My Switches Don’t Switch

[abstruse1]

I have a PWM signal generator that requires 3-30vdc and puts out a bit less voltage than its input.  Its load capability is measured in milliamps.  I want to use it to modulate the input to various transformers that will power CW multipliers.

I need some way to increase the load capability of this PWM.  I’ve tried transistors, MOSFETs and IBGTs (including “bricks”).  No luck.

My assumptions about these devices is that with a small power input, as with my PWM, they should be able to switch much larger loads, just like an old fashioned mechanical relay would do, and be able to switch them at several kHz frequencies. Is this what they can do?

After reading lots of info on these devices, I’m lead to believe that IGBTs may be the best solution.  I have a few, and tried a 30J121, rated at 600V and 30A (hard to believe for such a small device):
 
I’m using the output of a Variac, run through a bridge rectifier, for the load voltage. Seeing as to how the IGBTs have only three leads, and I need four points of electrical contact (2 signal inputs and 2 for the load circuit), I had to guess.  I connected the signal input to the Gate and Emitter, and connected one leg of the load across the Emitter and Collector.  Is this correct?

When I used an LED as a load, it seemed to work.  I could only see LED flickering at or below ~40Hz.  Then I tried a 3K, 5W resistor for a load. Again, it may have worked, except that the frequency measured across the load bore little resemblance to the freq. setting on the PWM and seemed to be affected by the load voltage input to the PWM.  I was using a Yitensen VC3165 to measure the frequency (BTW, the 3165 agreed with the output setting on the PWM when they were connected directly to each other).

The 3K resistor heated up as I would have expected.  The IGBT stayed cool.  Then I used a NST for a load and the IGBT popped at some low voltage, ~30VDC.

I’ll stop here.  I can go on about the MOSFET trials if anyone cares.

I expect I’m doing a lot of wrong things here.  My electronic knowledge is not very good.

Any help greatly appreciated!

[V-Troxi]

Hi,

Your problem is caused by your lack of a decent driving circuitry for your IGBT.
A function generator cannot provide enough current to quickly charge the gate of an IGBT.
When you are trying to switch at several kHz, the transistor is most likely not really turning on at all.

What you need is a so called gate driver. This is a circuit which acts as a current amplifier to charge
the gate in order of a few microseconds or even nanoseconds (the necessary rise time of the gate voltage depends on your switching frequency but it should generally be as fast as possible).
This gate driver can be realized as a push-pull stage with a NPN and PNP BJT transistor or you can simply use one of the many available integrated circuits, which are very easy to use (IXYS has some quite powerful ones, capable of up to 20Apk of charging current - that's probably a bit overkill for your project ). They usually feature an input with a specific hysterieis - this means that any voltage in a given range is enough to make the output of the driver turn high (most of the time a few volts is enough).

A gate driver IC also needs a ceramic filtering capacitor (~100nF) across it's supply and ground pin.
This filters the HF currents that the IC is drawing in operation.
In addition to that you should add a small resistance like 2-10 ohms in series with the IBGT gate and the output of the gate driver. This limits the charging current and helps to reduce unwanted oscillations of the gate signal.

When you are only planning to use one transistor, you should use it as a low side switch, meaning the emitter is connected to ground, the supply is connected to the (+) connection of your load and the (-) connection of the load is connected to the collector of the IGBT.
The signal output of the gate driver goes via a gate resistor to the gate and the GND pin of the gate driver is connected to the emitter.

The advantage of a low side switch is that you can use an unisolated driving signal with ground reference.
(High side switches require more complicated concepts like bootstrapping, isolated power supplies or gate drive transformers...).

Another important aspect is the gate voltage. Most gates of IGBT bricks should be switched at around 20V which is is also the input voltage of your gate driver.

And one last thing: you said that you are planning to switch a transformer.
Because a transformer is an inductive load, you need to protect your transistors against high voltage spikes which can appear at the inductor turn off.
This should be accomplished by snubber networks or simply snubber capacitors.
Snubber networks consist of a series resistor (which is sometimes left out) and a capacitor.
These absorb the voltage spikes and protect your transistors.
There are also lot of guides on snubber design online.

I hope I could help with my explanation.

Best regards,

Vtroxi

[abstruse1]

Vtroxi, thanx for your elaborate explanation of what I need to do.  I'll have to read it carefully since some of it is over my head, but I can probably figure a lot out.  Stay tuned.