Chopper Transformer Problem (High Freq. Inverter)

Thread Starter

Elhassan

Joined Jul 17, 2022
7
Hi, Currently I am Designing a High Frequency Inverter, I do have a Components from some Commercial Inverters as a Reference for me while designing so that I can follow or Reverse engineer that Inverter.
as you surely know, on high frequency inverter, at first I step up the voltage from 12VDC or 24VDC to 311VDC then I do the SPWM Switching to generate the pure sine wave, I have 2 Chopper Transformers, the first from12VDC inverter, and the second from 24VDC inverter(input voltages).
I am using SG3525 IC to generate the PWM for the MOSFETs, soft start and dead time is taken in Consideration with the MOSFET that I'm using, the Generated pulses is measured correctly using my oscilloscope, and I can Change the Frequency using a Potentiometer, so that I can Test the Circuit in different frequencies, The Circuit is Quite Simple I guess, Something like that (the SG3525 Circuit doesn't matter because I checked it with the Oscilloscope, and it's working fine)
1664136125283.png
I event Connected MOSFET Driver to Double Check that there is nothing Affecting the SG3525.
My power source is 12VDC 10A Power Supply, acting as a Input Battery
When I tried to use the Transformer taken from 24V Inverter, Which is Supposed to Convert 24V to 311V on about 30Khz I guess, Every thing is Working Fine, Fine I mean that the MOSFETs not heating when there is no load
but when I connect the Transformer taken from the 12V Inverter on the same inverter, the same MOSFETs are Heating so Bad! and Drawing Very High Current, I Guessed that the Transformer is Saturated, so I tried to increase the frequency, but same problem occurred, I tried different 12v input chopper transformer taken from another 12v inverter, and same result happened.
I don't have that much knowledge in chopper Transformers, and I don't even have the right tools to measure it (like the inductance of each coil), but I am Trying Hopefully to just get things right by try and error to check that the steps are roughly right, then I'll Buy the right tools to measure and calculate to improve the efficiency.
Notes:
- the Output Voltage from the Transformer is then Rectified and there is high Voltage 1000F Capacitor is then Charged, and the Voltage is Then Converted to pure sine wave using SPWM Pulses from Microcontroller, Everything is Working fine, I Checked it using Oscilloscope.
- My testing prototype design is very Close to the Commercial Product and similar to Designs I saw on internet.
- My whole design is Build on a new High Quality breadboard, the signals are not distorted or anything, everything is working fine, the problem is from the transformer or the frequency (IDK)
- SPWM Carriage Frequency is 20Khz (just in case)
- for Rectifying the Output from the Transformer, I use Fast Recovery Diodes Taken from another Inverter
so the Question is,
- What Might be the Problem in my case that causing the MOSFETs to Heat up so fast?
- If there is any Materials that I can stick to it to know more about chopper transformers?
Thanks in Advance for even reading my Problem
 

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LowQCab

Joined Nov 6, 2012
2,515
You probably don't have adequate Heat-Sinking,
and probably don't have the Current-Limiting-Resistor set to a safe level,
and probably don't have a large enough Transformer-Core,
and possibly don't have the "Dead-Time" set correctly, causing
a partial "Shoot-Through" situation with the FETs.

Switching-Transformers are highly-specialized for a particular application,
so, as a general hack, use the largest Transformer-Core that You have space and Money for,
that is rated for twice the Power that You expect it to transfer.

You can also "double-stack" identical Toroidal-Transformer-Cores for
additional Power-Handling-Capacity.
.
.
.
 

Ian0

Joined Aug 7, 2020
6,331
You took the transformer out of another product, but you don't know what frequency it was designed for, and you are using it at 20kHz "to be on the safe side".
20kHz is a very low frequency for an inverter: my guess is that the original would be running around 100kHz. If you run it at 20kHz when it was designed for 100kHz, it's going to saturate, and overheat the driving transistors.
 

Thread Starter

Elhassan

Joined Jul 17, 2022
7
Thank you all for the Reply, the Problem was solved!
The problem was that the Transformer is Switching so Fast Causing Ringing, so I Just Added a 100 ohm + 2.2nF Snubber to the Circuit as Shown
1665445156693.png
 
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