First DRSSTC 1.3b DRIVER

[Alex_1996]

Hello! This question may be too quick to answer and at the moment it is the only one I have. Maybe in a few days I will have more questions, you know how it works 

I am building a small DRSSTC, and for this I am using a half bridge from FGH60N60SMD and a driver and switch from Aliexpress (I attach picture of them).

I have tested the system with a dummy load (a metal can) and the system oscillates when I test it at 50VAC input. The can gets very hot. The system consumes at that voltage about 350W with the pulse width at maximum.

To cut to the chase, I was interested in measuring the voltage of the OCD resistor, which in this case is 10 ohms, and thus know how much current was flowing through the circuit using Ohm's Law. My TC's are 1:25:25, total 1:625. I limited the OCD comparator voltage to about 4.5V so that 300A max. When I tested the system at 50VAC and measured with my multimeter the voltage on the 10 ohms resistor of the OCD, the multimeter on DC voltage scale marked 0 volts and the voltage on the comparator also marked 0 volts, something very strange, because if the diodes at the output of the CT of the OCD rectify the AC current, my multimeter should throw some voltage value from the 10 ohms resistor of the ocd and it does not do it.

That has led me to think that this driver, which looks like a 2.7 (the manufacturer hardly gives any data), is in bad shape.

Should the OCD resistor of a DRSSTC driver give a DC voltage reading? And this could be measured with a multimeter / voltmeter on DC voltage scale?

I hope you can help me, so I can implement another driver or revise the one I have  :'(

Thanks a lot

[davekni]

Should the OCD resistor of a DRSSTC driver give a DC voltage reading? And this could be measured with a multimeter / voltmeter on DC voltage scale?

Meter is measuring average voltage.  OCD trips on peak current (converted to voltage across 10 ohm resistor).  For a full wave rectified sine wave, average is 2/PI times peak.  If interrupter duty cycle is 1%, meter should read 0.01 * 2/PI of peak voltage.

[Alex_1996]

Thank you very much for your quick response

It is a subject that I was curious about, so I really appreciate the explanation

Best regards 

[davekni]

Thank you very much for your quick response

You are welcome.

BTW, It is also possible that the driver board OCD circuitry is not just like UD2.7.  I've seen designs, at least one from EVR, that place the burden resistor directly across CT secondary, with bridge rectifier after burden resistor.  Minimum current limit is then 2 diode forward voltage drops across burden resistor.  And of course, voltage across burden resistor is all AC, no DC.

[Alex_1996]

Hello, you are right! It had passed me by that there are two types of OCD circuits

I will keep all this in mind for this DRSSTC design and for other future designs 

Thank you very much for your explanation

[Alex_1996]

Hello, I have not found another post similar to the question I am asking.

I am building my first DRSSTC. Right now I am using the 1.3b driver schematic as is, without making any modifications. The only modification, so to speak, is the use of UCC27425 instead of UCC27423 as originally used in this driver.

When I feed the driver I can't get it to work. The GDT does not hum at all. In testing, I found that it only works if I electrically connect pins 3 and 4 of the 74HC14N externally. If I do the latter, the GDT does emit a hum. I have tested the OCD with this mode of leaving pins 3 and 4 of the 74HC14N electrically connected externally and when I supply a voltage higher than the OCD limit voltage, the red LED lights up, so it seems to work.

I don't know what is happening. The integrateds are new and original. I don't understand what could be happening.

I have followed the 1.3b driver schematic as originally designed.

I hope you can help me. Attached is an image of the driver

[klugesmith]

Did you pay attention to logic polarity difference between UCC27423 and UCC27425? 

[Alex_1996]

Did you pay attention to logic polarity difference between UCC27423 and UCC27425?

Thank you for your quick response

As I saw the internal diagrams of both UCC's I didn't seem to see any difference between them.

Attached is an image of the diagrams of the UCC27423 and UCC27425.

If you can explain a little more I would appreciate it

[Alex_1996]

Did you pay attention to logic polarity difference between UCC27423 and UCC27425?

Thank you for your quick response

As I saw the internal diagrams of both UCC's I didn't seem to see any difference between them.

If you can explain a little more I would appreciate it

Okay. Checking as you told me, I have seen that in the UCC27425 pins 4 and 5 are connected by an IF gate, while in the UCC27423 they are connected by a NOT gate.

That is why when I connect electrically externally pins 3 and 4 of the 74HC14N the driver works completely, since I don't deny the signal coming in through pin 3 of the 74HC14N. Not being denied, it enters without problems through pin 4 of the UCC27425 and continues without being denied, leaving through pin 5 and going to one of the Mosfets.

I hope I understood it well