• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

GDT - limitation

MacIntoshCZ

Member
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
1606333674805.png
 

MacIntoshCZ

Member
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

Member
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

Well-Known Member
Most Helpful Member
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

Member
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

Well-Known Member
Most Helpful Member
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

Member
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
 

ChrisP58

Well-Known Member
Most Helpful Member
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.
 

Latest threads

EE World Online Articles

Loading
Top