GDT - limitation

[MacIntoshCZ]

Hello,
I just drawn this schematic.
I would like to have 500khz usable. But i am little bit aware of storage time of npn/pnp transistors.
VCC 15V; PWM input max 5V
thanks

 

[ChrisP58]

Have you considered modeling your circuits in a spice simulator? That way you could see how all of the parts work together and make adjustments of component values without needing to build the circuit.

Many members here use LTspice. https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html

 

[MacIntoshCZ]

Have you considered modeling your circuits in a spice simulator? That way you could see how all of the parts work together and make adjustments of component values without needing to build the circuit.

Many members here use LTspice. https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html

Hello,
In past i used to work with multisim (High School) . IT was ok but sometimes IT drives me crazy. I Lost trust in simulations. Maybe sometime i Will try

 

[MacIntoshCZ]

I am having issue with low peak current. It should be around 3 amps. (PWM voltage is 12V peak). VCC is 15V. Input current to base is around 20mA, gate current is 200mA at peak. Are 2n3904 any limited by peak currents? 20mA * 70hfe at lest 1,4Amps

 

[rjenkinsgb]

1N4148 diodes are only rated at 100mA or so & the forward voltage is up to about 0.9V at 100mA; it will be a lot! higher when the peak current flows.

Try putting some Schottky or ultrafast rectifiers in those positions, something that has a low forward voltage at the peak gate current, when it matters.

Also, D1 & R3 appear redundant; those may be needed with transformer drive to prevent excess base current, but not with the emitter follower.

 

[MacIntoshCZ]

1N4148 diodes are only rated at 100mA or so & the forward voltage is up to about 0.9V at 100mA; it will be a lot! higher when the peak current flows.

Try putting some Schottky or ultrafast rectifiers in those positions, something that has a low forward voltage at the peak gate current, when it matters.

Also, D1 & R3 appear redundant; those may be needed with transformer drive to prevent excess base current, but not with the emitter follower.

Diodes arent Gering any hot. Average current Is OK.
About D1 And R3 i was thinking about his usage. Thanks

 

[rjenkinsgb]

The average current is irrelevant in this type of application, the only time it (and the voltage drops) matter is at the instant the FET gate capacitance is charging, each cycle.

They have a relatively high internal resistance, so limit the peak current.
Try swapping them in one driver and see how the response changes?

 

[MacIntoshCZ]

I made some measurements and i think problem is Q1. R3 is not connected. D1 is bypassed. R1 IS 10ohms instead 3r3.

So when peak gate current appear values are:
Ur1 = 336mA / Ud2 = 2.45V / Uq1ce ( from 15V to 10V in 5ns, settle at 3.6V in 100ns) / Ur2 = 2.8V

Calculated values.
Ibase = 18.6mA
IGate = 336mA
Rd2 aprox 7.3 ohms
Rq1ce = 31.45 ohms

 

[MacIntoshCZ]

Also tried bat42 instead 1n4148 probably bad choice.
I got also mur6060pt and mur880e. Not tried

 

[ChrisP58]

In your original post you said that the PWM voltage was 5V max. In post # 4 you said it is 12V. Which is correct?

I'm asking about the signal driving the base of Q1.

 

[MacIntoshCZ]

For this thread i stick with 12V as its emitor follower.