Snubbers are RC networks that are really useful for protecting components (transistors, diodes, etc.) and reducing EMI, especially in switching applications.
It is common practice in power electronics to use simple resistor-capacitor (RC) networks to reduce the stress in power transistors and diodes due to voltage spikes created by parasitic inductances and capacitances in transformers, inductors, diodes, transistors, layout, and other components.
A co-lateral effect of those spikes is related with EMI/EMC problems in both conducted and radiated emissions because the existence of some resonance (ringing) between inductances and capacitances.
In my previous article “Ringing with High Quality Components” (In Compliance, May 2017) I explained that ringing is the origin of emissions, distortion, damage, instability, etc., and introducing losses using resistors or ferrites is a common solution, especially when the ringing frequency is not high enough from the switching frequency, situation where the RC snubber network explained in this article is not so useful or possible.
The Rs-Cs snubber network looks like Figure 1, where both components are connected in parallel with transistors and diodes, especially in switching applications as inverters or DC/DC converters.
The proper values for Rs and Cs are usually obtained by experimental procedures and/or by trial and error using a real prototype. Because ringing is related with parasitics, it is not easy to optimize the network by simulation (note I write it is not easy, but it is possible).
A typical procedure to obtain optimum values for Rs and Cs in applications is explained in the following procedure:
- Consider reducing power in your application to minimize the risk of damage to the prototype (and increasing safety for user). But note, in some situations the ringing with low power could be different from the ringing at higher power (especially if parasitic capacitances in diodes or transistors change with power). Check the final result.
- Locate the origin of the ringing frequency with near field probes (if necessary).
- Look at the waveform with your scope. But…
- Be careful with pigtails in your setup.
- Avoid loading with input impedance of your probes/scope (my preferred method is to use an electric near field probe so you do not need to touch the net of interest).
- Measure the ringing frequency fo. This is the natural frequency for our “second order circuit”. We model the underdamped situation considering we have unknown inductance (Lstray) and capacitance (Cstray). Obviously, if you have ringing it is because you have an underdamped resonance (low loss). We will try to add losses to kill that ringing.
- If you are not able to estimate directly Lstray or Cstray…
- Add a shunt capacitor Cshunt across the device to reduce the frequency of ringing to half the original value. With that test you will be able to calculate the parasitic capacitance creating the ringing as Cstray = Cshunt/3.
- The parasitic inductance can be calculated then from the equation of resonant frequency between inductance and capacitances in parallel:
- Lstray = 1 / [(2πfo)2 x Cstray]
- A second order resonant circuit has a characteristic impedance: Zo = √(Lstray x Cstray) and a good damping situation is obtained with a resistor Rs = Zo. So:
- If you have a very low power application, add the resistor Rs in parallel to kill the resonance.
- If you want to avoid dissipation in that resistor, add the series Cs capacitor. The capacitor must look like an “open” for low frequencies (avoiding losses) and a “short” for high frequencies (ringing). A typical value for this capacitor try to obtain the cut-off frequency for the RC filter as ωc = ωo/4 = 1/RsCs so we can start with Cs = 2/(Rsπfo). If ringing is not decreased enough, we can try a bigger capacitor but the result will be more losses. Check the requirements for your application.
- Finally, the RsCs network must be connected with high frequency techniques: small packages, short connections, small loops to be effective.
I have prepared an example so you can understand the procedure and some typical results.
In this example, ringing is detected in MOSFET transistors switching at 108kHz.
The waveform in the transistor (drain-source) was measured with a x10 passive probe (Figure 2).
The measured ringing frequency was fo = 18.5MHz. Some capacitors were connected between drain and source till the frequency was reduced to 9MHz approximately. The capacitor needed for that was Cshunt = 2200pF so the estimated stray capacitance for our resonant system is Cstray = Cshunt/3 = 733pF. From that value and the ringing frequency, the stray inductance can be calculated: Lstray = 100.7nH.
From Lstray and Cstray, the characteristic impedance for our resonance can be calculated:
Zo = 11.72ohm
Following the procedure described previously, the RC snubber values were calculated as Rs = 11.72 ohm and Cs = 3nF. In our prototype we used Rs = 12 and Cs = 4.7nF and the ringing was removed as you can see in Figure 3.
My final advice: Avoid ringing in your circuits. They are dangerous and create a lot of instability and EMI/EMC problems.
