AN4129 Datasheet, PDF (26/39 Pages) Fairchild Semiconductor – Green Current Mode PWM Controller FAN7601

Circuit description and design guidance
Figure 32. Flyback converter input current waveform
AN4129
The peak current is about 0.7 A, and the on-time is about 3.5 microseconds. The charge
that must be delivered by the capacitor directly across the converter each cycle is about:
Equation 6
The capacitance needed was determined experimentally - 0.1 µF is a good starting point at
this power level.
So, ripple voltage for the 0.1 µF capacitor is about:
Equation 7
Values as small as 0.047 µF can be made to work, but the inductor value must increase to
keep the noise on the AC line low enough. The resulting inductors either become physically
large, or the resistive loss in the winding affects efficiency.
Next we examine the input capacitor. We use 0.1 µF here as well.
At 150 kHz, the 0.1 µF capacitor has a reactance of about 10 Ω. This reactance shunts away
90% of the noise current from the LISN, leaving only about 10% to the input.
So 12 Vp-p must be reduced to about 12 mVp-p (half the noise appears on each line
terminal relative to ground). The differential inductor must have a reactance of about 1000
Ω. At 150 kHz the needed inductance is about 10 mHy. This can be split into two small
chokes, one in each line, so that some common mode noise is also attenuated. 2 x 4.7 uHy
was used. The selected inductors have about 5.5 Ω of DC resistance, so I²R loss is
relatively low.
The inductors have a self-resonance in the 2 to 5 MHz range. This is a common mode
resonance that can cause some trouble. It is damped by placing a resistor across each
inductor. Self-resonance is frequently stated on inductor datasheets, or it can be measured.
The resistor value used is the same as the inductor impedance at resonance:
Equation 8
The resistor has no effect on the rest of the filter at lower frequencies - it's swamped by the
lower impedance of the inductor.
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Doc ID 023314 Rev 1