I have a circuit where MOSFET drives signal pulse transformer, and I want to make variable rising time from (100 to 300 ns). How can I make it?
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1\$\begingroup\$ Do you have the part number/spec of the transformer & FET?.. What is the application? Often, solution is found somewhere else. \$\endgroup\$– jayCommented Nov 20, 2021 at 2:59
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1\$\begingroup\$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. \$\endgroup\$– Community BotCommented Nov 20, 2021 at 6:49
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\$\begingroup\$ I added the picture of circuit. What should I add to the circuit to be able to adjust rising time of signal. (probably use current source or something like that). \$\endgroup\$– DanielCommented Nov 20, 2021 at 15:02
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\$\begingroup\$ @Daniel See the next example with a simple current generator. Just change R40 value. \$\endgroup\$– Antonio51Commented Nov 21, 2021 at 19:36
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2 Answers
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Without pulse specifications, it is relatively "complicated" to answer the question.
The following "theoretical" circuit (almost similar to that of the OP), has been "evaluated" with the addition of setting of a current source.
It seems to suit the principle requested. The problem, for now, (only an example of testing) is that the adjustable rise time is much longer than 300ns (may be changed by C2), and a problem may arise with the transformer (coupling: here, ideal).
NB: some diodes are perhaps not needed. To be verified.
I have added a "real" simple current generator (Q3) and here is the resulting rise time of ~ 200 ns.
And here, the powers requirements for that case.
(all specifications must obviously be met ... Ib(Qx), Ic(Qx), pow(Qx), en zo voort ...).
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\$\begingroup\$ There isn't any base or emitter resistance in your circuit diagram which would make current extremely big and possibly fry the BJT. \$\endgroup\$ Commented Nov 21, 2021 at 20:06
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\$\begingroup\$ Where? The examples are for "testing" only. If you need a real circuitry, it is up to you to dimension all elements for all specifications and powers ... \$\endgroup\$ Commented Nov 21, 2021 at 20:12
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\$\begingroup\$ My simulations indicates that ib max = 80 mA with a very short peak at 150 mA. Ic max = 265 mA with a very short peak at 1.6 A. Power Q3 peak 220mW, mean 70 mW. Well choosen, Q3 will not fry. \$\endgroup\$ Commented Nov 21, 2021 at 21:28
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\$\begingroup\$ Tried old BJT BD139 for fun. R37 can be 500 Ohm. Ok for all specs Ib max = 0.5 A, Ic max pulse = ~ 2 A, @Tcase=25°C Pmax = 12.5 W, @Tamb=25°C Pmax= 1.25W, pulse operation 10 us, here 20 us. \$\endgroup\$ Commented Nov 21, 2021 at 21:41
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What you are looking is probably slew rate limiter. Units are V/s and similar derivatives. You can't say 100-300ns of rise time, because it has to include also the magnitude of such rise.
TI app note: slew rate limiter
But it could be also made simpler with RC elements if the goal is not a perfect ramp. Keep in mind that you need a very fast opamp, the TI note may not use the most suitable as it produce approx. 20V/s. But the terminology is the correct one, I think.
EDIT:
Perhaps the simplest solution would be a N-MOSFET instead of BJT and variable gate resistor.
simulate this circuit – Schematic created using CircuitLab
answered Nov 20, 2021 at 16:43