Issue 290: EQ Answers

Problem 1—What is an R-C snubber, and what is a typical application for one?

Answer 1—An R-C snubber is the series combination of a resistor and a capacitor that is placed in parallel with a switching element that controls the power to an inductive load in order to safely absorb the energy of switching transients.

The problem is that a load that has an inductive component will produce a brief very high-voltage “spike” when the current through it is interrupted quickly. This spike can cause semiconductor devices to break down or even mechanical contacts to arc over, reducing their lifetime. The snubber absorbs the energy of the spike and dissipates it as heat, without ever allowing the voltage to rise too high.


Problem 2—How do you pick the resistor value in an R-C snubber?

Answer 2—To pick the resistor value, you first need to know what the maximum voltage you want to allow is. For example, if you have a MOSFET that has a drain-to-source breakdown rating of 400 V, you might choose to limit the snubber voltage to 200 V. Call this VMAX. Next, you need to know the maximum current that will be flowing through the load (and the switching element). Call this IMAX. At the instant the switching element opens, this current will be flowing through the resistor, and this will determine the initial voltage that appears across the switching element. Therefore pick the resistance: R = VMAX/IMAX.


Question 3—How do you pick the capacitor value in an R-C snubber?

Answer 3—Picking the capacitor can be more tricky. The key concept is that you need to pick a capacitor that can absorb the energy stored in the inductance of the load while keeping its terminal voltage under VMAX. Since loads don’t often specify their values of inductance, this may require some experimentation. Let’s call the load inductance LLOAD. The energy that it stores at the maximum current is: E = 0.5 IMAX2 LLOAD.The energy that a capacitor stores is: E = 0.5 V2C.

So, if we say that we want the capacitor to store the same energy that’s in the inductance when its terminal voltage is at VMAX, we can combine the twe equations and then solve for C:

0.5 VMAX2C = 0.5 IMAX2LLOAD

C = (IMAX2/VMAX2)LLOAD

This value will actually be somewhat conservative, because some of the initial energy of the inductance will be dissipated in the resistor during the initial transient, before it even gets to the capacitor. After that, the inductance and the capacitor will behave as a series-resonant circuit, with the current oscillating back and forth until all of the energy is gone.


Problem 4—What additional concern is there with regard to an R-C snubber when switching AC power?

Answer 4—When switching DC, the snubber absorbs the energy stored in the load’s inductance, and after a while, no current flows and the capacitor is charged to the supply voltage. However, when switching AC, the snubber has a finite impedance at the AC frequency, which means that it “leaks” a certain amount of current even when the main switching element is open. While this may or may not cause a problem for the load (usually not), there is also the issue of the continuous power being dissipated in the snubber resistor. The resistor must be rated to withstand this leakage power in addition to the energy of the switching events.