DI-55
Key Design Points
TRANSFORMER PARAMETERS
• For good cross-regulation, minimize transformer leakage
- use foil for 3.3 V and 5 V outputs; minimize peak
primary currents by designing transformer for continuous
conduction mode.
EEL25, N67 or equivalent,
Core Material
gap for ALG of 202 nH/T2
7 pin + 7 pin
Bobbin
• Shunt regulator (temperature compensated) between 5 V
and 3.3 V outputs in dotted box on schematic is optional.
It is only necessary where min and max load conditions
do not occur simultaneously on both outputs.
• Feedback is taken from both the 3.3 V and 5 V outputs to
the reference (U3) via R9, R11 and R12. The 12 V output
is DC stacked on the 5 V output for enhanced regulation
and voltage centering.
• Primary clamp components VR1, D5, R7, R1 and C2 limit
the leakage inductance induced peak drain voltage spike.
• D5 is a slow recovery diode to recover some of the clamp
energy. It must be a glass passivated type to guarantee a
defined trr.
Shield 1: 32T, 2 x 32 AWG
Primary: 63T, 2 x 32 AWG
Bias: 6T, 4 x 32 AWG
Winding Details
Shield 2: 4T, 4 x 32 AWG
3.3 V/5 V foil: 2T + 1T, 0.12 mm foil
-24 V: 13T, 2 x 32 AWG
+12 V: 4T, 4 x 32 AWG
Shield (NC-1), tape, primary
(1-4), tape, bias (5-7),
tape, shield 2 (13-NC),
foil (10,11-9-13), -24 V
(14-10,11), 12 V (12-8).
Winding Order
(pin numbers)
Primary: 800 µH ±10%
Leakage: 80 µH (max.)
Inductance
Primary Resonant
Frequency
• Use a Zener clamp for lowest zero load input power
consumption.
300 kHz (minimum)
Table 1. Transformer Design Parameters.
Load
Range
(A)
Regulation (%)
Voltage
(V)
-5 -4 -3 -2 -1
0
1
2
3
4
5
6
7
3.3
5
12
0.3-0.6
0.3-1.2
0.1-0.2
-24
0.03-0.05
Table 2. Worst Case Output Cross-Regulation - all Outputs Taken
from Minimum to Maximum Loads.
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