AN-6014 Datasheet, PDF (9/15 Pages) Fairchild Semiconductor – Green Current Mode PWM Controller

AN6014
shown in Fig. 24. A resistor is connected between MOSFET
gate and ground to increase V_CC current when duty
increases. Too small R_d can cause UVLO operation at no
load and too large R_d can cause OVP operation at full load.
APPLICATION NOTE
OUT ⁵
R_{d RS}
Figure 24. Dummy Resistor Connection
2*Tr
3.4 Diode RC Snubber Design
The snubber of the secondary diode can be designed by the
following steps.
(a) Measure the original resonance period (Tr) of the diode
voltage without RC filter, as shown in Fig. 25.
(b) Find a capacitor (Cdsn) value that makes the resonance
period double (2*Tr) when connected in parallel with the
diode, as shown in Fig. 26.
(c) Calculate the snubber resistor (Rdsn) by (14).
Rdsn = ^{3 ⋅Tr}
(14)
2π ⋅Cdsn
Figure 26. Diode Voltage Connecting C_dsn
3.5 MOSFET RCD Snubber Design
AN4147 describes the design guidelines for RCD snubber of
the flyback converter, but it does not consider the snubber
capacitance of the secondary diode and the MOSFET Coss.
In a real case, when the MOSFET turns off, the energy
stored in the leakage inductance discharges the snubber
capacitor of the secondary diode and the MOSFET Coss
until Vds is charged to Vin + nVo. During this discharging
time, the energy stored in the leakage inductance decreases
and the peak value of the snubber charging current decreases
from Ipeak to Isn_peak, as shown in Fig. 28. Therefore the
peak voltage of the snubber is lower than the calculated
value, using the equations in AN4147. If you use the values
calculated by AN4147, the snubber design is most conserva-
tive. Use the values calculated by AN4147, then increase
R_SN value step by step, checking the voltage of the C_SN to
reduce the power loss of the R_SN. If R_SN is too high, the
MOSFET V_ds voltage spike is too high. The MOSFET V_ds
voltage should not exceed the MOSFET rating even though
the converter output is shorted.
R_dsn C_dsn
V_in
n:1
+
V_sn
R_sn
iD
Vo
Tr
C_sn ⁺
D_sn
i_sn
L_ik
Figure 25. Diode Voltage without C_dsn
V_ds ⁺
i_d
C_oss
© 2006 Fairchild Semiconductor Corporation
FAN7602 Rev. 1.0.1
Figure 27. Flyback Converter with Snubbers
www.fairchildsemi.com
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