# dc damping

#### vks_foe

Joined Jan 11, 2009
61 Last edited:

#### crutschow

Joined Mar 14, 2008
25,267
Can somebody please explain how we design the damping RC network for DC circuits with inductive load as shown in fig #### BillB3857

Joined Feb 28, 2009
2,533
• thatoneguy

#### gootee

Joined Apr 24, 2007
447
I have read most of the snubber-design papers that are on the web. At least two of them gave a practical procedure to use for an existing circuit, which I have paraphrased elsewhere and will paste below:

PRACTICAL SNUBBER AND TERMINATION DESIGN

Below is a pretty slick and practical way to determine parasitic capacitance and/or inductance, and the characteristic impedance and optimal damping or termination resistance (and optional series capacitance) needed when ringing or reflections (as the case may be) are present.This is a very simple method for determining the capacitance and the inductance that are causing an LC resonance in a circuit or a transmission line or PCB trace, which also gives the characteristic impedance for the resonant circuit, which is often everything needed in order to know how to damp it, optimally. (I refer to the C and L as parasitic. But this method and the math may be used when either one, or both, or neither is parasitic, in case an installed inductor and/or capacitor are involved.)

This assumes that there is a ringing condition, already, such as might occur on a digital buss or a transmission line or PCB trace, or in a switch-mode power supply or an AC-to-DC transformer/rectifier circuit, and in many other types of circuits. (If you don't have ringing and just want to determine some of these parameters, I guess maybe you could try hitting your circuit with a pulse train and decrease the rise and fall times until it rings.)

1. Measure the frequency of the resonance or ringing, using an
oscilloscope (or a circuit simulator, if you've modeled the parasitics well).

2. Add a shunt capacitor and adjust the value of this capacitor until the frequency of the ringing is reduced by a factor of two. I've left out the math but the value of this resulting capacitor will be three times (3X) the value of the parasitic capacitance that is creating the resonance.

3. Because the parasitic capacitance is now known, the parasitic inductance can be determined using the formula:

L = 1 / [(2 · π · f)² · C]

where f = (original) resonant frequency and C = parasitic capacitance.

4. Now that both the parasitic capacitance and inductance are known, the
characteristic impedance of the resonant circuit can be determined using the following formula:

Z = √(L/C)

where L = parasitic inductance and C = parasitic capacitance.

5. The resistor value used for the terminator or for the RC snubber circuit should be equal to Z, the value of the characteristic impedance, and the capacitor, if used, should be sized between four and ten times the parasitic capacitance. The use of larger (than 4X) capacitors slightly reduces the voltage overshoot at the expense of greater power dissipation in the resistor.

NOTE: The resistor, alone, is all that is needed to prevent or damp-out the ringing (or reflections, as the case may be). But if power dissipation in the R would then be too high, a C is added in series with the R, so that only the unwanted frequencies cause currents in the resistor. (And that, boys and girls, is the only reason there's a capacitor in a snubber.)

Cheers,

Tom

#### tinamishra

Joined Dec 1, 2012
39 