- #1
jrive
- 58
- 1
Turning FETs "off" in presence of AC signal at Drain or Source
Hello,
I have a very interesting problem that surprised me, and as of yet have not been able to find a solution for.
I have two coupled coils. One is being driven by a pulses. The other coil has FET switches to switch the high load path out and instead divert the induced current through a lighter load. This is necessary so that the reflected load onto the primary is minimized. The problem is that the parasitic caps (and perhaps the parasitic internal BJT) on the FETs, configured as 2 back-to-back N-channel mosfets to block any current flowing through the body diodes, cause current to flow through the FETs (remember, the voltage at the node between L2 and R2 is positive and negative). So, I cannot fully turn the FETs off, and the load on the undriven coil is not the 100k, but the 100K in parallel with the 75ohm plus the parasitic impedance presented by the FETs.
I've tried other switching elements, but I still can't get this to work. I can't use a relay (which would work fine, by the way).
So, I'm stumped. Any suggestions are welcome. Again, the objective is to be able to turn mosfets M1 and M2 FULLY OFF such that current in the secondary is minimal so that the reflected load back onto the primary is minimal as well.
Thanks!
Hello,
I have a very interesting problem that surprised me, and as of yet have not been able to find a solution for.
I have two coupled coils. One is being driven by a pulses. The other coil has FET switches to switch the high load path out and instead divert the induced current through a lighter load. This is necessary so that the reflected load onto the primary is minimized. The problem is that the parasitic caps (and perhaps the parasitic internal BJT) on the FETs, configured as 2 back-to-back N-channel mosfets to block any current flowing through the body diodes, cause current to flow through the FETs (remember, the voltage at the node between L2 and R2 is positive and negative). So, I cannot fully turn the FETs off, and the load on the undriven coil is not the 100k, but the 100K in parallel with the 75ohm plus the parasitic impedance presented by the FETs.
I've tried other switching elements, but I still can't get this to work. I can't use a relay (which would work fine, by the way).
So, I'm stumped. Any suggestions are welcome. Again, the objective is to be able to turn mosfets M1 and M2 FULLY OFF such that current in the secondary is minimal so that the reflected load back onto the primary is minimal as well.
Thanks!