Discrete FET driver turns off slowly

[eem2am]

Hi,

I have come up with two cheap FET gate driver circuits for driving the gate of a power mosfet in a switch mode solenoid driver.

The PWM signal is provided by the output pin of a microcontroller.
The solenoid current is 200mA.
The FET Vgs(th) should be between 0.8V and 3.5V


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Complimentary pair gate driver schematic:-
http://i56.tinypic.com/2zh136o.jpg

...the above "Complimentary pair gate driver schematic" gives a slow turn off of the FET since there is a lingering Vce(sat) voltage of the PNP BJT.

...here is the FET gate voltage showing the slowly declining FET gate voltage at turn off..........................

FET gate voltage with complimentary pair drive:-
http://i52.tinypic.com/xn5h8z.jpg

.....in order to make the FET turn off quicker, the following adjustments were made.................


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Adjusted Complimentary pair gate driver:-
http://i55.tinypic.com/v7t5sj.jpg

having the diode D2 in the Adjusted Complimentary pair gate driver gives a faster turn off of the FET.

..................the diode D2 in the "Adjusted Complimentary pair gate driver" also ensures that there is no shoot-through current by 'slamming' off the PNP BJT when the NPN conducts.

I assume that since the driver devices are BJT's, they will offer more resilience to transients coupled through the Drain-Gate capacitance than circuits which directly drive the FET from the microcontroller port.?

(its my belief that its FET gates that are the most susceptible thing to damage by transients and ESD....BJT's are more robust?)


Does any reader know of a technique of adjusting the above schematics so as to make the FET gate voltage go to zero quicker when turn-off occurs?

 

[duke37]

I am not too sure what turn off speed you require, a solenoid is not a rapid device, but the circuit you show does not pull the FET gate to ground. It only pulls down to about 0.6V. What is the voltage output of the microcontroller? If it is 5V then it can drive CMOS which can pull up to 5V and down to ground.
Alternatively, use an NPN transistor (grounded emitter) to drive the base of a PNP transistor with emitter connected to 5V (limit the current to about 1mA) and connect the collector to the FET. A pull down resistor of 1k will give a rapid turn off.
You could try a 1k pull down resistor, gate to ground, with your existing circuit

 

[Laplace]

I can't see the need for a gate driver circuit, especially if the microcontroller interface has a CMOS driver where the two states will be Vgs=0V and Vgs=5V. Why not a direct connection to the gate?

Addendum: Apologies. When I first looked at the picture of the PWM signal, I saw no signal. It just looked like a totally black screen. On closer examination there is a 20 kHz pulse wave where the Vgs signal tail going to zero approximates an RC discharge with an RC time constant of about 5 microseconds or thereabouts. So with a gate input capacitance of 650 pF and R4=10K, the RC time constant is (650 E-12 x 10 E+3) 6.5 microseconds. I would try making R4 a lower value, not lower than 1K. But realize that it doesn't matter what the gate voltage looks like, it only matters what the drain voltage looks like.