Hi All,
I am designing a Class D audio amp and started to ask myself what happens at the source of the switching after the top MOSFET switches off (i.e. M1 in the picture).
I have not depicted the bottom MOSFET other than its flyback diode.
My original query was how to size diodes D1 and D2. I was following a reference design from International Rectifier where they seem to have used diodes with an If=1A and an Iffsm=40A.
My original understanding was, that when M1 switches off, a current path through L1 must be maintained to drive the speaker until L1 is discharged of energy. This as I thought naturally comes through D2, which I was correct.
The thing is, D2 sees basically the full load current during upper FET off stage. For the reference design of a 250W @ 50V amplifier, this should be approximately 5-7A. So how could International Rectifier choose diodes which can only sustain 1A continuous? I mean, even if we consider off cycles etc.. etc.. 1A still seems too small.
When I sat down to simulate this, I was caught off guard by the ripple at the source of M1. It appears to be once L1 is primarily discharged of energy. This just seems odd to me and hard to explain. Is this normal? Is it not going to be a huge EMC issue? Although this is just a hobby, any half bridge circuit I work on in the future will be impacted similarly.
Thanks in advance for your interpretations.
James
I am designing a Class D audio amp and started to ask myself what happens at the source of the switching after the top MOSFET switches off (i.e. M1 in the picture).
I have not depicted the bottom MOSFET other than its flyback diode.
My original query was how to size diodes D1 and D2. I was following a reference design from International Rectifier where they seem to have used diodes with an If=1A and an Iffsm=40A.
My original understanding was, that when M1 switches off, a current path through L1 must be maintained to drive the speaker until L1 is discharged of energy. This as I thought naturally comes through D2, which I was correct.
The thing is, D2 sees basically the full load current during upper FET off stage. For the reference design of a 250W @ 50V amplifier, this should be approximately 5-7A. So how could International Rectifier choose diodes which can only sustain 1A continuous? I mean, even if we consider off cycles etc.. etc.. 1A still seems too small.
When I sat down to simulate this, I was caught off guard by the ripple at the source of M1. It appears to be once L1 is primarily discharged of energy. This just seems odd to me and hard to explain. Is this normal? Is it not going to be a huge EMC issue? Although this is just a hobby, any half bridge circuit I work on in the future will be impacted similarly.
Thanks in advance for your interpretations.
James
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