Electronic – Pulse transformers and V.dt ratings

I think you may be exceeding what is known as the volt-microsecond limit for the transformer. Basically this figure tells you how long you can apply a pulse of a certain voltage before the voltage reverses. Real transformers will saturate and the V.\$\mu s\$ limit tells you, in practical terms how far you can push the transformer.

The waveform initially looks good on the rising edge - it attains a peak of 4 or 5 volts (it looks that way from your pictures) but after about 10 us it goes wrong - this means your transformer is probably rated as having a volt-microsecond product of about 50.

Other problems with the circuit are: -

• You need more gate drive voltage to adequately and efficiently turn the mosfet on
• As you vary mark-space ratio, the peak voltage seen by the gate (from the transformer) will vary considerably - you need to have a clamp circuit on the secondary that keeps the gate drive at a decent level for varying mark-space.
• I don't think that transformer is man-enough to provide the current needed by the gate to efficiently switch the mosfet without incurring heavy losses.
• The transformer drive circuitry also isn't powerful enough to drive a transformer should you have a transformer (and clamp) that would fit the bill efficiency-wise.

Here is a helpful (in parts) article on the subject. And here is a helpful guide on designing transformer circuits for driving mosfets: -

You can probably see in one of the diagrams (fig 3) the problem you are getting.

I do not know the details of your mosfets, so these answers are general guidelines. You seem to already have a good grip on the important issues:

1) You are correct in assuming 10mH is way too high of an inductance for any switching power supply. 10mH is so high you could use it in audio circuits. Your chart (Table 3) for guidelines is good. Stay below 4mH if possible, but it is a good value if you want minimum power consumption on these drive transformers @100KHZ.

2) For a given frequency, small ferrite toroid cores will have a limited range of inductance in which they are efficient at resonating with the frequency and transferring power to the secondary windings. At 500uH it will work but the idle current will be on the high side.

At 2mH the idle current will be much lower, but if the toroid is very tiny and you go above 2mH you may saturate the core, causing the primary to act like a short circuit.

Current limiting and / or fusing of the primary driver IC or mosfet is recommended. I would select a toroid at least 1cm diameter with a cross section of 2 x 5 mm. Avoid way over sized toroids as they have a lower resonate frequency, about 30KHZ to 50KHZ.

3) For 12 volts and minimum inductance that is safe for the mosfet driver, 6 turns on the primary and 2 (or 4) x 6 turns on the secondary will do just fine if you wind it yourself. Each set of 6 turns should wrap around the entire toroid to avoid eddy currents that create 'dead spots' in the core.

Be sure to put a 10 or 22 ohm 1/4 watt resistor right at the gate of each mosfet. This reduces ringing at the drain pin and parasitic oscillation. The basic rise/fall time for a given mosfet is specified by the manufacturer.

To calculate the combined gate rise/fall time use: 2 x pi x R x L x C x 12(volts), where L is the inductance of your secondary winding, C is the mosfet gate capacitance, and R is the series resistor I mentioned. Don't forget your dealing with square waves with sharp rise and fall times (a few 100nS at most).

There is no 'perfect' rise and fall time, just the fastest times possible with inductive values between min and max limits as per Table 3, and the resistor to prevent distortion at the mosfet drain pins.

4) For a combination of efficiency and enough drive current for the mosfets, I recommend starting out with a toroid or 'pot' core with a primary and secondary inductance of about 2mH. I would ask you to buy a LCR meter if you wind your own, but they are expensive.