Hi,
can anyone help with this one? I am running an H-bridge supplied at 80V with four mosfets (2xZVP3310F on the high side, 2x ZVN3310F on the low side) triggered by a PIC running at 5V - No intentional inductances, just tracks, the load is resistive.
The triggers are pulses about 5V 60 - 100 usecs on alternate gates to reverse the output polarity. (they are not simultaneous!!!!)
Two more ZVN3310F provide the inverted pulses for the ZVPs.
On the scope I can see a fast largish spike (about 1 - 2 usec, around about 10-15V amplitude) at the very beginning of the output pulse across the load.
(rough outline bottom right of the attached drawing)
This voltage is more than the drop due to the current through the load (rectangular outline).
I have asked a few other people but they are puzzled as well as to what exactly is going on. That pulse is mains sensitive, although the circuit is battery powered. Running the pulses at 100 Hz it goes up and down slowly, if I run them at say 30 Hz they go up and down faster, so it looks as if a mains pickup is involved. In fact, it also grows enormously if I touch the ground, making me thing it could be related to the scope ground . However, the pulse is rather solid, does not go away with any decoupling I have tried and it is still there, although reduced, if I use a battery powered scope.
I have substituted a bipolar for the ZVP (MMBTA92) and the effect is still there.
I am attaching a schematic of the output stage for clarity.
Thanks to any kind bright brain out there
can anyone help with this one? I am running an H-bridge supplied at 80V with four mosfets (2xZVP3310F on the high side, 2x ZVN3310F on the low side) triggered by a PIC running at 5V - No intentional inductances, just tracks, the load is resistive.
The triggers are pulses about 5V 60 - 100 usecs on alternate gates to reverse the output polarity. (they are not simultaneous!!!!)
Two more ZVN3310F provide the inverted pulses for the ZVPs.
On the scope I can see a fast largish spike (about 1 - 2 usec, around about 10-15V amplitude) at the very beginning of the output pulse across the load.
(rough outline bottom right of the attached drawing)
This voltage is more than the drop due to the current through the load (rectangular outline).
I have asked a few other people but they are puzzled as well as to what exactly is going on. That pulse is mains sensitive, although the circuit is battery powered. Running the pulses at 100 Hz it goes up and down slowly, if I run them at say 30 Hz they go up and down faster, so it looks as if a mains pickup is involved. In fact, it also grows enormously if I touch the ground, making me thing it could be related to the scope ground . However, the pulse is rather solid, does not go away with any decoupling I have tried and it is still there, although reduced, if I use a battery powered scope.
I have substituted a bipolar for the ZVP (MMBTA92) and the effect is still there.
I am attaching a schematic of the output stage for clarity.
Thanks to any kind bright brain out there