Two switch forward based on UC3845

Silvio

Member
Its not a question of re-doing the pcb but getting a good Strong waveform. It is clearly seen that the GDT is absorbing a lot of power and exhausting the available power from the drive. The waveform clearly shows this and charging the gates is taking forever, hence the slow rise and fall times. Your best option I guess is to make another totem pole after the drive to add some current to the primary of the GDT. If you like to try something else well considering the GDT primary inductance is only 500uH then you can put a 10 to 20 ohm resistor in series with the primary to limit current. (remove everything else and feed from the drive directly) This may cure the problem. Do not forget that with a 2 switch converter you are driving 2 gates simultaneously. Experimenting with the resistor value may get you a good balance to get enough current and not loading the driver chip more than necessary.
 
Do not forget that with a 2 switch converter you are driving 2 gates simultaneously
Well, it makes sense since peak currents are basically double that of a half bridge where transistors are driven one at a time. But I was thinking, a BC546/BC556 totem pole can only sink 100mA continuously, and a UC3845 can do 500mA. Even a TL494 can do 200mA. So how is it helping?
 

Silvio

Member
Well, it makes sense since peak currents are basically double that of a half bridge where transistors are driven one at a time. But I was thinking, a BC546/BC556 totem pole can only sink 100mA continuously, and a UC3845 can do 500mA. Even a TL494 can do 200mA. So how is it helping?
Well it all seems that the primary inductance is low for this purpose and loading the chip. I myself made GDT and lower inductances than 1mH and using 15v did not gave me good results. I also learnt that symmetry is also important in GDT and you get best results using toroidial cores. If you try a higher inductance I guess you will get better results. All I know is that when loading the driver the waveform gets deformed. The series resistor I mentioned I seen it used in a pro amp having a GDT of 470uH. This was driving a couple of IGBTs. To tell you the truth I monitored the output waveform from this GDT and the wave was not that square either. Regarding the totem pole well in that case use something more beefier like BD139/140.
 
Forgot to mention, this is the GDT

 

Silvio

Member
You had an EE16 in your schematic and that was what I was imagining you had as a GDT. The ring core is a good choice. The one I have also has more or less the same inductance. well you can drive that directly without any series capacitor. Just put a 1k load resistor in its outputs and check waveform before hooking up any mosfets. If you are happy with the result just put a series resistor to the gates and all should be well.
 
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