FAN100_09 Datasheet, PDF (11/16 Pages) Fairchild Semiconductor – Primary-Side-Control PWM Controller

Temperature Compensation
Built-in temperature compensation provides constant
voltage regulation over a wide range of temperature
variation. This internal compensation current
compensates the forward-voltage drop variation of the
secondary side rectifier diode.
Green-Mode Operation
The FAN100 uses voltage regulation error amplifier
output (VCOMV) as an indicator of the output load and
modulates the PWM frequency as shown in Figure 25
such that the switching frequency decreases as load
decreases. In heavy-load conditions, the switching
frequency is fixed at 42KHz. Once VCOMV decreases
below 2.8V, the PWM frequency starts to linearly
decrease from 42KHz to 550Hz to reduce the switching
losses. As VCOMV decreases below 0.8V, the switching
frequency is fixed at 550Hz and FAN100 enters into
“deep green” mode, where the operating current reduces
to 1mA, reducing the standby power consumption.
Figure 26. Frequency Hopping
Figure 25. Switching Frequency in Green Mode
Leading-Edge Blanking (LEB)
At the instant the MOSFET is turned on, a high-current
spike occurs through the MOSFET, caused by primary-
side capacitance and secondary-side rectifier reverse
recovery. Excessive voltage across the RCS resistor can
lead to premature turn-off of the MOSFET. FAN100
employs an internal leading edge blanking (LEB) circuit
to inhibit the PWM comparator for a short time after the
MOSFET turns on. External RC filtering is not required.
Frequency Hopping
EMI reduction is accomplished by frequency hopping,
which spreads the energy over a wider frequency range
than the bandwidth measured by the EMI test equipment.
FAN100 has an internal frequency-hopping circuit that
changes the switching frequency between 39.4kHz and
44.6kHz with a period of 3ms, as shown in Figure 26.
Startup
Figure 27 shows the typical startup circuit and
transformer auxiliary winding for FAN100 application.
Before FAN100 begins switching, it consumes only
startup current (maximum 10μA) and the current
supplied through the startup resistor charges the VDD
capacitor (CDD). When VDD reaches turn-on voltage of
16V (VDD-ON), FAN100 begins switching, and the current
consumed increases to 3.5mA. Then, the power
required for FAN100 is supplied from the transformer
auxiliary winding. The large hysteresis of VDD provides
more hold-up time, which allows using small capacitor
for VDD.
Figure 27. Startup Circuit
© 2009 Fairchild Semiconductor Corporation
FAN100 Rev. 1.0.2
11
www.fairchildsemi.com