do mosfets without a paralleled diode exist ? - Page 1
Forum section: electronics / general

author: Simon
author_rank: Unknown
timestamp: November 30, 2011, 12:43:13 pm
content: now from memory the reverse diode in a mosfet is part of the mosfet construction, but, are there mosfets with no parallel reversed diodes ?

author: Zero999
author_rank: Super Contributor
timestamp: November 30, 2011, 12:47:18 pm
content: The solution is to use two connected back-to-back but the on resistance will be double.

author: Simon
author_rank: Unknown
timestamp: November 30, 2011, 12:51:34 pm
content: and presumably I'd need to control the gates seperately ? hm could be done

author: slateraptor
author_rank: Frequent Contributor
timestamp: November 30, 2011, 01:59:50 pm
content: Quote from: Simon on November 30, 2011, 12:43:13 pmTo my surprise, 4-terminal FETs in discrete package are practically nonexistent.I guess those lectures on the body effect in e-circuits were unknowingly targeted towards future IC designers.Learn something practical everyday.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 30, 2011, 02:33:46 pm
content: Quote from: Simon on November 30, 2011, 12:51:34 pmNot necessarily - you can often connect the gates together, e.g. if you have a control voltage that will be more positive than either end.

author: Simon
author_rank: Unknown
timestamp: November 30, 2011, 02:37:37 pm
content: well you see the line connected to the source of the original mosfet will first be negative and then becom positive so not so simple, I think i'll need to look at other ways around this.

author: Zero999
author_rank: Super Contributor
timestamp: November 30, 2011, 03:35:15 pm
content: Quote from: Simon on November 30, 2011, 12:51:34 pmThe gates can be connected together.It's common for MOSFETS to be connected in this manner in a solid state relay. I showed an example in the post linked below:https://www.eevblog.com/forum/index.php?topic=5470.msg71441#msg71441

author: Simon
author_rank: Unknown
timestamp: November 30, 2011, 03:43:05 pm
content: hm I thgink the problem here is that I have a very specific situation, possibly needs solving with some built in logic or a standard relay

author: Zero999
author_rank: Super Contributor
timestamp: November 30, 2011, 04:32:17 pm
content: Quote from: Simon on November 30, 2011, 03:43:05 pmNot quite.Look at the schematic again.Hint: a MOSFET can still turn on and bypass the body diode if the gate voltage is positive, with respect to the drain, which will act as the source i.e. the MOSFET will work in reverse.

author: amspire
author_rank: Super Contributor
timestamp: November 30, 2011, 11:06:04 pm
content: Quote from: Simon on November 30, 2011, 03:43:05 pmCan you give us some details? How much reverse voltage do you want to put across the mosfet? What current? What ON resistance do you need?Rereading your posts, I have no idea whether you are talking about power switching or small signal switching.If it is power switching, then you need back-to-back mosfets. If it is signal switching, then there are lots of examples of FETs and MOSFETs being used for switching bipolar signals. Junction FETs actually do a pretty good job, mosfets are fine as long as the AC amplitude is below the diode turn-on voltage, and there are 4 wire mosfets like the 3N173, but they seem rare nowadays.Check out multimeter manuals such as the HP 3455A. I seem to remember they went mad with using mosfets for signal switching. Not only that, they used diodes as a precision switch for the +/- 10V dc reference to the dual-ramp ADC, but probably only HP can manage to do that trick.When you are talking about the signal switching, a lot comes down to the circuit characteristics each side of the switch. Is one side low impedance (ie not affected by any currents imposed by the switch)? Is one side ground?Richard