Hi All,
I have already raised this question but never really got the answer I was after so hoping to raise it again, this time attaching the circuit.
Just to summarize the intent, when the lower FET of U$2 is switched, C4 is allowed to charge. C4 is then intended to provided the gate driver in U$1 for the upper FET power through some cycles while the lower FET remains off.
This is for Class D audio and what I am yet to understand when sizing C4, is how to accomodate for instances where the lower FET is 'off' for longer periods of time and the upper FET is demanded to do quite a few on/off switchings. Any reference I have found for sizing this 'bootstrap' capacitor in half-bridge topologies, don't really seem to account for this. Mind you they do multiply by about a 20x safety factor, after you calculate how much charge it would need to drive the circuit and switch the upper FET on once. I just feel that 20x may be too small if for example the upper FET must be turned on and off 300x for a given upper half of the sinusoid http://en.wikipedia.org/wiki/FileWM,_3-level.svg.
I'm sure my C4 cap is big enough, I have referenced a similar design from International Rectifier and they only used 0.33uF, but I just hate not understanding before I proceed.
Cheers,
I have already raised this question but never really got the answer I was after so hoping to raise it again, this time attaching the circuit.
Just to summarize the intent, when the lower FET of U$2 is switched, C4 is allowed to charge. C4 is then intended to provided the gate driver in U$1 for the upper FET power through some cycles while the lower FET remains off.
This is for Class D audio and what I am yet to understand when sizing C4, is how to accomodate for instances where the lower FET is 'off' for longer periods of time and the upper FET is demanded to do quite a few on/off switchings. Any reference I have found for sizing this 'bootstrap' capacitor in half-bridge topologies, don't really seem to account for this. Mind you they do multiply by about a 20x safety factor, after you calculate how much charge it would need to drive the circuit and switch the upper FET on once. I just feel that 20x may be too small if for example the upper FET must be turned on and off 300x for a given upper half of the sinusoid http://en.wikipedia.org/wiki/FileWM,_3-level.svg.
I'm sure my C4 cap is big enough, I have referenced a similar design from International Rectifier and they only used 0.33uF, but I just hate not understanding before I proceed.
Cheers,
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