AN4129 Datasheet, PDF (24/39 Pages) Fairchild Semiconductor – Green Current Mode PWM Controller FAN7601

Circuit description and design guidance
AN4129
The injected voltage needed at the divider tap should be just sufficient to shut off the FET at
the line zero crossing. The average of this voltage should therefore be equal to the average
of an equivalent DC-input converter supplying the LEDs. The actual value of the lower
resistor is determined experimentally to give the best compromise between high line and
low line THD.
With the 270 kΩ upper divider resistor and the impedance of the ILED pin in parallel, a 3 kΩ
resistor gives good results. THD is acceptable across the 90 V to 132 V voltage range. The
divider values are not critical - 5% resistors are adequate.
A small capacitor across the lower divider resistor smooths the current pulses from the duty
cycle measurement circuitry and helps keep switching noise out of the system. This should
be in the range of 10 nanofarads for the switching frequency used in the demo.
4.12
EMI filter design
In the US, conducted noise is measured between 150 kHz and 30 MHz. Low frequency
noise is the most difficult to filter. The operating frequency was selected so that the
fundamental is just below the measurement band, and the second harmonic frequency is as
high as possible.
The noise injected into the line can be broken into two components, differential mode and
common mode. We consider the differential mode noise first. Common mode noise is a very
different problem.
Conducted emissions testing is done with a standard line impedance simulation network
(LISN). A grossly simplified diagram is shown below.
Figure 29. Simplified LISN schematic
,).%
(IGH :
,
#
,OW :
/HMS
'2/5.$
'2/5.$
/HMS
#
,OW :
.%542!,
(IGH :
,
!-V
Differential impedance in the noise spectrum (150 kHz to 30 MHz) is 100 Ω line-to-line, 50 Ω
line-to-ground.
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Doc ID 023314 Rev 1