Questions about half bridge BJT driving

[ben7]

Some questions:

1. What is the purpose of the extra winding on the GDT? (Circled in red) Does it have something to do with the crossover? (I.E. making sure that both transistors aren't going to be turned on at the same time)
2. Are C7 and C8 (green) there to turn the transistors on and off faster? What would be the purpose of the diode and resistor in series with them? To discharge them?
3. What are R2 and R3 (orange) doing? They certainly don't allow enough current to flow to be turning the transistors fully on.

-Ben

[japlytic]

Re: Questions about half bridge BJT driving

1: The winding circled in red is used for current sensing.
2: C7 and C8 help speed up turn-off timing while D12 and D13 help speed up turn-on timing.
3: R2 and R3 are startup resistors, since this design has the PWM controller on the secondary side but not on the primary side.

[rievax_60]

Re: Questions about half bridge BJT driving

The winding circled in red is for current feed back into the driver transformer. It causes the Base drive current to Q1 and Q2 to increase with increasing load current. This keeps the transistors in saturation at high load currents and prevents the drive current from being unnecessarily high at low load. When BJTs are driven too hard, they become more difficult to turn off quickly.

[rievax_60]

Re: Questions about half bridge BJT driving

Q3 and Q4 supply very little drive power into the driver transformer. They only supply an initial turn on pulse to the output transistors, then the feedback current takes over.
During the dead time, Q3 and Q4 are both on, short circuiting the primary of the driver transformer, holding Q1 and Q2 off. Q3 and Q4 alternately turn off to cause Q1 or Q2 to turn on.

[ben7]

Re: Questions about half bridge BJT driving

Thanks for the explanations!

-Ben