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Resistor-Capacitor (RC) Snubber Design for Power Switches

[Editor's Note: This tech bite was originally posted on the Fairchild Semiconductor website. Richard Chung, the applications manager at Fairchild, would like to bring these tech briefs from his team to the Planet Analog audience.]

Power Switches are the heart of every power converter. Their operation will directly determine the reliability and efficiency of the product. To enhance the performance of the switching circuit of power converters, snubbers are placed across the power switches to suppress voltage spikes and damp the ringing caused by circuit inductance when a switch opens. Proper design of the snubber can result in higher reliability, higher efficiency and lower EMI. Among many different kinds of snubbers, the resistor-capacitor (RC) snubber is the most popular snubber circuit. This article explains why the snubber is need for the power switches. Some practical tips for an optimum snubber design are provided as well.

Figure 1

Four basic power switching circuits

Four basic power switching circuits

There are many different topologies used in power converters, motor drivers and lamp ballasts. Figure 1 shows four basic power switching circuits. Within all of these four fundamental circuits, and in fact most power switching circuits, the same switch-diode-inductor network is shown within the blue lines. The behavior of this network is the same in all these circuits. Therefore, a simplified circuit as shown in Figure 2 can be used for the switching performance analysis for the power switches during switching transient. Since the current in the inductor almost does not change during switching transient, the inductor is replaced with a current source as shown in the figure. The ideal voltage and current switching waveform of the circuit is also shown in Figure 2.

Figure 2

Simplified power switching circuit and its ideal switching waveform

Simplified power switching circuit and its ideal switching waveform

When the MOSFET switch turns off, the voltage across it rises. The current, IL , however, will keep flowing through the MOSFET until the switch voltage reaches Vol . The current IL begins to fall once the diode turns on. When the MOSFET switch turns on, the situation is reversed as shown in the figure. This type of switching is referred to as “hard switching.” The maximum voltage and maximum current must be supported simultaneously during the switching transient. Therefore, this “hard switching” exposes the MOSFET switch to high stress.

Figure 3

Voltage overshoot at the MOSFET switch turn-off transient

Voltage overshoot at the MOSFET switch turn-off transient

In practical circuits, the switching stress is much higher because of the parasitic inductance (Lp ) and capacitance (Cp ) as shown in Figure 4. Cp includes the output capacitance of the switch and stray capacitance due to PCB layout and mounting. Lp includes the parasitic inductance of the PCB route and MOSFET lead inductance. These parasitic inductances and capacitances from the power devices form a filter that resonates right after the turn-off transient, and therefore superimposes excessive voltage ringing to the devices as shown in Figure 3. To suppress the peak voltage, a typical RC snubber is applied across the switch as shown in Figure 4. 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 in order to keep the power dissipation of the resistor to a minimum.

Figure 4

Resistor-capacitor snubber configuration

Resistor-capacitor snubber configuration

Where power dissipation is not critical, there is a quick design approach for the RC snubber. Empirically, chose the snubber capacitor Csnub equal to twice the sum of the switch output capacitance and the estimated mounting capacitance. The snubber resistor Rsnub is selected so that

Rsnub =Vol/IL .

The power dissipation on Rsnub at a given switching frequency (fs) can be estimated as:

When this simple and empirical design does not limit the peak voltage sufficiently, then the optimizing procedure will be applied.

Optimized RC snubber
In those cases where power dissipation is critical, a more optimum design approach should be used.

First, measure the ringing frequency (Fring ) at the MOSFET switch node (SW) when it turns off. Solder a film type 100 pF low-ESR capacitor across the MOSFET. Increase the capacitance until the ringing frequency is half of the original measured value. Now the total output capacitance of the switch (the added capacitance plus original parasitical capacitance) is increased by a factor of four as the ringing frequency is inversely proportional to the square root of the circuit’s inductance capacitance product. So the parasitic capacitance Cp is one third of the externally added capacitor value. The parasitic inductance Lp now can be obtained by using following equation:

Once the parasitic inductance Lp and parasitic capacitance Cp are figured out, the snubber resistor Rsnub and capacitor Csnub can be chosen based on following calculation.

The snubber resistor can be fine turned further to reduce the ringing if it is found to be insufficient.

The power dissipation on Rsnub at a given switching frequency (fs ) is (Csnub Vol2 xfs ).

Looking for more practical design tips for the power electronics circuit? Check out this link.

11 comments on “Resistor-Capacitor (RC) Snubber Design for Power Switches

  1. Sachin
    July 31, 2014

    @Richard, thanks for the post. Lot of useful info about RC snubber design. I also visited the Fairchildsemi link that you shared. There are so many useful Application notes. Thank you for sharing the link.

  2. Richard.Chung
    July 31, 2014

    Great to hear @SachinEE that this kind of topic is applicable to you. It motivates us to create more relevant material like this. Give us more ideas what issues you haven't seen addressed enough.

  3. geek
    July 31, 2014

    @Richard: Thank you for the blog. It was indeed quite informing about the power switches and how they technically work. Can you give me more examples on which areas these power swtches are being worked at?

  4. chirshadblog
    July 31, 2014

    @tzubar: Well is there a separate voltage level for different categories ? 

  5. Richard.Chung
    August 1, 2014

    @tzubair these power switches are used in switched mode power supplies

  6. amrutah
    August 1, 2014

    @tzubair: The snubbers are something like filters to snap-up any surge due to sudden switching activity. Any hard switching,  or hot plug of a device to power supply.  The snubbers protect these power supply switches.  

    Richard has explained nicely in this blog, for more on the design you can also visit http://www.ti.com and search for snubber circuit design.

  7. amrutah
    August 1, 2014

    @Richard: Thanks for the blog and the details on the snubber.

      Is there any reason that the parasitic resistance is not considered here, like the DCR of the inductor and the parasitic bond wire resistance on the supply and load?  I think even these parasitic resistance dampen in the same direction as snubber Rsnub

  8. Richard.Chung
    August 2, 2014

    great question @amrutah! It's not always known the parasitic R of the bond wire. It is common for some MOSFETs device structure and package parasitic to act as a snubber. In terms to account for DCR of inductor, supply, and load as a snubber – the configuration of the supply, parastic L or C, and also the oscillation signature (frequency and amplitude). In my experience, I prefer (not a must) to depend on component values known for snubber and don't solely depend on DCR of the inductor to fully dampen an oscillation due to sometimes 20% variation in DCR of inductors and who knows what other parasitics that exists in line with the inductor (because they are difficult to measure due to manufacturing variations). love to discuss it offline sometimes

  9. etnapowers
    August 7, 2014

    Good post, thank you. I think that also the equivalent parallel capacitance of the inductor plays a role here, the typical values of Rs and Cp should be accurately evaluated during the design phase.

  10. etnapowers
    August 7, 2014

    The circuit shown in figure 2 is usually realized with a power MOSFET instead of the diode, because this solution allows the designer to keep in control the switches by mean of a driving voltage and to avoid energy returns from the output to the input of the circuit.

  11. RedDerek
    August 20, 2014

    The other perspective is to be able to control the clamping. Being dependent on the parasitics can be a problem since parasitics can change from lot-to-lot of parts, as well as if the component supplier changes the manufacturing process to optimize yield or cut cost.

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