Hi folks,
This post is related to my previous one
http://forum.allaboutcircuits.com/showthread.php?p=273818#post273818
but is an entirely different problem hence the new thread.
I have a generator control circuit that drives a FET (N-Channel Enhancement mode) with a PWM signal to regulate the field current.
The circuit is needs to be supplied from a stator winding that, due to a couple of small permanent magnets on the field rotor, only outputs about 12VAC at start-up. Obviously this is quite enough to adequately drive the FET gate but once energised, when the FET turns on, the same stator winding produces anything up to 130VAC and will be delivering several amps, all be it pulse width modulated via the FET, to the field rotor.
My drive circuit is currently simple and ineffective:-
Across the rectified output of the stator winding I have a large dropping resistor in series with a 10V zener with the fet gate at their junction. The FET source and zener cathode share the ground rail.
To turn the FET off I have to sink all the current available from the resistor.
There are several big issues with this configuration that I am aware of but cant work out how to cure.
I am sure I am just missing something simple here
A voltage reg that will drop 100V or so at a couple mA perhaps?
(I did think of using a depletion mode FET but can't find one that will handle the voltage / power)
Please help if you can, I am stuck
Thanks for reading
Al
This post is related to my previous one
http://forum.allaboutcircuits.com/showthread.php?p=273818#post273818
but is an entirely different problem hence the new thread.
I have a generator control circuit that drives a FET (N-Channel Enhancement mode) with a PWM signal to regulate the field current.
The circuit is needs to be supplied from a stator winding that, due to a couple of small permanent magnets on the field rotor, only outputs about 12VAC at start-up. Obviously this is quite enough to adequately drive the FET gate but once energised, when the FET turns on, the same stator winding produces anything up to 130VAC and will be delivering several amps, all be it pulse width modulated via the FET, to the field rotor.
My drive circuit is currently simple and ineffective:-
Across the rectified output of the stator winding I have a large dropping resistor in series with a 10V zener with the fet gate at their junction. The FET source and zener cathode share the ground rail.
To turn the FET off I have to sink all the current available from the resistor.
There are several big issues with this configuration that I am aware of but cant work out how to cure.
- The high impedance of the circuit, imposed by the maximum zener current, makes the FET turn on slow. (Probably, I am finding it hard to measure)
- My drive circuit has to cope with the entire potential gate drive current whilst discharging the gate again I suspect slow.
- The drive circuit must have a very high impedance during power-up to allow the gate to charge initially without any positive assistance.
- The system constantly dissipates a relatively large amount of power, which is exactly what the FET is supposed to avoid the need for.
I am sure I am just missing something simple here
A voltage reg that will drop 100V or so at a couple mA perhaps?
(I did think of using a depletion mode FET but can't find one that will handle the voltage / power)
Please help if you can, I am stuck
Thanks for reading
Al
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