Bootstrap capacitor overcharging

[dknguyen]

Hi. THere is a section in the International Rectifier Application Notes that talks about sizing boostrap capacitors for gate drivers. It says that the operation of the boostrapping circuitry can lead to overcharging of the capacitor which will damage the IC and to prevent this the capacitor must be (rule of thumb) at least 15x larger than the minimum size which is actually needed to sustain the gate drive.

DOes anyone know anything more about this overcharging? Because I am wanting to use a boostrap capacitor (rather than an isolated supply) with the Adum1234 isolated gate drivers from Analog Devices and the single capacitors large enough cost a bundle (ceramics, I am not using electrolytics for this).

Also, 4.7uF capacitors cost $0.67 each while 22uF capacitors cost $4.89 each. But I don't have enough space in the PCB to place a bunch of 4.7uF capacitors. Is it a bad idea to "stack" SMD capacitors onto each other?

https://www.electro-tech-online.com/custompdfs/2008/03/dt98-2.pdf

 

[Roff]

Hi DK,
I'm not implying that I can help, but you really should post a link when you refer to something that obviously has one (the IR app note).

 

[dknguyen]

Hi DK,
I'm not implying that I can help, but you really should post a link when you refer to something that obviously has one (the IR app note).

*fixed* - filler

 

[ronsimpson]

Dknguyen,

Onsemi has an application where they set the boost strap capacitor by p-p ripple voltage on the capacitor.
In short if you want to drive the gate of a MOSFET form 0 to 10 volts and you know the total input capacitance, then if the boost strap cap is 10x then the ripple on the cap will be 1 volt. (1/10) I think 0.5 to 0.1 volts ripple is fine.

I check the boost supply to prove it is not starving out at minimum and maximum duty cycle.

 

[dknguyen]

I need to supply an average of 5mA for 1/10,000th of a second. I'm charging up the capacitor to 14V (diode drop already accounted for) and it must stay above 12V. So the bare minimum capacitor I need is 1.75uF. I am using a minimum of 4.7uF, but I'm worried about the overcharging thing. 2V ripple is quite a bit but yeah...4.7uF ripple would be around 1V. But yeah...again...

It shouldn't be starving out since the capacitor will only need to support one switching once per commutation cycle. It will have another 1/10,000th of a switching cycle to get recharged (but unlike a bootstrap IC it will be drawing 1mA the entire time from the capacitor since it is designed to run off an isolated supply rather than a boot-strap.