# When should I use a low ESR capacitor

I am in the middle of a new design and I need to choose the right capacitor.

What is the impact of equivalent series resistance (ESR) in a capacitor?

When should I use a low ESR capacitor?

If the ESR of the capacitor is high relative to the reactance of the capacitor ($X_C= \frac{1}{2\pi fC}$) at frequencies of interest, then you might want a low-er ESR capacitor.

The requirement for "low-ESR" capacitors normally arises in output filters of switching power supplies, where the frequency is relatively high (kHz to MHz). It's less important in mains filters (including the input filter of a SMPS) where a large capacity electrolytic tends to have a proportionally small ESR so that 100Hz or 120Hz ripple is not greatly affected by the ESR.

ESR also causes $I^2R$ heating, which can dramatically shorten the life of electrolytic capacitors (half life for every 10°C rise is a rule of thumb).

They're also useful in building ultra-low noise analog power supplies, because the low ESR part can reduce the noise with fewer filter stages when you use a high-quality low-ESR polymer electrolytic rather than alternatives.

ESR is just what it says, resistance in series with your capacitor.

Low ESR is important if there's a lot of ripple current in your capacitor. The RMS ripple current will cause heating (I^2R) losses in the capacitor, and additional ripple voltage.

It will also affect the frequency response of your capacitor. The ESR zero formed by the RC circuit can actually help stability in a power supply control loop, at the expense of higher output ripple.

So if your application has high ripple current and you don't need the ESR zero for stability then a low ESR cap will likely be the way to go.

If you are looking for energy storage without large di/dt then a high capacity electrolytic is more appropriate.

• What do you mean by if you don't need the ESR zero ? (#ExplainLikeI'm5) – JinSnow Jul 11 '19 at 8:15
• @JinSnow That would be a good topic for a new question rather than trying to answer in the comments of a different question. E.g. "What is an ESR zero in a power converter's output filter and what are its implications for control loop stability?". You might even find the answer with Google. – John D Jul 11 '19 at 14:31
• @ John Thanks! I asked it here (but without the second part, since I don't know either what a "control loop stability" means) electronics.stackexchange.com/questions/447964/… – JinSnow Jul 12 '19 at 3:38

You should use a low ESR capacitor when the expected I^2 R heat loss (ripple current, squared, times the ESR), is too much heat for the component.

Power-supply capacitors smooth ripple on DC power supplied from AC sources. When the AC source is low frequency (50 Hz, 60 Hz, 120 Hz...) the capacitors are physically large, and could tolerate high ESR (like, 1 ohm for a 1A supply with a 1000 uF filter capacitor). That's because a one-amp ripple current only created one watt of heat, and a large (over a square inch of surface area) 1000uF capacitor can shed that heat.

When switchmode supplies went up to 50 kHz, and a suitable ideal-capacitor value was (again for 1A output) about 2.2 uF, 1 ohm ESR means that same 1W would be dumped into a capacitor the size of a pea. It'd fail, because it's too small to dissipate one watt of heat.

That isn't the full story (there might be local heating 'hot spots' even if the average dissipation seems supportable), so there are separate ESR and ripple current specifications.

Capacitor ESR, outside of DC power supply filtering, is also of concern because it is an unwanted stray impedance. It may be discussed as 'dissipation factor', or 'loss tangent', or Q, and have significant frequency dependence.

Another situation where you would want a low ESR capacitor is when there's a lot of current draw in small bursts (like a sub-woofer amplifier). In such case a higher ESR will limit the maximum current that can be drawn hence limiting power output and performance of the circuit.