EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Chris Wilson on October 21, 2015, 09:56:09 am
-
I have an IRFP460 used as a switch and I have had a few failures, possibly as a circuit feeding the gate is going resonant, or seeing a lot of RF about. It was suggested I fit a Zener diode as opposed to an IN4007 across gate to source, and also fit a ferrite bead on the gate lead. Now, I have had no dealings with ferrite beads, but the few I have seen are too small to over the leg of a fairly big FET like an IRFP460. So do they come in different ID's? Or do I buy a leaded one and solder its shortened ead as close to the gate leg of the FET as possible?
What about different material mixes, are they available in different mixes like toroids?
The circuit it is switching is the feed to the output FET's in a Class D 136kHz amp, so what should I be looking for in buying ferrite beads, can anyone give me a bit of insight please?
Many thanks for any replies.
-
Funny you mention toroids. Both powdered iron mixes and ferrites are available in that shape. Fair-Rite's products are numbered, so be careful not to confuse them with Micrometals'! (Others use alphanumeric designations, e.g. Ferroxcube's 3F3 and etc., TDK's N97 and etc..)
FBs are available anywhere from microscopic (SMT chips well under 0402) to over 6" across. Beads are intended for use on component leads or over cables; the cable kind come pretty large, so somewhere in the 1-3" range they kind of cross over to being toroids for RF and power transformers. But they're all the same thing, just in different heights and diameters, it doesn't matter.
Closed ferrite shapes, like beads, toroids, and ungapped shapes, are not useful under DC bias.
The general purpose is to add impedance, especially at RF, to the conductor or cable. Obviously, an important first step in making a transformer, hence that application as well. For signal applications, this adds damping and high frequency filtering, especially in combination with some capacitors and maybe resistors. This doesn't help much for single power lines (because of saturation), but if the current is equal and opposite (as should be the case for the sum of all conductors in a cable), saturation is avoided and common mode impedance is introduced. This is especially helpful for EMC, dampening the resonances of cables, and to some extent filtering noise (from within the device, or induced on it from ambient sources).
FBs can be used on transistors, usually the input (gate/base) or common (source/emitter) terminals to act to slow down switching speed, or help neutralize unstable impedances (low input resistance tends to result in oscillation in the 10-100MHz range).
Whether applying a FB is right for you, would require a study of just what's oscillating (if anything), and some analysis of whether it'll help at all, or actually make things worse (FBs aren't by any means a miracle).
Tim
-
https://www.youtube.com/watch?v=81C4IfONt3o (https://www.youtube.com/watch?v=81C4IfONt3o)
-
.. but the few I have seen are too small to over the leg of a fairly big FET like an IRFP460.
They seems like custom made, attached below are the FB I got while ago from a local electronics recycling shops, and they were used at the gate pin at high power mosfets.
The blue ones are encased in blue plastic jacket and taller than the naked one.