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Registered Member #6857
Joined: Sat Sept 22 2012, 08:25AM
Location: Srbija, Novi Sad
Posts: 40
Hi,
I'm trying to isolate the mosfet (IRFP460) gate from driver circuitry. However, the sources of ferrite toroids in my country are pretty limited, I wasn't able to find any in the electronic stores. I did however manage to scavenge a lot of ferrite from broken devices. Most of the SMPS transformers are gaped, so the best candidates were these few, at least considering their sizes:
I have no idea about any of these, I don't know what these color codes mean, and I have no oscilloscope.
The first one looks like plain ferrite, it's not colored, and I found a bunch of these inside an old printer. Those were used as some kind of filters, mostly power filters, coming from the printer PSU. The other two were scavenged from an old ATX power supplies. The first one's color is olive green, and the second one is light green with one side painted bluish-green.
My question is, does anyone know whether any of these could work? Are the other two powdered iron or ferrite? Also, I have the highest hopes in the first one. Would stacking a few of them increase the flux and driving power transferred to the mosfet gate?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The ferrite cores used for common mode input filters (two separate isolated windings) are suitable like the core on the left in your photo's To be sure, with a multimeter on ohms range there should be some measurable resistance which indicates MnZn .. the good for GDT type no measurable resistance indicates NiZn which are low permeability .. not good for a gdt.
Registered Member #6857
Joined: Sat Sept 22 2012, 08:25AM
Location: Srbija, Novi Sad
Posts: 40
Thank you for your reply.
When touching the multimeter leads the resistance is 0.2Ohm, when measuring the winding on 3 stacked rings it's 0.4Ohm, is that ok, or is it too little?
Registered Member #6857
Joined: Sat Sept 22 2012, 08:25AM
Location: Srbija, Novi Sad
Posts: 40
OK, I'm going to expand this thread a bit.
First, is the transformer core on the picture below big enough for isolating IRFP460's gate? It's from an old input filter, as Sulaiman suggested, and it has no gap. Only one mosfet is isolated.
Next, could anyone suggest me the best way to amplify the drive signal, and discharge the mosfet gate fast? I drew three options that popped in my mind.
(sorry about the image, threw away the paper before realizing how bad the photo was, so I made it clearer in paint a bit)
The first one (a) and the third one (c) seems OK to me, but I'm probably wrong.
The second one (b) concern's me a bit, I did saw some people use it on GDTs, but shouldn't the stored energy be allowed some time to pass onto the secondary circuit during the OFF time? Or does that only applies to the gaped cores? Wouldn't the PNP transistor take a part of the energy, not giving enough to the mosfet?
Also, what should the transformer ratio be? Is 1:1 good enough?
I know I ask too much, but I'm trying to learn and understand while I'm playing, and any help is greatly appreciated :)
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
A lot depends upon the duty cycle; for ANY transformer (or inductor) winding volt.second integral for positive signals MUST be the same as for negative signals or else direct current will build up in the winding and the core will saturate. So, for example, in 'flyback' mode the ON voltage may be 12V for 90us and OFF for 10us so the average OFF voltage would have to be 12V x (90/10) = 108V which would destroy a 50V rated BC337.
Also, in general, it is best to have some resistance in both the primary circuit to limit current if the transistor is turned on for too long and in the secondary to 'damp' oscillations between transformer inductance and transistor capacitance and to limit the peak transistor drive current (I doubt a BC337 could over-drive an IRFP460)
in a circuit like your b) diagram you also need series capacitance between transistors and transformer to allow negative as well as positive voltage swings accross the transformer primary (integral of v.t = 0) SO what is the duty cycle range for the IRFP460 ?
the cores with low resistance are suitable for GDTs, I consider ANY low resistance a good sign (opposite for r.f. ferrites)
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
To design a GDT you need to have a reasonable idea of the maximum volts.time required to drive the transistor. If the volt.seconds applied is more than designed for then the core will saturate, so you could make a GDT with a large number of turns to allow for long on times but if you have too many turns the leakage inductance will be too large to drive the transistor at higher frequencies.
So for any GDT there is a range of on times that will work well but above and below that the GDT will not work well
so you need to have a fairly good idea of what your operating range will be. Unfortunately it's not practical to make a 'universal' GDT SO if you have a specific application in mind, explain it and you will get lots of specific advice
if you want a general purpose any frequency/any duty cycle driver then I suggest that you use direct drive to the gate, non-isolated, and use an isolated supply for whatever it is that the transistor is driving (e.g. isolation transformer + rectifier(s) + capacitor etc.) OR have a separate isolated gate drive supply (e.g. 12V dc) and use an opto-coupler.
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