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Showing posts with label RCD Snubber. Show all posts
Showing posts with label RCD Snubber. Show all posts
Thursday, April 9, 2009
Snubber Circuit Design
Mosfet RCD Snubber Circuit Design
Design the MOSFET RCD snubber circuit
Push-Pull Snubber Circuit
Mosfet Snubber Circuit in Flyback Converter Circuit
Switch Protection Design - Fast-Recovery Diodes
Wednesday, March 25, 2009
Design the MOSFET RCD Snubber Circuit
When the power MOSFET is turned off, there is a high
voltage spike on the drain due to the transformer leakage
inductance. This excessive voltage on the MOSFET may
lead to an avalanche breakdown and eventually failure of the
FPS. Therefore, it is necessary to use an additional network
to clamp the voltage.
The RCD snubber circuit and MOSFET drain voltage
waveform are shown in Figure 10 and 11, respectively. The
RCD snubber network absorbs the current in the leakage
inductance by turning on the snubber diode (Dsn) once the
MOSFET drain voltage exceeds the voltage of node X as
depicted in Figure 10. In the analysis of snubber network, it
is assumed that the snubber capacitor is large enough that its
voltage does not change significantly during one switching
cycle. The snubber capacitor used should be ceramic or a
material that offers low ESR. Electrolytic or tantalum
capacitors are unacceptable due to these reason
Circuit diagram of the snubber network
Figure 11. MOSFET drain voltage and snubber
capacitor voltage
voltage spike on the drain due to the transformer leakage
inductance. This excessive voltage on the MOSFET may
lead to an avalanche breakdown and eventually failure of the
FPS. Therefore, it is necessary to use an additional network
to clamp the voltage.
The RCD snubber circuit and MOSFET drain voltage
waveform are shown in Figure 10 and 11, respectively. The
RCD snubber network absorbs the current in the leakage
inductance by turning on the snubber diode (Dsn) once the
MOSFET drain voltage exceeds the voltage of node X as
depicted in Figure 10. In the analysis of snubber network, it
is assumed that the snubber capacitor is large enough that its
voltage does not change significantly during one switching
cycle. The snubber capacitor used should be ceramic or a
material that offers low ESR. Electrolytic or tantalum
capacitors are unacceptable due to these reason
Circuit diagram of the snubber networkThe first step in designing the snubber circuit is to determine
the snubber capacitor voltage at the minimum input voltage
and full load condition (Vsn). Once Vsn is determined, the
power dissipated in the snubber network at the minimum
input voltage and full load condition is obtained as
the snubber capacitor voltage at the minimum input voltage
and full load condition (Vsn). Once Vsn is determined, the
power dissipated in the snubber network at the minimum
input voltage and full load condition is obtained as
where Ids-peak is specified in equation (8), fs is the FPS
switching frequency, Llk is the leakage inductance, Vsn is the
snubber capacitor voltage at the minimum input voltage and
full load condition, VRO is the reflected output voltage and
Rsn is the snubber resistor. Vsn should be larger than VRO
and it is typical to set Vsn to be 2~2.5 times VRO. Too small a
Vsn results in a severe loss in the snubber network as shown
in equation (23). The leakage inductance is measured at the
switching frequency on the primary winding with all other
windings shorted.
Then, the snubber resistor with proper rated wattage should
be chosen based on the power loss. The maximum ripple of
the snubber capacitor voltage is obtained as
switching frequency, Llk is the leakage inductance, Vsn is the
snubber capacitor voltage at the minimum input voltage and
full load condition, VRO is the reflected output voltage and
Rsn is the snubber resistor. Vsn should be larger than VRO
and it is typical to set Vsn to be 2~2.5 times VRO. Too small a
Vsn results in a severe loss in the snubber network as shown
in equation (23). The leakage inductance is measured at the
switching frequency on the primary winding with all other
windings shorted.
Then, the snubber resistor with proper rated wattage should
be chosen based on the power loss. The maximum ripple of
the snubber capacitor voltage is obtained as
where fs is the FPS switching frequency. In general, 5~10%
ripple of the selected capacitor voltage is reasonable.
The snubber capacitor voltage (Vsn) of equation (26) is for
the minimum input voltage and full load condition. When
the converter is designed to operate in CCM under this
condition, the peak drain current together with the snubber
capacitor voltage decrease as the input voltage increases as
shown in Figure 11. The peak drain current at the maximum
input voltage and full load condition (Ids2 peak) is obtained as
ripple of the selected capacitor voltage is reasonable.
The snubber capacitor voltage (Vsn) of equation (26) is for
the minimum input voltage and full load condition. When
the converter is designed to operate in CCM under this
condition, the peak drain current together with the snubber
capacitor voltage decrease as the input voltage increases as
shown in Figure 11. The peak drain current at the maximum
input voltage and full load condition (Ids2 peak) is obtained as
where Pin, and Lm are specified in equations (1) and (6),
respectively and fs is the FPS switching frequency.
The snubber capacitor voltage under maximum input voltage
and full load condition is obtained as
respectively and fs is the FPS switching frequency.
The snubber capacitor voltage under maximum input voltage
and full load condition is obtained as
where fs is the FPS switching frequency, Llk is the primary
side leakage inductance, VRO is the reflected output voltage
and Rsn is the snubber resistor.
side leakage inductance, VRO is the reflected output voltage
and Rsn is the snubber resistor.
Figure 11. MOSFET drain voltage and snubbercapacitor voltage
From equation (26), the maximum voltage stress on the
internal MOSFET is given by
internal MOSFET is given by
where VDC max is specified in equation (3). Check if Vds
max is below 85% of the rated voltage of the
MOSFET (BVdss) as shown in Figure 12. The voltage rating
of the snubber diode should be higher than BVdss. Usually,
an ultra fast diode with 1A current rating is used for the
snubber network.
In the snubber design in this section, neither the lossy
discharge of the inductor nor stray capacitance is considered.
In the actual converter, the loss in the snubber network is
MOSFET (BVdss) as shown in Figure 12. The voltage rating
of the snubber diode should be higher than BVdss. Usually,
an ultra fast diode with 1A current rating is used for the
snubber network.
In the snubber design in this section, neither the lossy
discharge of the inductor nor stray capacitance is considered.
In the actual converter, the loss in the snubber network is
Less than the designed value due to this effects
Monday, March 9, 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:
more(pdf)
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.
more(pdf)
MAGNETIC SNUBBER FOR 200W PFC
WITH UNIVERSAL MAINS
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
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:
more(pdf)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.
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.
more(pdf)
MAGNETIC SNUBBER FOR 200W PFC
WITH UNIVERSAL MAINS
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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