MOSFET Switch Configuration

It's easy, but we have to understand what you want. Do you have +26V and -14V sources and want to switch them to a load that will draw up to 1.5A, controlled by a logic level, etc.? How rapidly? What about short-circuit protection? Etc.?

Maybe you could use a solid-state relay. NEC has gadgets up to 5 amps. Try CEL.COM.

John

I'm feeling the need to write a few ASCII diagrams. Here's a simple bipolar switch that will connect either a positive supply or a negative supply voltage to an output, controlled by a 5V CMOS logic level. The circuit uses MOSFET switches, with gate voltages created by current-source level shifters to make the FET gate voltages independent of supply voltage.

Each gate has a diode and pulldown transistor. Combined with the MOSFET's high gate capacitance, this gives the switches a fast shut-off, slow turn-on property, which insures that both switches will never be on at the same time during switching, avoiding high supply-to-supply shoot-through current spikes. In the case of relay switches, that's called break before make.

. High-Current Bipolar Voltage Switch . +Vin . o-----------+-------+------- S D --------------, . | | _|___|_ | . +12 to 150V 4.7k | ____ | . | C G Q1 | . +---- B npn | P-type MOSFET | . | E -------+ | . | | | . +----|

John Larkin, i cannot use such relay, because its costly for our board design than mosfets. We will use hundreds of it : (about 512-to-1024 per board)xNumber of board of the design.

Winfield, The requirement is that the control that will switch off is a 3.3V logic, while the voltage input (VIN) of the mosfet pair would be -14V to +26 (+/- 1.5A max), this is much like an AC Switch.About the switching speed, that would be 50ns(20Mhz) full speed.

Regards, YY

Looks like he has a single Vin terminal varying between -14 to +26, to be chopped at up to 20MHz. Of course it has to be low component count, come in under a penny, and use no more than 10mm2 of board space...too typical.

Oh psshaaaw.

RL

OK, that's a 40V signal swing, and you need a Ron of no more than say 0.5 ohms? Is more OK? What about Roff (and C_off)? Is there an OFF feedthrough spec (looking at the 20MHz below)?

And you want low cost and simplicity. What're the source and load impedances like? That's important in order to understand what shortcuts can be taken. Perhaps you'd better just tell us what you're really trying to accomplish. Where does the 1.5A spec come from, what's its relevance? Is that a current spike? What about switch charge injection, do you have a limit there?

For example, depending on the exact configuration, it might be possible to use a single MOSFET. It could be an enhancement mode with 50-volt maximum gate voltage, or depletion mode like Supertex dn3525, controlled by a simple diode-resistor scheme.

OTOH, if your Ron spec isn't so low, e.g. 20 ohms, you could consider the Supertex 8 or 16-channel 200V CMOS switches.

Whew!

You mean the 40V input signal has up to 20MHz in it? Or do you mean you want to switch 1.5A in 50ns, and do so up to 20 million times a second (BTW, that's 25ns max per transition)? Either way that's a tough spec, considering the issue of high MOSFET capacitances. Not to mention inductance! This is a place to be careful and explicit about your requirements, and not overstate.

What in the world are you building?

John

The starter controller for this?

martin

In message , dated Sat, 2 Sep 2006, martin griffith writes

I bet no-one manages to put one of those into a Beetle!

true, did you see the worlds fastest (diesel) JCB video?

martin

Cool. I bet it's noisy.

John