question for people on EC

119

01:39:18

So assume I have a bjt with the load (3.3k resistor) connected series between pwr and the collector. And I want to use a mosfet with a pwm to switch current on/off to the base of the Bjt, would both these circuit operate the same irl? Both topologies work the same in the circuit sim, but in reality would one work better? Its important that the current thru the load is switch fully off for half the cycle and i was wondering if by using the topology on the left, the base-emittor junction being parallel to the drain-source of the mosfet would lock the voltage across the mosfet (when on) at 0.7v to 1v and result in a higher rds-on so that some small current still enters the base preventing the bjt from switching fully off. Any advice is greatly appreciated.

published 6 years ago

crake

6 years ago

Well, the one on the left is inverted. Not the same there, between the two. Also, the one on the left has about 18mA running through the bjt with no input signal, and pull down resistor. Thats quiet a bit of current for being off. The one on the right seems better but youll have to deal with Vto + Vbe which may suck for your application.

Bushmills

6 years ago

The one on the right has no good means to discharge B-E capacitance, keeping it longer then necessary in transition. During that time will transistor dissipate more power, therefore heat up. A resistor of around 47k between B and E may give a sharper switching characteristic.

crake

6 years ago

Put a 10k resistor on the base of the bjt on the right and it has the same current draw as the circuit on the left. You don't need the resistor on the drain of the circuit on the right.

crake

6 years ago

Why 60V DC?

crake

6 years ago

You making this thing active high, or low? Should have probably asked those questions up front lol.

Bushmills

6 years ago

See the effects of B-E resistor in http://everycircuit.com/circuit/5759064618041344. Dimensioning and frequency changed to show effect more clearly.

BIPfx

6 years ago

@crake thanks for the help i went with your advice and went with the circuit on the right with and made the changes you suggested and it worked for my purposes. Thanks

crake

6 years ago

Glad I could help. Just be sure, if havent already, check the datasheet for maximum collector-emitter voltage. For example 2n3904 is 40V max across collector-emitter.

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