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pulse transformer fet drive problem

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dom444

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I am testing this gate drive pulse transformer circuit but the top 2n2222 connected to positive rail runs hot the ground one runs cool should they not have used higher current transistors to drive s coil?.
and should there not be a 1k resistor on its base as well as the other if anyone can help?.

63841-fig1.jpg
 
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Input voltage swing too low? How are you driving the circuit?
 

just using signal generator at the moment 20khz square wave
 

Transformer selection is the problem, You should select transformer based on ET value, the transformer’s field would not store enough energy to ensure the proper turn-off of the device.

If the signal is AC then better to go with capacitive coupled driver circuit or direct coupled circuit
22255.jpg
 

the reason i am looking at this circuit is that the drive transistor circuit makes it flexible on the transformer you use.


**broken link removed**
 

The dual switch topology in post #1 isn't bad. If you drive it correctly, the transistors won't run hot.

Do you drive it with 0/12V square wave as required? Did you check the waveforms with an oscilloscope?
 

FvM said:
The dual switch topology in post #1 isn't bad. If you drive it correctly, the transistors won't run hot.

Do you drive it with 0/12V square wave as required? Did you check the waveform with an oscilloscope?
Click to expand...

thing is i have to drive it from 5v pwm chip so i might have to boost the voltage thats not a problem i have tested the circuit works fine gives a nice sharp fast turn off and rise time just the transistor issue thats all, and it doesn't get instantly hot takes a thew minutes but ground transistor is dead cold.
 

Well that answers your question then. If you only drive it with 5V then the top BJT will have a ~7.5V drop across it, while the lower one will be saturated. Your options are to either use a 12V drive, lower the supply on the BJTs to 5V (and maybe use a 1:2 transformer), or modify the circuit itself.
 
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mtwieg said:
Well that answers your question then. If you only drive it with 5V then the top BJT will have a ~7.5V drop across it, while the lower one will be saturated. Your options are to either use a 12V drive, lower the supply on the BJTs to 5V (and maybe use a 1:2 transformer), or modify the circuit itself.
Click to expand...

My signal generator has variable voltage output tried 12v drive voltage even did totem pole transistors with 12v substituted a larger transistor for the top one still gets hot the only thing have left to try is different transformer and adjust pulse width.
 

Ok I am getting somewhere now its the transformer only runs a bit warm now but have overshoot on the wave form needs a damping resistor where would that go?
 

dom444 said:
I am testing this gate drive pulse transformer circuit but the top 2n2222 connected to positive rail runs hot the ground one runs cool should they not have used higher current transistors to drive s coil?.
and should there not be a 1k resistor on its base as well as the other if anyone can help?.

View attachment 131870
Click to expand...
What does you GDT look like? parameters? DCR, LP, #turns , µe
It looks like you driving equivalent of 560Ω high 360Ω which is // ( Ciss +Rg) low.
If you made your own then a turns ratio direct from 50Ω transformer could be around 1:3 with 5Vpp drive. Compare bipolar and single sided with DC offset. It is the DC content of your drive that makes the transformer warm.

The high side NPN in your circuit has larger Vce and with the excitation current of your primary and larger Vce of the emitter follower is limited by the impedance of 2pi f L which is affected by saturation and operating frequency and inductance.

The reference design is lacking all these details, which are essential to get good results.
 
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SunnySkyguy said:
What does you GDT look like? parameters? DCR, LP, #turns , µe
It looks like you driving equivalent of 560Ω high 360Ω which is // ( Ciss +Rg) low.
If you made your own then a turns ratio direct from 50Ω transformer could be around 1:3 with 5Vpp drive. Compare bipolar and single sided with DC offset. It is the DC content of your drive that makes the transformer warm.

The high side NPN in your circuit has larger Vce and with the excitation current of your primary and larger Vce of the emitter follower is limited by the impedance of 2pi f L which is affected by saturation and operating frequency and inductance.

The reference design is lacking all these details, which are essential to get good results.
Click to expand...

Thank you SunnySkyguy its the high side transistor that was getting hot not the transformer but i made a new transformer with different ferrite ring higher inductance runs much better
just a bit warm, then i discovered the wave form has overshoot bad, so a bit of reading said i should place a damping resistor in series with transformer so i placed a 10 ohm for now and got a perfect square wave with just a bit of drop at the top and now the high side transistor runs cold.
but will play a bit more with this, and might post some oscilloscope images if i get a chance.
 

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