Capacitor selection ceramic vs electrolytic for bootstrap

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deer

Joined: 22 Sep 2009
Posts: 6

07 Nov 2009 11:46 Capacitor selection

Hi All,

My question may be simple!!!. What type of capacitor need to be used for the bootstrap capacitor to drive high side mosfets. Is it ceramic or electrolytic?
I am using IR2131 MOSFET driver. My calculation gives me to have 2.2uF capacitor for the bootstrap circuitry, but i am not sure what capacitor to be used since this value is coming in the territory between ceramic and electrolytic.

My understanding is electrolytic cannot be used since this capacitor needs to be charged and discharge at the rate of 100Khz and may not be desirable for the electrolytic capacitor than the ceramic. Otherwise the electrolyte will get heated up more. Are these assumptions correct?

Thanks in advance.
Deer

svhb

Joined: 07 Mar 2006
Posts: 100
Helped: 14

07 Nov 2009 13:51 Re: Capacitor selection

I see no reason why an electrolytic capacitor can not be used at 100kHz. Look at all the smps power supplies around...

There is however some more involved. Due to a pulsed current in the capacitor, it heats up. Heat is a big lifetime shortener for these type of capacitors.

For doing some estimations, it can be good to try to calculate the RMS-ripple current that flows through the capacitor. The maximum RMS-ripple-current is specified in the datasheet (mostly toghether with the frequency band). Use this information together with the lifetime specification of the capacitor noted.

For example : a 85°C capacitor with a specified life of 2000 hours can have a maximim RMS-ripple current of 500mA --> this means that if you put an ac-current through the capacitor of 500mA RMS at 85 degrees, the capacitor is expected to go of spec after 2000 hours.

This seems to be not so much, but you can multiple the lifetime by 2 for every 10 degrees Celsius your ambient is less (Arrhenius). So in the previous example, for an expected maximum ambient (of the capacitor!) of 45 degrees Celsius, it will last 2000*2^4 = 2000*16 = 32000 hours. If you have a lower ripple current, you can expect again some longer life, but ambient temperature is the biggest factor.

These calculations are normally necessary for SMPS-output (and input) capacitors where ripple currents can be rather high.

For your case, be sure to select a capacitor for high-frequency use (like in SMPS), and not for standard 50/60 Hz use. If you expect problems, you can select lon-life types with for example a lifetime of 5000Hours at 105°C (the multiplication factor for the lifetime will be much bigger).

Stefaan

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FvM

Joined: 22 Jan 2008
Posts: 5150
Helped: 766
Location: Bochum, Germany

07 Nov 2009 14:46 Capacitor selection

You possibly can keep the RMS current specification (I'm not sure about). But remember, that the capacitor should
be able to source a several A gate current pulse. I didn't check, but at worst case, the voltage drop will trigger the undervoltage
protection of the gate driver. If not, you get at least a considerable gate current degradation by the capacitor's ESR.
You better use a 1-2 uF ceramic capacitor, as everyone does.

deer

Joined: 22 Sep 2009
Posts: 6

10 Nov 2009 15:57 Re: Capacitor selection

Thank you FvM and svhb,

I could understand the svhb calculation and thus the possibility of the use of electrolytic capacitor. But at the same time as FvM explained, i was concerned about the initial peak gate current requirement during every switch on time of the MOSFET thought the time of peak current is interms of microseconds. Also i was having a doubt that, whether the eletroltic capacitor could delover this high peak current within short duration as other (like ceramic) capacitor can do.

The next think is the inherent high ESR of electrolytic capacitor. Actually i could not do any calculation for the justification. I have considered ceramic by looking at the above factors only.

Thanks alot.

Deer