Gate Drive
uzzors2k, Tue Sept 12 2006, 02:25PMI'm having some problems getting a nice drive signal. Without a load I get crisp PWMable squarewaves, but when running both mosfets the wave gets sloppy with incredible 5µs rise/fall times.
At 40 KHz...How can I drive the IRFP450s better? Also could it be a solution that doesn't involve driver chips?
This is the schematic. Sorry I don't have any scope pics, but my camera is charging.
Re: Gate Drive
HV Enthusiast, Tue Sept 12 2006, 03:14PM
A few things. Check to see if those transistors can provide the necessary current you need to swing the gate in the time you need.
Also, the 39 ohm series resistor is probably a bit high. I'd try something around 5 ohms.
Also, with the large number of turns you have, you may have a lot of leakage inductance which is hurting you, especially if the windings aren't wound trifillar.
HV Enthusiast, Tue Sept 12 2006, 03:14PM
A few things. Check to see if those transistors can provide the necessary current you need to swing the gate in the time you need.
Also, the 39 ohm series resistor is probably a bit high. I'd try something around 5 ohms.
Also, with the large number of turns you have, you may have a lot of leakage inductance which is hurting you, especially if the windings aren't wound trifillar.
Re: Gate Drive
Steve Conner, Tue Sept 12 2006, 04:08PM
Try using fewer windings on your GDT and make sure it's multifilar wound rather than just two piles of wire.
Steve Conner, Tue Sept 12 2006, 04:08PM
Try using fewer windings on your GDT and make sure it's multifilar wound rather than just two piles of wire.
Re: Gate Drive
Steve Ward, Tue Sept 12 2006, 06:34PM
Id drop that 100 ohm resistor on the primary to maybe 5-10 ohms, then lower the gate resistors to about the same 5 ohm range. Right now you just have far too much resistance in there.
Steve Ward, Tue Sept 12 2006, 06:34PM
Id drop that 100 ohm resistor on the primary to maybe 5-10 ohms, then lower the gate resistors to about the same 5 ohm range. Right now you just have far too much resistance in there.
Re: Gate Drive
Sulaiman, Tue Sept 12 2006, 06:51PM
I too would change the 100 Ohms for 5 to 10 Ohms,
I'd also reduce the 39 Ohm resistors to 0 to 10 Ohms.
With a turns ratio of 20:80 = 1:4
the 100 Ohm resistor 'seems' like 100 x (4^2) = 1.6 kOhm to the gates,
1.6 kOhm with TWO gate capacitances is what's giving you 5 us risetime.
10 Ohms instead of 100 would give about 0.5 us etc.
Sulaiman, Tue Sept 12 2006, 06:51PM
I too would change the 100 Ohms for 5 to 10 Ohms,
I'd also reduce the 39 Ohm resistors to 0 to 10 Ohms.
With a turns ratio of 20:80 = 1:4
the 100 Ohm resistor 'seems' like 100 x (4^2) = 1.6 kOhm to the gates,
1.6 kOhm with TWO gate capacitances is what's giving you 5 us risetime.
10 Ohms instead of 100 would give about 0.5 us etc.
Re: Gate Drive
Marko, Tue Sept 12 2006, 06:54PM
What are you using that design for?
For duty cycle different than 50% or modulation you will have trouble shutting the transformer off, and if you put too small resistor at input you will be stressing transistors and wasting a lot of power.
Anyway at 50%dc you should get somewhat better ON times. Drecrease resistors and number of turns as said.
With push-pull drive you need diodes across the transistors to conduct the inductive kick produced after each cycle to supply rail.
That's why I think such design is messy.
Seeing your schematic, your number of turns ratio is completely wrong. You have 1:4 GDT with 25V input, and you are forcing huge voltage (100V) on gate making zeners suffer and having very low current capability (you stepped the current down by factor of 4).
For mosfet drive you should use simple 1:1:1 GDT with some 10..15 turns (IIRC) and supply voltage of some 15volts or etc.
I would wind it out of CAT5 cable or shielded cable, use shield as primary and parallel remaining wires for minimal resistance, while also keeping extreme coupling.
Wire must be thick for that use!
For drive, would use a fullbridge of complemewntary transistors, or better SG3525 and it's output bridge stage.
I think it sould work for few nf of capacitance at 40kHz..
Marko, Tue Sept 12 2006, 06:54PM
What are you using that design for?
For duty cycle different than 50% or modulation you will have trouble shutting the transformer off, and if you put too small resistor at input you will be stressing transistors and wasting a lot of power.
Anyway at 50%dc you should get somewhat better ON times. Drecrease resistors and number of turns as said.
With push-pull drive you need diodes across the transistors to conduct the inductive kick produced after each cycle to supply rail.
That's why I think such design is messy.
Seeing your schematic, your number of turns ratio is completely wrong. You have 1:4 GDT with 25V input, and you are forcing huge voltage (100V) on gate making zeners suffer and having very low current capability (you stepped the current down by factor of 4).
For mosfet drive you should use simple 1:1:1 GDT with some 10..15 turns (IIRC) and supply voltage of some 15volts or etc.
I would wind it out of CAT5 cable or shielded cable, use shield as primary and parallel remaining wires for minimal resistance, while also keeping extreme coupling.
Wire must be thick for that use!
For drive, would use a fullbridge of complemewntary transistors, or better SG3525 and it's output bridge stage.
I think it sould work for few nf of capacitance at 40kHz..
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