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Pulse transformer rating exceeded?

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Jun 22, 2008
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i wish to use this pulse transformer to turn a mosfet on and off in a SMPS.

(i wish to use the 1:1 one, top line)

Unfortunately, near the top, it says that its Maximum Pulse current is 200mA.

This appears to be bad for me as a MOSFET is driven on by a high current of some 1.5A , which flows for a few 100ns.

Since this 1.5A current is higher than 200mA, does that mean that i cannot use this pulse transformer for my purpose?

The SMPS is a two transistor forward converter with switch frequency 100KHz

i dont know much about the High Freq Transformers but a jump from 200ma to 1.5a is well bad you can always try it and see how hot the Transformer gets if its hot with in room temp i would not trust it but to me this sounds a big jump in Current. i would try to find another solution.

thankyou for your recommendations,

the thing is, though, you see, on the datasheet it talks of "thyristor & triac control", i believe that thyristor drivers keep the pulse on for the entite thyristor on maybe for that 200mA is too much...

....but with a MOSFET....the 1.5 Amps literally only flows for 200ns.

..and i just can't see that heating it up enough.........though i also don't know if i can rule out damage even with a short 200ns pulse, and i share your scepticism of its capabilities.

the magnet wire of it may just blow like a quick blow fuse?...even though the 1.5Amp current pulse only flows for about 200ns

Assume for the present, that the transformer can manage the current pulse.

But there is another, more basic problem. Your MOSFET may have an effective input capacitance of 1 to 5 nF. It forms a series resonance with the 19 uH transformer leakage inductance at 0.5 to 1 MHz. Now tell me, how to transmit a 100 ns rise time gate pulse accross the transformer? It simply doesn't work.

thankyou FvM, i had forgotten about that.

actually i believe i will stick in some small series resistance in the primary and secondary circuits.

this should , i hope, damp out the resonant oscillation.

as such here is an example, with the series resistance...


The current pulse into the Mosfet will still be insignificantly small ?

And i hope the 1:1 pulse transformer in the datasheet will be able to manage.?

You should add a 19 uH series inductance to reflect the transformer leak inductance and watch the results.
1N4148 also isn't good for 1.5A, by the way.

I am new to power electronics so my question might be simple....but what I want to know is when a pulse is applied to primary the current will rise as V/Lm (V is amplitude of pulse, Lm is magnetizing inductance of transformer). When the pulse goes to zero the current will remian constant (or almost constant it might decrease slightly due to parasitics and leakages), but then the next voltage pulse comes and so current will increase again, so ultimately the core will saturate, correct? I'm sure I am missing some concept here. Can you please tell me how you will prevent the core from saturating?

By applying a DC balanced pulse (as achieved by the capacitive coupling in the above circuit), the flux is allowed to zero.

Thanks FvM,

Could you pleasee elaborate on "DC balanced pulse" and how is that achieved by capacitive coupling? Aren't C17 and C18 d.c. blocking capacitors?

Aren't C17 and C18 d.c. blocking capacitors?
Yes, so the voltage is DC balanced - having a zero DC component.

As another aproach, you're considering about an increasing transformer current respectively flux (it's just the same, as long as we ignore the rather small core hysteresis). But the capacitor simply enforces a current mean value of zero, so it can't increase, what ever you supply to the circuit input.

FvM, thankyou for pointing out the 19uH of leakage inductance for this pulse transformer.

I added this 19uH to the simulation, in series with the primary winding.

It really, really slows up the rise of the FET gate drive current. -It's horrendous, literally.

The power dissipation in this upper FET is thus considerably increased unfortunately.

Also, the FET gate charge current is so heavily filtered by the 19uH leakage inductance, that i end up being able to get away with using the 1N4148 diodes since the current doesn't get anywhere near above the 1N4148'S Max. current rating.

Also, i had to decrease R17 to 220R as this was needed to dampen the horrendous LC oscillation of the Gate voltage caused by the leakage inductance and the capacitance.

Does any reader know how anybody ever gets by using pulse transformers for high side FET drive?
-with the above said problems.

I was working on a similar thing a while back and the leakage inductance is a terrible problem. I had a pulse transformer with 10uH leakage, which somehow came into resonance with FETs gate. The gate drive signal looked more like a sine wave than anything else.

The solution is to use a high permeability toroid core, something with Al factor > 1500nH and use bifilar winding (twist two wires together and wind them on the core). With a winding of about 20 turns you should get leakage inductance lower than 0.5uH which is good enough. Also, choose the right core and number of turns to get a resonable magnetising current (don't go over 200mA).

One more thing; use a toroid and don't even bother with E-type core, since they are terrible for these purposes. They just give you too much leakage. Tried three types of them, all failed.

Best regards,

Added after 7 minutes:

This was the first one I made:

I took a torroid core from an ATX power supply, so I don't really know what kind it is. Anyways, it has a magnetisig inductance of about 500uH; I used it at 250kHz to drive a SPP20N60C3 mosfet.

I see farnell is you supplier; they have some toroid cores from Epcos and ferroxcube that could get the job done.

Hope it helps.


I assume, that the rather high capacitive coupling and low voltage insulation strength of the bifilar winding design contradicts most gate drive applications of practical importance.

I don't think, that direct drive pulse transformers have a real chance for fast switching applications with larger MOSFETs.

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