RC Snubber Network Design – A Practical Approach
An RC Snubber Network consists of two passive components, a Resistor and Capacitor. These components are connected in series across the output of switching components. The switching components are usually configured in a push-pull arrangement, and the active devices are generally either IGBT’s (Insulated Gate Bipolar Transistors) or MOSFET’s (Metal Oxide Semiconductor Field Effect Transistors).
In a Switch Mode Power Supply, the switching components and primary transformer windings have a parasitic Capacitance and Inductance associated with them. There combination forms an LC resonant circuit. When the switching components are gated, it is this parasitic based self resonance that creates ringing on the rising and falling edges of the switching waveform. The ringing appears as decaying amplitude oscillations. More commonly, when considered singularly they are referred to as Voltage spikes or Transients. The Voltage spikes can have an amplitude high enough to stress the switching components to eventual destruction. The ringing occurs at a frequency that is many times higher than the switching frequency. If an RF Spectrum Analyzer (Test equipment that shows voltage amplitudes in the frequency domain) is used to observe the ringing, it will be seen as Spurs (Spurious RF emissions) many times higher than the baseband frequency. These emissions can cause EMI (Electromagnetic Interference) / RFI (Radio Frequency Interference). Because of this, Noise and process problems can occur to Audio equipment, Communication Networks, Computer Systems, Radios, Televisions and Video Systems.
When designing a Snubber Network for switching circuits a practical approach is the best method for reducing Spurs. It involves the use of a DSO (Digital Storage Oscilloscope), Scientific Calculator, and a small selection of components. To work out the required values of Resistance and Capacitance for the Snubber Network it is necessary to determine the value of the parasitic Inductance, parasitic Capacitance, as well as the frequency of the Spurs. This can be achieved by taking measurements and calculating the unknown quantities. All necessary precautions should be taken while doing this to avoid contact with live circuits. High Voltages are normally present in Switch Mode Power Supplies and other types of Power Inverters. Contact with these Power Supplies can Kill, or cause serious injuries. If you are not a competent Electronics Technician or Electronics Engineer treat this information as Reference material only.
Spur attenuation, is achieved by performing the following steps:
1. Utilizing a DSO, measure the natural resonant frequency of the Spurs, and the peak Amplitude.
2. Connect a low value capacitor (100pF or less) across the switching device. Keep increasing the capacitance until the peak amplitude of the Spur is observed to have halved from what was originally observed (this occurs at the -6dB point). Take note of the Capacitor value used to achieve this, and divide this value by three. This value represents an approximation of the parasitic Capacitance.
3. Now that we know the frequency of the Spurs (f), and the parasitic Capacitance (C) of the switching circuit, we can calculate the Parasitic Inductance (L), where L = 1/ [(6.28 x f) squared] x C.
4. The Impedance (Z) of the switching circuit can now be calculated, where Z = square root (L/C). The Snubber resistor is selected to match this impedance. This allows maximum power transfer between source (Switching device) and load (Snubber network).
5. The Snubber Capacitor is generally made to be ten times the value of the parasitic capacitance. This provides a very high attenuation of the Spur (past the previously measured -6dB point).
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