APPLICATION NOTE

AN4145

REV. 1.0 4/6/04

7

Figure 13. Primary Voltage Waveform Without Ringing

Circuitry (C2, R2)

Figure 14. Primary Voltage Waveform with Ringing

Circuitry (C2, R2)

As a first pass, one method of determining the values for

C2 and R2 is as follows:

1.

Determine the frequency of the ringing waveform and

calculate the period.

2.

Multiply the period, determined in Step 1, by 5.

3.

Assume a value for the resistor (usually less than 100R).

4.

Calculate the size of the capacitor by dividing the

number obtained in Step 2 by the size of the resistor

in Step 3.

Note: The advantage of using the resistor R2 and the capac-

itor C2 network is that it reduces the ringing as shown in Fig-

ure 11, but the disadvantage is that the high frequency ripple

passing through capacitor C2 gets dissipated as heat in resis-

tor R2. If noise reduction is more important than efficiency

then proceed, otherwise efficiency will drop off some.

Printed Circuit Board Guidelines

1.

Take the time to place and orient the components

properly.

2.

If heat sinks are used, make sure they are grounded.

3.

A component shield maybe required.

4.

Common mode capacitors should have low ESR values

as well as maintain a short lead length to the ground

place.

5.

If a snubber circuit is being used across the transformer

to slow down the rise time of the MOSFET switch turn-

ing off, make sure that the trace lengths from the drain

and two primary transformer pins are short. If possible

place the snubber circuitry between the two primary

pins.

6.

avoid slots in the ground and power (if used) planes

7.

Under 50 MHz (don’t forget to consider the harmonics

of the PWM controller) traditional decoupling methods

are effective. Use one or two decoupling capacitors

(often 0.1 or 0.01 µF) placed close to the IC power and

ground pins. Consider the loop area formed between the

decoupling capacitor and the IC, and place the capacitor

for minimum loop area.

8.

Keep ground runs as short as possible and as thick and

large as possible.

9.

Avoid sharp corners on traces.

10. Try to group all noisy component in the same area just

in case shielding is required.

11. Use multi-layer printed circuit board if possible.