Types of snubber circuits | Design of snubber for flyback converter | Simulation of flyback converter in Matlab simulink

1. Types of passive snubbers and
design of a snubber circuit for
flyback converter
Kaushik Naik
naikkaushik93@gmail.com

2. Contents
2
• Introduction
• Passive Snubber Types
• Snubber Design for Flyback Converter
• Simulation of flyback converter without snubber
• Simulation of flyback converter with snubber
• Comparison of power loss across MOSFET
• Conclusions
• References

3. Introduction
• Snubbers are small networks of parts in the power switching
circuits whose function is to control the effects of circuit
reactance.
• The basic intent of a snubber is to absorb energy from the
reactive elements in the circuit.
• A snubber limits the amount of stress which the switch must
endure and this increases the reliability of the switch.

4. Introduction Contd.
4
• Snubbers may be either passive or active networks.
• Passive snubber network elements are limited to resistors, capacitors,
inductors and diodes.
• Active snubbers include transistors or other active switches, often
entail a significant amount of extra circuitry and introduce another
level of parasitic which must be dealt with (usually with a passive
snubber).
• This presentation is limited to the main types of passive snubbers and
design of a snubber for flyback converter.

5. Passive Snubber Types
5
• Passive snubbers are called dissipative snubbers. Dissipative snubbers are
those which dissipate the energy they absorb in a resistor.
• Dissipative snubbers may be designed to control the rate of rise of voltage
or current or be designed to clamp the voltage.
• Types of passive snubbers:
1. Simple RC Voltage Snubber
2. The RCD Voltage Snubber
3. Simple RL Current Snubber

6. Passive Snubber Types
6
1. Simple RC Voltage Snubber
• The RC snubber is used to reduce the peak
power dissipation in the switch and commonly used
in push-pull converters.
• If the values of R and C are chosen correctly the switching losses can be reduced by
up to 40% including both the loss in the switch and the loss in the resistor over the
complete switching cycle.
• In applications where damping is required the value of the resistor must be close to
the impedance of the parasitic resonance which it is intended to damp.
• The snubber capacitance must be larger than the resonant circuit capacitance but
must be small enough so that the power dissipation of the resistor is kept to a
minimum.

7. Passive Snubber Types
7
2. The RCD Voltage Snubber
• A typical application of a resistor-capacitor-diode
snubber is to control the rate of rise of voltage on the
drain or collector of a switching transistor in a forward,
flyback or boost converter.
• At turn-off, the snubber will carry a major portion of the switch current (if not all of it) and
this transfers the power dissipation of the switch into the snubber.
• The reliability of the switch increases since its peak power dissipation is reduced.
• When the RCD snubber is used, the RC time constant must be short compared to the
switching frequency because the capacitor must be charged and discharged on each cycle.
• First choose the capacitor C1 large enough so that it contains negligible switching ringing,
and then choose R1 so that the power dissipated in R1 at V1 (voltage across the capacitor
C1) is equal to the switching loss caused by the leakage inductance.

8. Passive Snubber Types
8
3. Simple RL Current Snubber
• The purpose of a current snubber is to control the rate of rise of current in
the switch.
• The current snubber for the switch also benefits the turn off of the diode on the output. It
provides a controlled rate of change of current in the diode and the low dI/dt lowers the
dissipation in the diode and reduces the peak reverse current.
• The inductor functions normally when the switch turns on and current only flows through
the resistor when it is needed to dissipate the energy which is stored in the inductor.
• The amount of power dissipated by the resistor is equal to 1/2LI2F, where I is the peak
current in the inductor and it includes the diode reverse recovery current as well as the load
current.
• This snubber can be very effective in increasing the overall efficiency of the circuit and in
increasing the reliability.

9. Snubber Design for Flyback Converter
9
• The excessive voltage due to resonance between Llk
and the output capacitance of the MOSFET should
be suppressed to an acceptable level by an
additional circuit to protect the main switch.
• The RCD snubber circuit absorbs the current in the
leakage inductor by turning on the snubber diode
(Dsn) when Vds exceeds Vin+nVo.
• The value of snubber capacitor can be calculated
from,
• The value of snubber resistance can be calculated
from,
𝐶𝑠𝑛 =
𝑃𝑅𝑠𝑛
0.5 × 𝑉𝑠𝑛
2
× 𝑓𝑠𝑤
4𝑅 𝑠𝑛 𝐶𝑠𝑛 = 𝑇𝑂𝑁 𝑀𝑂𝑆𝐹𝐸𝑇

10. Snubber Design for Flyback Converter
10
Calculations:
• Snubber capacitor:
• Snubber resistance:
𝐶𝑠𝑛 =
𝑃𝑅𝑠𝑛
0.5 × 𝑉𝑠𝑛
2
× 𝑓𝑠𝑤
4𝑅 𝑠𝑛 𝐶𝑠𝑛 = 𝑇𝑂𝑁 𝑀𝑂𝑆𝐹𝐸𝑇
𝐶𝑠𝑛 =
0.1
0.5 × 3252 × 100𝑘
= 18𝑝𝐹
𝑅 𝑠𝑛 =
𝑇𝑂𝑁 𝑀𝑂𝑆𝐹𝐸𝑇
4𝐶𝑠𝑛
=
16𝑛
4 × 18𝑝
= 222Ω
𝑃𝑅𝑠𝑛 = 0.1W
𝑉𝑠𝑛 = 325V
𝑓𝑠𝑤 = 100kHz
𝑇𝑂𝑁 𝑀𝑂𝑆𝐹𝐸𝑇 = 16n sec
(You will get this in
datasheet)
Parameters:

11. Parameters for simulation of flyback converter
11

12. Simulation of flyback converter without snubber
12

13. Simulation of flyback converter without snubber
13
The peak voltage overshoot across MOSFET was found to be 7kV.
Voltage across MOSFET:

14. Simulation of flyback converter with snubber
14

15. Simulation of flyback converter without snubber
15
The peak voltage overshoot across MOSFET was found to be 900V.
Voltage across MOSFET:

16. Comparison of power loss across MOSFET
16
As per the simulated results, it is found that the power dissipation
reduces by 0.1W when snubber is used.

17. Conclusion
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• The use of snubber in a switching circuit can be helpful in reducing the switching
losses and increase the life span of the switch by reducing the voltage stress
across the switch.
• Different types of passive snubber circuits and their applications are studied.
• Design calculations of RCD snubber for flyback converter is carried out.
• The benefit of using snubber circuit is evaluated by simulating the flyback
converter in simulink tool.
• It is found that by using snubber circuit the voltage spike across the switch is
removed and the power dissipation in the switch is also reduced by 0.1 watts.

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