# Determine minimum working frequency from the V*usec value of a pulse transformer?

1. Sep 27, 2016

### Honk

Hi.
I need some help in explaining in a simple way how to calculate the minimum allowable
frequency at a certain voltage when selecting a pulse transformer.
The one I'm looking at has an ET value of 45Vusec.

I'm feeding it 22.5KHz at +/-6VAC square wave. It seems to work fine but I fear it is operating out of spec.
Can someone nice her please help me (in s simple way) how to calculate the data from the ET value?

Regards / Honk

2. Sep 27, 2016

### Baluncore

3. Sep 28, 2016

### jim hardy

I really like that Butler link.

I'm no pulse transformer expert, but hre's the basic physics you start from.

Volts in a coil is proportional to Δflux /Δtime. dΦ/dt

so Φ = ∫volts dt
for one half cycle of your square wave volts is constant so it can move to other side of integral sign
Φ = volts X time
and they're telling you your transformer can hold enough flux to withstand 45 volt- microseconds, that is 45 volts for 1 microsecond or 1 volt for 45 microseconds
or 6 volts for 7.5 microseconds.

So what is frequency of a square wave with half cycle duration of 7.5 microseconds ?
Any lower frequency saturates it.

Note that is for unidirectional applied voltage.
Since your bipolar square wave drives the core between negative and positive Φmax not zero and max, you probably would get away with running it at half that frequency provided wave is square ie 50% duty cycle ..