SCR Gate Drive Transformer

[ZeroResistance]

I am trying to trigger an SCR using a gate drive transformer.

The SCR datasheet says

Igt (gate trigger current) as
Typ 2mA
Max 15mA

Vgt (gate trigger voltage) as
Typ 0.6V
Max 1.5V

Currently I have hand wound a ferrite toroid core with 30Turns primary and 10 turns secondary and am giving a 10uS @ 12V pulse to the primary via a transistor, I also have a free wheeling diode across the primary.
However at the secondary output I get around 8uS @ 0.5V.

Its seems the upper limit I get for the pulse width is around 8uS no matter how long I stretch the pulse to the primary, so probably I'm hitting saturation limits of the toroid.
The toroid cross sectional area is around 15sq.mm.

I was expecting around 4V at the secondary due to the 3:1 turns ratio, so I'm wondering what's going wrong here, I'm just measuring the open circuit voltage across the secondary.

Probably a hint or 2 would help me to proceed in the right direction.

TIA

[T3sl4co1l]

Right, 15 mm^2 * 0.3T * 30t = 135 uV.s, so at 12V and 10us you're very close to that.  Give or take remenance, exact point you're calling saturation, and so on.

I don't know why you're not measuring 4V on the secondary.

That's when it's not connected to the gate, right?  Also, you have a series current limiting resistor on the gate?

Tim

[ZeroResistance]

As of now I am meauring open circuit voltage across the secondary.
Not connected the gate yet.
I wound another toroid this time 1:1, 30T primary and 30T secondary and this time when I give 12V pulse in the primary I get 12V in the secondary.

To clarify regarding the last transformer, I had taken 5 equal lengths of wire and then wound 10 T will all of them then I wired 3 of these in series to get a 30T primary taking care that I wired the end of the 1st to the start of the second. So using this I finally had a 30T primary and 2 10T secondaries.

With regard to the resistor for gate i wasn't planning any resistor except for a reverse diode for SCR gate protection.

Another point that I'm not clear with is I'm operating this transformer in unipolar pulse mode. Generally speaking transformers are operated in bipolar mode right?. So will this unipolar pulse eventually cause problems in the circuit. how do typical pulse transformers handle this?

[schmitt trigger]

You very likely are exceeding the core's Volt-microsecond capability. Perhaps remanence is higher and you are reaching saturation earlier than calculated.

But even then 135Vus seems to me a little on the low side to properly drive an SCR.
If I remember correctly, they are at least 300Vus.

[T3sl4co1l]

Without secondary side current limiting, you need it on the primary side then. Which is even better, as that reduces EMF --> more microseconds during the pulse.

Tim

[ZeroResistance]

You very likely are exceeding the core's Volt-microsecond capability. Perhaps remanence is higher and you are reaching saturation earlier than calculated.

But even then 135Vus seems to me a little on the low side to properly drive an SCR.
If I remember correctly, they are at least 300Vus.

How do you determine if a specific volt-second rating can drive a particular SCR, like i said the SCR datasheet says Igt as 2 to 15mA ?

[ZeroResistance]

Without secondary side current limiting, you need it on the primary side then. Which is even better, as that reduces EMF --> more microseconds during the pulse.

Tim

How does that work out ? I mean I don't quite understand why current limiting on the primary side is better than the secondary and also regarding the EMF reduction

[T3sl4co1l]

EMF means the voltage drop across the winding due to induction.  That is, minus DCR and stuff.

On a normal, low-loss transformer, it's only slightly less than the measured terminal voltage.

It's a more precise term, because say you used very fine wire for the primary, and relatively thick wire for the secondary, so that the primary DCR provides that current limiting already: you'd find the secondary saturates at the same point even though you're apparently driving it with much more flux.  Because most of that isn't flux, it's I_pri * DCR dropped across that primary resistance, not the actual induced EMF.

You can also use a reverse diode on the secondary side, for the same reason: this allows more flux so the flux discharges faster than when using a diode on the primary side.  (You'd still want some damping on the primary side, to deal with leakage inductance -- an R+C across the primary should do fine here.)

Tim

[Zero999]

Without secondary side current limiting, you need it on the primary side then. Which is even better, as that reduces EMF --> more microseconds during the pulse.

Tim

How does that work out ? I mean I don't quite understand why current limiting on the primary side is better than the secondary and also regarding the EMF reduction

Putting the resistor on the primary side will damp any leakage inductance in the transformer.

I've done this before, but it was a long time ago, so can't remember the details. I used a ferrite bead, some very thin enamelled wire for the primary and polyamide insulated (good enough for mains isolation) wire for the secondary. It was a sensitive gate TRIAC, so would have triggered more easily than your SCR. I didn't use a back EMF diode on the gate, since a TRIAC's gate will conduct in both directions. There was no current limiting: to limit the energy to the gate of the TRIAC, I used a 100nF capacitor, on the transformer's primary.

I don't know why your transformer works as expected, with a 1:1 turns ratio but not with 1:3.

[T3sl4co1l]

You very likely are exceeding the core's Volt-microsecond capability. Perhaps remanence is higher and you are reaching saturation earlier than calculated.

But even then 135Vus seems to me a little on the low side to properly drive an SCR.
If I remember correctly, they are at least 300Vus.

How do you determine if a specific volt-second rating can drive a particular SCR, like i said the SCR datasheet says Igt as 2 to 15mA ?

Depends on application.

Is the SCR ready to fire (V_AK positive)?  If so, a few microseconds at, say, 10 * I_gt will do.

Why 10 times?  Because Igt is the guaranteed minimum, and it will turn it on slowly.  You get faster switching, lower switching loss, and higher peak dI/dt capacity, with higher drive.  In short, it's cooler and safer.  And it doesn't cost much drive power.

If it's not ready to fire, like in an AC application, then the better method is AC drive, using an antiparallel diode to deliver pulses of gate current while clamping the transformer's flyback voltage.  In a mains application, the delay between firing pulses won't matter much (i.e., if it's being driven for 10us, then the transformer resets for 10us, then repeat, it will only fire in that 10us window, every 20us; but that's fine for mains that's moving in the 10s of ms).

If it's an inverter application, better to use a voltage doubler circuit, to deliver continuous gate current, so the SCR turns on as soon as it becomes forward biased.  The primary winding should be driven with a full wave bridge (any will do: half bridge, full bridge, push-pull).

The trigger circuit is then mostly the same, but instead of the enable signal producing a single pulse, an oscillator is gated by the enable signal.

Tim

[ZeroResistance]

You can also use a reverse diode on the secondary side, for the same reason: this allows more flux so the flux discharges faster than when using a diode on the primary side.  (You'd still want some damping on the primary side, to deal with leakage inductance -- an R+C across the primary should do fine here.)

Tim

So a R in series with the primary should be okay for currrent limiting on the primary side right? You say R + C across the Primary instead of the free wheeling diode, also Hero999 said he used a cap across the primary. What is this cap or R + C meant for?

Is the SCR ready to fire (V_AK positive)?  If so, a few microseconds at, say, 10 * I_gt will do.

So for the SCR i'm using that comes to around 150mA, for a few 10's of microseconds. Question is, how do I validate that my GDT can supply 150mA to the gate. Do I measure the current in the gate of the SCR when I trigger it or will the Volt second rating of the GDT give an estimate of the secondary drive?

[T3sl4co1l]

Take small-signal measurements of your transformer, and toss together a representative schematic.  We can walk through all of these figures as a design-time matter.  Then if you like, you can take measurements as well to see how it works out.

Tim

[Zero999]

You can also use a reverse diode on the secondary side, for the same reason: this allows more flux so the flux discharges faster than when using a diode on the primary side.  (You'd still want some damping on the primary side, to deal with leakage inductance -- an R+C across the primary should do fine here.)

Tim

So a R in series with the primary should be okay for currrent limiting on the primary side right? You say R + C across the Primary instead of the free wheeling diode, also Hero999 said he used a cap across the primary. What is this cap or R + C meant for?

Sorry, I didn't make myself clear. I put the capacitor in series with the primary, not in parallel.

How are you driving the transformer? Are you just using microcontroller IO pin, a signal generator or a single transistor?

I just connected the transformer to a constant voltage power supply, via a capacitor and push-button switch, to see if my pulse transformer worked. I know this isn't ideal: there will be switch bounce and the capacitor will only discharge via its internal leakage. Unfortunately the project got shelved, so I didn't do any more scientific tests.