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Monday, March 09, 2009

Mosfet RCD Snubber Circuit Design

Design Guidelines for RCD Snubber of Flyback Converters
Application Note AN-4147
Fairchild Semiconductor
Snubber design
The excessive voltage due to resonance between Llk1 and
COSS should be suppressed to an acceptable level by
an additional circuit to protect the main switch.

The RCD snubber
circuit and key waveforms are shown in Figures 2 and 3.
The RCD snubber circuit absorbs the current in the leakage
inductor by turning on the snubber diode (Dsn) when Vds
exceeds Vin+nVo. It is assumed that the snubber capacitance
is large enough that its voltage does not change during one
switching period.

When the MOSFET turns off and Vds is charged to Vin+nVo,
the primary current flows to Csn through the snubber diode
(Dsn). The secondary diode turns on at the same time.
Therefore, the voltage across Llk1 is Vsn-nVo. The slope of
isn is as follows:


Snubber Circuits Suppress Voltage Transient Spikes in
Multiple Output DC-DC Flyback Converter Power Supplies
RCD Voltage Snubber
This snubber is applicable to rate-of-rise voltage control
and/or clamping. The presence of the diode in the
configuration makes this a polarized snubber. The two
possible configurations for this resistor-capacitor-diode
(RCD) snubber are shown in Figure 2. The configuration
shown in Figure 2A can only act as a voltage clamp.
The variation shown in Figure 2B is applicable to either
rate-of-rise control or clamping of the drain voltage of
the switch.

RCD Clamp
In the clamp mode the purpose of the snubber is to
clamp the voltage during turn-off at the drain of the
MOSFET. The parallel RC circuit may be returned to
ground or to a voltage other than ground (i.e., input voltage
if the drain can go above input voltage) since this will
reduce the power dissipation in the resistor. The MOSFET
switch itself will have to sustain the peak power dissipation
during turn-off. The value of the capacitor, CCLAMP,
and resistor, RCLAMP, is based on the energy stored in
the parasitic inductance, as this energy must be
discharged into the RC network during each cycle.
The voltage across the capacitor and resistor sets the
Clamp voltage, VCLAMP.

Rate-of-Rise Control RCD Snubber
When the RCD snubber is used to control the rate of
voltage rise at the MOSFET drain, the capacitor must be
completely charged and discharged during each cycle to
be able to control the rate-of-rise of the drain voltage.
The RC time constant of the snubber should, therefore,
be much smaller than the switching period (consider the
effect of duty cycle on pulse width). Typically, the time
constant should be about 1/10th the switching period.
When the switch turns off, the inductor current is diverted
through the snubber diode to charge the capacitor to
the rail. At that time, it is expected that the output rectifier
will turn on.


In high voltage continuous mode boost converters,
a significant part of the power mosfet switching
losses is related to the turn-on edge.
In fact, at turn on, the power mosfet has to sustain
both the boost diode reverse recovery and
the stray capacitances associated energies.
Moreover, the additional peak current due to the
recovery of the diode can be significantly high, in
particular at high temperature, thus increasing the
high frequency noise, the E.M.I. filter requirements
and reducing efficiency.
The turn on peak current, generating all the
above mentioned problems, has been dramatically
reduced by using the magnetic snubber we
propose at Fig. 1b.
The concept of this snubber is to reduce (and
control) the turn-on di/dt of the mosfet to the most
convenient value, considering the voltages and
switching frequency applied to the system.

Voltage Snubber

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