferroxcubed P/N: EFD25 3F3
10-pin EFD25 surface mount bobbin
Bobbin
Key Design Points
Primary 8T + 8T, 2 x 24 AWG
Bias 5T, 1 x 34 AWG
Winding Details
• For the nominal under-voltage set point VUV:
R1 = (VUV – 2.35) / 50 µA
+12 V 5T, 4 x 27 AWG
Primary-1 (4-NC), Bias (1-5), + 12 V
(9, 10-6, 7), Primary-2 (NC-2)
Winding Order and
Pin Numbers
VOV = (R1 x 135 µA) + 2.5 V
• For highest efficiency designs: use continuous conduction
mode operation, this design uses approximately 0.5 KRP;
minimize turns in the transformer and at this (60 W) power
level, keep AC flux density BM < 1100 Gauss; fully fill a
single layer for each winding to minimize leakage
inductance and maximize copper fill factor; if possible use
a Schottky rectifier diode (D2) with a low-forward drop.
• The transformer primary is split in order to minimize
leakage inductance to obtain better efficiency.
• Tighter output voltage tolerance is possible by using a
TL431 (and associated components) in place of the Zener
diode VR2.
Primary Inductance
21 µH ±25% (at 400 kHz)
Primary Resonant
Frequency
3.0 MHz (minimum)
Leakage Inductance
1 µH (maximum)
Table 1. Transformer Design Parameters.
• Follow good layout practices:
- Forlengthofthesecondarycurrentloopfromtransformer
pins 9 & 10, diode D2 and capacitors C10 to C18 and
back to pins 6 & 7 of the transformer: ensure identical
path length for each of the capacitors C10 to C18 to
ensure theyequally share the ripple current.
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. ©Copyright 2003, Power Integrations, Inc.
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