Electronic – How important is the gate threshold voltage on a MOSFET

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.

The RED LED should not be glowing. Maybe something is wired wrong or the 2N7000 is damaged, but there should not be more than nA of current flowing with 10mV on the gate, which will not produce a visible amount of light in normal room lighting, even with the most efficient red LED. Try replacing Q2 after checking the wiring.

Of course if you are leaving some types of multimeter connected where you are measuring the 0.35V then there could be enough current through the meter to provide a faint light from the LED.

There is no problem in reverse biasing LEDs up to the rated voltage (usually 5V), and in fact far beyond that typically (but there are no guarantees so you shouldn't do that).