The Snubber circuit is one type of dv/dt protection circuit of the thyristor.
With the help of snubber circuit, the false turn-on of a thyristor due to large dv/dt can be prevented.

1. Ac Rc Snubber Calculator Free
2. Rc Snubber Calculator

Your RC Snubber is: V 0 = open circuit voltage I pk = R S = snubber resistance S C s = snubber capacitance And the peak dV/dt is: dV/dt pk = While for a sinewave excitation voltage, rms current in amps is the familiar: f = frequency in Hz I rms = 2πfCVrms x 10-6 C = capacitance in µF V = voltage in Vrms. For rc snubber I have used a 100nf@600v cap and I want to consider the 100 ohm resistor as a SMD (1/4 w). For max surge voltage (600v) max current that flows to the resistor is Xc=1/ (2.pi.60.100nf)-Imax=600/Xc 0.023 so the power that the resistor should resist is 100. (Imax)^2 0.06W. It seems to me that I can use the SMD resistor?

RC Snubber Circuit for SCR dv/dt Protection:

This type of snubber circuit consists of a series combination of resistance R and Capacitance C in parallel with a SCR.

• When a reverse voltage is applied, commutation process is initiated and the forward current flow through SCR approaches zero.
• Due to the inductance, current continuous to flow due to sweeping of charge carries at the external junctions.
• When it reaches a peak value it cannot be further supported by the charge carriers and falls very quickly to zero. This causes a voltage spike with the value of L(di/dt).
• Also when the supply is closed to the circuit( in the above figure say the switch S is closed), sudden voltage appears across SCR.
• Now, as the thyristor current is zero it can be considered as an open switch.
• At this moment, the capacitor C behaves like a short-circuit and therefore voltage across the SCR is zero.
• With the passage of time capacitor C gets charged at a slow rate such that dv/dt across the capacitor and therefore across SCR is less than the specified maximum dv/dt rating of the device.
• Thus the capacitor protects the SCR against high voltages and high dv/dt.

Based on the above discussion we can say that simply a Capacitor C is sufficient to protect the SCR against dv/dt false triggering.

Then what is the purpose of resistance R?

• In the RC snubber circuit, the resistance R limits the discharge current of capacitor at the instant of firing of SCR.
• Before SCR is fired, capacitor C charges to full voltage V.
• If SCR is fired, when the capacitor voltage is maximum, it discharges through the local path formed by capacitor C, Resistance R and SCR.
• During this time, if the resistance R is not included in the circuit, the discharge current will be high and consequently may damage the SCR due to large di/dt.
• Thus the Resistance R in the snubber circuit reduces the discharge current of the capacitor C and thus protect the SCR against large di/dt.

In actual practice, R, C and the load current parameters should be such that

1. dv/dt across C during its charging is less than the specified dv/dt rating of the SCR
2. Discharge current at the turn ON of the SCR is within reasonable limits.

Normally R,S and load circuit parameters form an undamped circuit so that dv/dt is limited to acceptable values.

In some RC snubber circuits, a diode D used to connect in parallel with the resistor R. It is used for the purpose of bypass and thus giving improved dv/vt protection.

Engineering Tutorial Keywords:

• snubber circuit
• snubber circuit basics
• snubber circuits
• How does a contact snubber work
• whats a snubber circuit
• capacitor snubber
• connect snubber circuit 1 so that snubber circuit 1 prevents the input and output contactors form producing dameage transient voltages
• construction of snubber circuit
• how non-discharge RCD snubber circuit works
• rcd snubber circuit

Ac Rc Snubber Calculator Free

RC Snubber Calculator Spreadsheet

In power switching applications, a designer often has to contend with spurious oscillations. These are the result of parasitic capacitance and inductance on the board and behave like the step response of an RLC circuit. These transients can induced undesired noise in neighboring circuits and create over voltage spikes that can compromise long term component reliability.

Figure 1 - Typical RC Snubber Circuit

The most common way to mitigate the effects of these transients is by using an RC snubber circuit. There are many excellent sources on the web discussing this topic so I won't repeat the discussion here (see references at the end for more information). Instead, I'm offering an Excel-based calculator that simplifies the components selection should you be faced with this task.

The Spreadsheet

The spreadsheet can be download through the link below:

And here's an Excel 2003 compatible version as some users can't open the latest excel formats.

The spreadsheet is quite simple to use (See Figure 2).

Figure 2 - The Spreadsheet Calculator 'Interface'

On the left side, colored in green, is the 'inputs' section. In order to precisely 'tune' the RC snubber, you should measure the frequency of the oscillations (without having the snubber) using an oscilloscope and enter it under fRING.

The next step is to add a know value test film capacitor in parallel with the MOSFET (CTEST) and measure the new oscillating frequency as a result. Finally, you must select the damping factor for the desired response depending on how much overshoot you can tolerate. There's a graph in this document illustrating the normalized effect of the damping factor on the response.

The 'results' section of the spreadsheet (colored orange) automatically calculate the recommended R and C values. The estimated parasitic capacitance CP and inductance LP are also displayed for reference. Power dissipated in the Rs resistor is also estimated using the maximum supply voltage (VS) and the maximum switching frequency (fS) so you can select a resistor with the appropriate power handling capability.

A table with standard E-series values is included for convenience.

Enjoy!

References

Rc Snubber Calculator

Comments, questions, suggestions? You can reach me at: contact (at sign) paulorenato (dot) com