Tl494 Gate drive circuit design

[MJC147]

Hello, i am trying to design a gate drive circuit with tl494, switching a gate drive transformer, which will switch two high voltage transistors on a half bridge power supply.

This is what i drew up then built, the switching transistors q1 and q2 are 2sc2655, with 33v 1n4752a diodes across them (hard to make out, i know)
At the moment i am trying to work out if its working properly. Its drawing about 0.2A of current at 24v from J1, which to me seems a little low for a gate drive transformer circuit…
This is the outputs of the gate drive transformer (secondaries) i measured.

Doesn't really look right, and im not sure what to think of the vpp.

Can anyone see anything wrong with my circuit , or that could be improved upon?

 

[rayma]

No load? Add capacitors that approximate the devices.

 

[MJC147]

Yes, at the moment just to test it, i dont have any load on the output.
Its a gate drive transformer, so surely it shouldnt have caps on the outputs? Unless im interpreting what you mean wrong

 

[rayma]

At the moment i am trying to work out if its working properly. Its drawing about 0.2A of current at 24v from J1, which to me seems a little low for a gate drive transformer circuit.

You need a load to exercise the circuit. Try adding a few nF to each secondary output.

 

[benb]

The gates of the MOSFETS you drive will look capacitive and it will be much more than just the gate capacitance due to the Miller effect.

 

[trobbins]

BJT's are being driven, not power fets.

 

[MJC147]

I will add, the transistors in the half bridge, are fjp13009

 

[rayma]

Here is a similar circuit using the TL494.

 

[MJC147]

Here is a similar circuit using the TL494.

Ah nice, thats just what i was looking for. They have some interesting stuff going on with the dead time control, i wander if mine will work alright at 50% dtc, as there voltage on dtc seems to float a little above 0v.

 

[samponek]

Transformátor vypíná tranzistory v samooscilačním obvodu s proudovou zpětnou vazbou.
Na transformátoru chybí vinutí pro proudovou zpětnou vazbu. Bez něj nemůže fungovat, výstupní průběhy jsou v pořádku.
Přidejte mrtvý čas, aby přes 13009 neprocházel žádný boční proud

 

[MJC147]

Samponek, i have implemented your reccomendation of increasing dead time to avoid shoot through 👍.
However, i am not sure if you are reffering to overcurrent protection or current controlled feedback.
This TL494 circuit will be used on an unregulated supply i am making, so overcurrent protection is not needed.

If you are reffering to overcurrent protection, surely measuring the voltage drop across a small 1 ohm resistor would be more effective than adding on a new winding to measure current. Could you expand on what you mean by adding a new winding for current feedback?

Or you are reffering to current controlled feedback, but this seems unnecassary especially with the tl494.

 

[samponek]

Podle potřeby přidejte napěťovou a proudovou zpětnovazební smyčku a ochranu proti zkratu. (transformátor je původní, bez převíjení)

 

[samponek]

Trimpot P2 = regulace proudu, Trimpot P3 = regulace napětí, Kondenzátor na pinu 4 = softstart, Odporový dělič připojený na pin 15 = ochrana proti zkratu. Výstupní usměrňovač lze zapojit jakkoli, symetricky nebo asymetricky. Mělo by to být vždy kolem 20V na odbočce pro TL494 pin 12 a kolem 5V na děliči zkratové a podpěťové ochrany pin 15 TL494.
Odporový dělič podpěťové ochrany je 2k2 / 470R a vede na pin 15 TL494 a rezistor 33k mezi piny 14 a 15 spouští zdroj při odstranění zkratu a zároveň nespouští zdroj při zkratu .
Výsledkem těchto opatření je, že zdroj nemůže být poškozen a po odstranění zkratu se sám spustí.