Switch 40VDC @3A up to 20kHz

JonnyMac

I have a possible Propeller-based project that needs to switch 40VDC at 3A, possibly as fast as 20kHz. This goes beyond the transistor circuits I've typically employed in the past and would welcome your suggestions. Ideally, I could connect directly from the Propeller to said device (transistor, FET, etc.) with nothing more than a current limiter. The external load is not inductive.

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Jon McPhalen
Hollywood, CA

Phil Pilgrim (PhiPi)

For the best performance you will probably want to use a MOSFET driver. These are designed to switch capacitive loads (e.g. a MOSFET gate) at high speed and to provide the necessary gate voltage levels for maximum conduction. The Micrel MIC4469 is an example of such a chip.

-Phil

Christof Eb.

Hi, Jon,
at the moment I don't know if there is a FET for 3V Gate voltage.
So I would use BD675A/677A/679A/681 darlington.
Best regards, Christof

Phil Pilgrim (PhiPi)

Darlingtons don't switch very fast. Also, the IRF3708 used by many Propellerheads is an nMOSFET characterized for gate voltages as low as 2.8V, but it will not handle 40V. With a MOSFET driver you have a much wider selection of transistors to choose from.

-Phil

Rayman

I think I'd go darlington too. It's simple and should be fast enough for 20 kHz...

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Christof Eb.

Hm, Phil, you are right not too fast,
http://www.datasheetcatalog.org/datasheet2/b/0fyqcqeis4u8k4pt38r7tr9pazcy.pdf states only 60kHz as limit - perhaps still enough for a simple solution?
Christof

Phil Pilgrim (PhiPi)

If he's PWMing at 20KHz with a resolution of, say, 8 bits, the transistor will have to be able to switch in less than 100ns to cover the entire duty-cycle range: 0.5 / (20000 * 256) .

-Phil

Bobb Fwed

JonnyMac said...
I have a possible Propeller-based project that needs to switch 40VDC at 3A, possibly as fast as 20kHz. This goes beyond the transistor circuits I've typically employed in the past and would welcome your suggestions. Ideally, I could connect directly from the Propeller to said device (transistor, FET, etc.) with nothing more than a current limiter. The external load is not inductive.

Are you just looking to control current? If so, what the others have suggested will work.
I have had many applications where I have to control the high-side of the voltage (i.e. when the control is off, the circuit's net voltage is Vss, not Vdd).

Attached is a couple simple solutions for this. The first one is very low part count (1 SOIC8 and 1 resistor), but the second will give you much higher frequency and higher current limits. Both can handle 40V@3A if you get FETs that have that kind of gate limit.

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