GDT loading Question

[zeus_threat]

Hi I am building the attached shcematic to learn more about half bridge SMPS. Currently it is powered by 30Vac. The problem i am getting is that the BD139 and BD140 are heating up a lot. If i remove the high side mosfet everything cools down. Any help is appreciated. The IC is a TL494. Thanks.

[metalsculptor]

I am not sure why the driver transistors are heating up but the gate drive circuitry does not appear correct. The gate needs to see 0 to +15V, in the circuit shown it will see +/- 25V because of the 1:2 turns ratio of the driver transformer. A sure way to kill mosfets.

[zeus_threat]

Hi thanks for replying I will recheck and post the measured voltages here and then rewind the GDT on 1:1 turns ratio and post the results here

[zeus_threat]

Hi i've adjusted the GDT turns ratio to 1:1. Now the high side mosfet is heating up and the BJT are still heating up. The high side mosfet gate resistor is heating up too.I measured the voltages across the GDT:

Primary +-10V square wave no ringing

Secondary +-10V square wave no ringing

Can anyone please help thanks

[zeus_threat]

I've checked the circuit the high side mosfet is dead. Am implementing the attached gate drive for both mosfets. I've sim'ed the circuit it looks ok. Any comments are welcome

[metalsculptor]

I assume the mosfets are failing with the gate shorted to the drain? If so the gate insulation was probably punched though by incorrect gate drive. A shorted gate would heat up the bjt's. The new gate drive circuitry looks better. no more negative voltage on the gate.

[zeus_threat]

You are right metalsculptor. The Mosfet had shorted gate and drain. I will replace the Mosfet and implement the circuit shown with a 1:1 GDT and report back. Thanks again for the reply

[zeus_threat]

I had an IRF540 handy which i've used and made the mods i mentionned. Everyting is working fine now i even applied 60Vac to the power section and everything remained cool. I'll get an IRF 840 and test from 30Vac up to 120Vac (2X 60 transformers in series) first before applying mains to the circuit.

[zeus_threat]

Placed an IRF840 in the circuit and went rom 30 to 210V isolated and finally 220V mains all went ok. No heating up at all. The EI-33 core is one i obtained from an ATX PSU and used it straight away. the unloaded voltage coming of the secondary is 200Volts on the transformer pins where normally 12V was expected. Would it be wise to load test the secondaries with a pair of 100W bulbs or would it better to implement rectification , common mode filter choke and Capacitor before testing?????

Thanks

[metalsculptor]

Good to see you got it working, If you are having no problems with it unloaded then using lamps to test it should be ok, Keep the leads to the lamps close together and short to reduce inductance and EMI.