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Registered Member #3451
Joined: Sun Nov 28 2010, 11:13PM
Location: United States
Posts: 100
Hi, all. I've been using this circuit to drive one of my power transformers:
(Despite its appearance, the top coil definitely has more turns than the center-tapped one.)
This design works satisfactorily. However, I would prefer to use an n-type MOSFET (such as the IRFP260) instead of a 2N3055. I already tried substituting the gate for the base, the drain for the collector, and the source for the emitter. As I expected, this approach didn't work.
Is there a way I can convert this circuit to make it suitable for a MOSFET?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Difficult with common fets as they require 6v gate-source or more to fully turn on though it may work due to the transformer windings. There are mosfets with low gate turn-on voltages but no part number comes to mind at the moment.
A zvs inverter with a center-tapped primary and a feedback winding works well with npn transistors .... most CCFL lamps use this arrangement.
Choose the primary : feedback turns ratio for about 2V peak on the feedback winding, definitely no more than 5 or 6 V !
There are logic level MOSFET that can be turned on at low gate voltages. Once turned on, the induce voltage is feedback to the switching device via positive feedback. The positive feedback can produce higher voltage than the supply so getting the MOSFET to fully turned on would not be a big issue.
That's how I ran my LED driver that has a hybid BJT/MOSFET from a single NiCd for the higher efficiency. I wish they also made the opposite of IGBT - a device that cab be driven a transistor, but with the switching characteristics of a MOSFET. ;)
Not sure if that is the right configuration for the Oscillator.
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Substituting a MOSFET for an NPN transistor
