AN-6014 Datasheet, PDF (10/15 Pages) Fairchild Semiconductor – Green Current Mode PWM Controller

AN6014
i_d
i_peak
i_peak
i_sn
Ideal case
t_s
I_{sn_peak}
Real case
i_D
APPLICATION NOTE
For more information, please refer to AN4148.
3.7 Power Limit Resistors Design
If the flyback converter operates in CCM mode, the maxi-
mum output current increases as the input voltage increases.
Figure 30 shows the CS/FB pin voltage when the input volt-
age is 85Vac and Fig. 31 shows the CS/FB pin voltage when
the input voltage is 265Vac with 4A load. With the same out-
put load, the feedback offset voltages are different. One is
236mV and the other is 418mV; a difference of 182mV. This
means that more current can be delivered when the input
voltage is 265Vac, compared with the 85Vac case, because
the feedback offset voltage decreases as the output load
increases. The maximum currents when the input voltages
are 85Vac and 265Vac, are 5A and 7A respectively. The
Plimit offset voltage should be subtracted from 1V as shown
in Fig. 8.
V_ds
V_sn
nV_o
V_in
Figure 28. Waveforms Related with RCD Snubber
3.6 Transformer Audible Noise
Because the FAN7602 operates in the burst mode at a light
load and no load, it has a switching period and a non-switch-
ing period. Figure 29 shows the gate voltage and the output
voltage at no load. The burst operation frequency is about
82~100Hz. The burst operation frequency varies according
to the load condition and the frequency is in the range of the
audible frequency. Therefore the transformer may generate
the audible noise. Because the audible noise comes from the
movement of the transformer coils, it is important to prevent
the movement of the coils. Varnishing the transformer tightly
helps reduce the audible noise. If the varnishing is not tight,
there can be some audible noise. If the current sense filter
size decreases, the MOSFET current peak decreases then
audible noise decreases, but the stand-by power increases.
CS/FB
236mV
Figure 30. CS/FB Waveform (Io=4A, Vin=85Vac)
CS/FB
Vout
418mV
Vgate
Figure 31. CS/FB Waveform (Io=4A, Vin=265Vac)
Figure 29. Burst Mode Operation Waveforms
© 2006 Fairchild Semiconductor Corporation
FAN7602 Rev. 1.0.1
10
www.fairchildsemi.com