Can't figure out how to wire IGBT to switch a load

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abstruse1

Joined Jan 18, 2021
11
Greetings. 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.
 

MisterBill2

Joined Jan 23, 2018
9,792
At least we need a schematic of the output circuit of your PWM generator.
If that is not convenient, what is certain is that you need more output power to drive other devices. Probably isolation will also be useful. So I am going to suggest feeding a simple amplifier IC with the pwm signal, That amplifier circuit can deliver enough power to properly drive an opto-isolator suitable for operating your transistor switch.
You do need to find some application notes for the power devices that you want to use. Appliction notes are usually found on the manufacturers website, never on yootoob or similar cartoon show sites. Application notes represent a manufacturers best efforts to show how to use their product successfully so they are quite useful. AND they give a better explanation than I do.
 

Ian0

Joined Aug 7, 2020
3,754
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?
Where did you connect the other terminal of the load? Where did you connect the Positive supply from the output of the rectifier? (A circuit diagram would help)
 
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