DI-56
• Good layout practices
Key Design Points
- For length of +12 V secondary current loop from
transformer pin 8, diode D30 and capacitors C30, C31
and back to pin 7 of the transformer: ensure identical
path length for C30 and C31 to guarantee they equally
share the ripple current.
• For the nominal under-voltage set point VUV:
R1 = (VUV – 2.35) / 50 µA
VOV = (R1 x 135 µA) + 2.5 V
• For highest efficiency designs: use continuous conduction
mode operation designed at approximately 0.4 KRP;
minimize turns in the transformer and at this (19 W) power
level keep AC flux density (BM) <1500 Gauss; fully fill a
single layer for each winding to minimize leakage
inductance and maximize copper fill factor; if possible use
Schottky rectifying diodes (D20 and D30) with a low-
forward drop.
• The transformer primary is split in order to minimize
leakageinductanceandthusobtainbettercross-regulation.
Note:minimizingprimaryleakageinductancewillimprove
output cross-regulation at load extremes.
• The -12 V output is not directly sensed as part of the
regulation loop. Cross-regulation may be improved by
adding a second sense resistor to work in conjunction with
R10. Both resistors R10 and the second sense resistor
would be changed to 76 kΩ each.
- The same is also true for the layout of the -12 V output.
• Choosing a larger DPA-Switch will increase efficiency at
low and medium input voltages.
TRANSFORMER PARAMETERS
Core Material
Bobbin
Epcos P/N: P 14x8 N87, ungapped
8-pin P 14x8 surface mount bobbin
Primary 7T + 7T, 2 x 29 AWG
Bias 5T, 1 x 36 AWG
Winding Details
+12 V 5T, 2 x 29 AWG
-12 V 5T, 2 x 29 AWG
Primary-1 (4-NC), -12 V (6-5), Bias (2-3),
+12 V (8-7), Primary-2 (NC-1)
Winding Order &
Pin Numbers
• Set resonant frequency of post-filter (L2, C22 or L3, C32)
beyond crossover frequency (typically 5% to 10% of
switching frequency).
Primary Inductance
22 µH ±25% (at 400 kHz)
3.8 MHz (minimum)
0.75 µH (maximum)
Primary Resonant
Frequency
Leakage Inductance
Table 1. Transformer Design Parameters.
A
9/03
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