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AN1300 APPLICATION NOTE

DESIGNING 36 WATT SERIES RESONANT CONVERTER

General Specifications:

Input Voltage:

85 - 130 Vrms

Output Voltage:

12 VDC +/- 2%

Rated Output Current:

3 A

Lightest Output Current:

0.5 A

The tank circuit will operate at a resonant frequency of 100 kHz.

Transformer Design

The E-E core size for a forward converter of this output power is about 25 mm. I will use TDK part number EE25/

19 core using the H7C1 material in this example.

The number of turns needed for the primary, operating at about 40 percent of the saturation flux density (1800

G), is about 49 turns. We will use a 2 section bobbin to "enhance" the primary’s leakage inductance, though, is

a very complex phenomenon, and this result will only be a rough estimation.

[Eq. 4]

Where:

K1 is equal to 3 for a simple primary and secondary winding

W1 is the length of the winding from end-to-end (in)

This relationship shows the largest factors in the amount of leakage inductance a winding will exhibit, is most

Plugging in the wire and core-related dimensions and solving, one arrives at a preliminary leakage inductance

of 44

µH. This will be used to calculate a preliminary value for the resonant capacitor.

The number of turns needed for the auxiliary and output windings will be 10 turns. I will use litz wire on the

secondary winding of 12 strands of 30 AWG to enhance the flexibility of the wire.

Design of the Tank Circuit

Calculating the value of the resonant capacitor to produce a resonance frequency of 100 kHz, one uses:

[Eq. 5]

Solving for this, using the leakage inductance found above, one gets a resonant capacitance of 0.056

µF.

Design of the Output Circuit

The value of the output capacitance to produce the desired level of ripple voltage is found by:

[Eq. 6]

This makes the output capacitance equal to 100

µF, which is needed to produce a ripple voltage of 30 mV when

the power supply is fully loaded at an output of 3 amperes.

L

lea k

K1 L

mt

()nx

2

100W

l

b

w

3

-------

+





≈

C

r

1

2

πrf

()

2

L

r

-------------------------

=

C

o

I

out m ax

–

[]

fV

rip ple p

p

–

()

()

-----------------------------------------

=