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Table 1. Transformer Design Parameters.

Key Design Points

• 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.

• 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 t

• Use a Zener clamp for lowest zero load input power

consumption.

Table 2. Worst Case Output Cross-Regulation - all Outputs Taken

from Minimum to Maximum Loads.

Voltage

(V)

Load

Range

(A)

3.3

0.3-0.6

5

0.3-1.2

12

0.1-0.2

-24

0.03-0.05

Regulation (%)

-5 -4 -3 -2 -1

0

1

2

3

4

5

6

7

TRANSFORMER PARAMETERS

Core Material

Bobbin

Winding Details

Winding Order

(pin numbers)

Inductance

Primary Resonant

Frequency

Shield 1: 32T, 2 x 32 AWG

Primary: 63T, 2 x 32 AWG

Bias: 6T, 4 x 32 AWG

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).

300 kHz (minimum)

EEL25, N67 or equivalent,

7 pin + 7 pin

Primary: 800

µH ±10%

Leakage: 80

µH (max.)