Hey all,
I am sizing a bootstrap capacitor for a half bridge mosfet class D audio amp. I am considering all factors (gate charge, high side driver level shift quiscent current etc..).
What I am uncertain of, is lets say I outputting a very low frequency but my switching (modulating) frequency is 400kHz. So for example, my high side will have to switch on and off 1000 times (400Hz output) before the signal modulates through the low side driver to recharge the bootstrap capacitor. Won't I have to account for the fact that each switching cycle, basically the gate charge will be lost (returned to the bootstrap cap source through the gate driver push pull stage) and thus my bootstrap must store up enough charge to allow for so much returned gate charge?
Any references I have seen only account for the quiscent currents and the total gate charge etc.., then multiply by 15 for a safety factor. I keep coming back to having to multiply nearly 1000 times, as the gate charge is generally the dominating factor in the calculation.
In a design I have seen 0.33uF did the trick for a 200W amplified channel. My calculations roughly matched this with a safety factor of only 15 but with 150+ it would be getting quite large to be charged up in nearly a microsecond.
Any assistance in settling my uneasy mind would be great. I don't want my amp to be suctacular if I decide to play some bass heavy tunes.
Cheerio!
James
I am sizing a bootstrap capacitor for a half bridge mosfet class D audio amp. I am considering all factors (gate charge, high side driver level shift quiscent current etc..).
What I am uncertain of, is lets say I outputting a very low frequency but my switching (modulating) frequency is 400kHz. So for example, my high side will have to switch on and off 1000 times (400Hz output) before the signal modulates through the low side driver to recharge the bootstrap capacitor. Won't I have to account for the fact that each switching cycle, basically the gate charge will be lost (returned to the bootstrap cap source through the gate driver push pull stage) and thus my bootstrap must store up enough charge to allow for so much returned gate charge?
Any references I have seen only account for the quiscent currents and the total gate charge etc.., then multiply by 15 for a safety factor. I keep coming back to having to multiply nearly 1000 times, as the gate charge is generally the dominating factor in the calculation.
In a design I have seen 0.33uF did the trick for a 200W amplified channel. My calculations roughly matched this with a safety factor of only 15 but with 150+ it would be getting quite large to be charged up in nearly a microsecond.
Any assistance in settling my uneasy mind would be great. I don't want my amp to be suctacular if I decide to play some bass heavy tunes.
Cheerio!
James