# ESR Capacitor ESR - Right selection

I have a PWM signal at 100Hz ("Vg" label at circuit) with variable duty cycle (10%-90%). Because I had voltage spikes I managed to eliminate them by placed three capacitors which types and values are below. My query is if the ESR of these capacitors is right for this frequency (100Hz) that I am using and if no which value of ESR is the right?

P.S. I don't want to place ceramic capacitors because they produces an acoustic noise/buzz.

Tantalum Capacitors - Solid SMD 35volt 0.1uF 10%
T494A104K035AT
Tantalum Capacitors - Solid SMD 35V 1.0uF 10% "A"
T491A105K035AT
Tantalum Capacitors - Solid SMD 10V 10uF 10% "A"
T491A106K010AT


• I see no capacitor type/value and I see no circuit diagram that shows what you did or anything about the spikes. Ah only 14 minutes to update the info, not bad LOL but what about the circuit and spikes? Sep 8, 2014 at 17:00
• My question is not about the spikes but for the ESR of the capacitors C1,C2,C3.
– 2 X
Sep 8, 2014 at 17:23
• Why did you pick tantalums? Sep 8, 2014 at 18:11
• What are LP1 and 2? I can't see that ESR matters very much here. What is that second FET doing? Sep 8, 2014 at 19:10
• LP1 and LP2 are car lamps.
– 2 X
Sep 9, 2014 at 12:57

When Q1 turns on there will be a current surge as it discharges capacitors C1 and C1(?). This puts extra stress on the FET and may induce noise into nearby circuits. Using low ESR capacitors will only make it worse.

The 0.1uF and 1uF capacitors you have chosen have a combined ESR of 10Ω||7.5Ω = 4.3Ω, so the peak discharge current will be about 12V/4.3Ω = 2.8A, which should be OK so long as the capacitors are placed close to the FET.

I presume Q3 is low power FET, so its higher on-resistance should limit the surge current in C3.

• The Q3 is the MMBF170LT1 which have RDS(on)=5Ω so the discharge current will be about 12V/5=2.4A and the I(drain) (pulsed) of the mosfet is 0.8A so the C3 capacitor is not suitable? A suitable capacitor will be a capacitor with an ESR approximately at I=V/R->R=V/I=12/0.8=15Ω? Is ok this capacitor "T491S106K010AT".
– 2 X
Sep 8, 2014 at 22:28
• Also, this ESR 15Ω don't increase the power dissipation of the capacitor and consequently the heating of the capacitor or in this frequency (100Hz) is not a problem?
– 2 X
Sep 8, 2014 at 22:38
• The time constant for C3 charging through R3 is much longer than your PWM period, so the voltage will only rise to about 1V At 90% PWM. Therefore the surge current should be less than 0.5 Amps (probably much less, when FET switching time is factored in). The current spikes are short, so average power dissipation in the capacitor is very low. Sep 9, 2014 at 9:29
• So which of two below capacitors it more suitable.
– 2 X
Sep 9, 2014 at 10:28
• The"T491S106K010AT" or "T491A106K010AT". If I understand right with the capacitor with ESR 4Ω I will not have the danger to damage the mosfet and with capacitor with ESR 20Ω I will not have a big power dissipation on the capacitor because of the pulses which produces a small surge current on the capacitor. Thanks for your all replies.
– 2 X
Sep 9, 2014 at 10:35