Would this bidirectinal switch ciruit work? - Page 1
Forum section: electronics / projects

author: hkBattousai
author_rank: Regular Contributor
timestamp: February 05, 2015, 09:48:38 am
content: I'm trying to design a general purpose bidirectional switch with MOSFET transistors.I have designed the following circuit.In my design, two MOSFETs are placed back-to-back. It is assumed that two isolated DC supplies are available. The two MOSFETs are simultaneously turned on and off by the same digital control signal. When the MOSFETs are on, the ports INOUT1 and INOUT2 will be short circuited in both direction. When the MOSFETs are off, the MOSFETs will block any current through the aforesaid ports.Would this circuit work? Is it necessary to isolate the digital pulse on both sides too? Please make some suggestions for me and correct my mistakes.

author: Paul Price
author_rank: Super Contributor
timestamp: February 05, 2015, 12:24:26 pm
content: is bidirectional: NoWill work: No

author: dannyf
author_rank: Super Contributor
timestamp: February 05, 2015, 12:31:50 pm
content: QuoteWhy do you think it will work?

author: penfold
author_rank: Frequent Contributor
timestamp: February 05, 2015, 12:52:25 pm
content: As long as that ground is floating, of course it should work. I personally wouldn't rely on the anti-parallel diodes built in the MOSFET and would add something with a low forward drop and fast recovery externally.

author: voja
author_rank: Contributor
timestamp: February 05, 2015, 01:07:44 pm
content: DC1 and DC2 are shorted, I guess? There's no need to isolate them. Same is with digital input signals.Check if MOSFETs already have Zener protection at input and add them externally if they don't.

author: mij59
author_rank: Frequent Contributor
timestamp: February 05, 2015, 01:14:13 pm
content: You need only one mosfet driver.In the on state the mosfet will conduct in both directions of current flow, no need for extra diodes.

author: hkBattousai
author_rank: Regular Contributor
timestamp: February 05, 2015, 01:16:54 pm
content: Quote from: penfold on February 05, 2015, 12:52:25 pmI get it. The ground must be only for this circuit block and must be isolated from the main circuit ground. It also means that the driving digital signal must be isolated by an optocoupler or something like that, isn't it?Quote from: penfold on February 05, 2015, 12:52:25 pmI didn't understand this part of your answer. When the switch is off, one of the MOSFETs and the other's body diode together will be blocking the current. It is not the body diode alone. Can you please explain your warning a little more.Quote from: mij59 on February 05, 2015, 01:14:13 pmBut if I use only one MOSFET, I can't only block the current in one direction. I want to be able to switch both AC and DC currents.Thank you for your answers. I will update my schematic soon and share it here.

author: penfold
author_rank: Frequent Contributor
timestamp: February 05, 2015, 07:52:11 pm
content: You're right there, you do need two mosfets, because it has two directional states,

author: hkBattousai
author_rank: Regular Contributor
timestamp: February 05, 2015, 09:22:46 pm
content: How about this one?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2015, 09:41:11 pm
content: This is all you need. Instead of a switch, you can use a logic source and single gate driver IC, and whatever value gate resistors you wish. The logic and supply must be isolated, that is all.Note that some snubber or protection circuitry may be desirable, especially with quick switching times and around inductive loads (e.g., AC line inductance). You could use a voltage peak, or dV/dt, type snubber per transistor, or An AC variation on either, connected from drain to drain; or just a boring R+C or MOV.Tim

author: tautech
author_rank: Super Contributor
timestamp: February 05, 2015, 10:00:00 pm
content: For what sort of "general purpose"?More than a *4066 can do?Current & voltage requirements?

author: hkBattousai
author_rank: Regular Contributor
timestamp: February 06, 2015, 07:55:59 am
content: Quote from: T3sl4co1l on February 05, 2015, 09:41:11 pmVery nice idea. I redesigned my circuit according to your suggestion. Is it OK now? Please tell me if still there is a problem in the circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2015, 02:50:58 pm
content: Yes, except you should still have independent Rg's because the transistors will switch at different voltages.Tim

author: MF-jockey
author_rank: Regular Contributor
timestamp: February 10, 2015, 09:47:27 pm
content: You should have a look to Panasonic's power PhotoMOS relays, it works itselfAQY211EH 30 V-AC/DC 1000 mA, ... AQY216EH 600 V-AC/DC 50 mAore bigger types likeAQZ204D 400 V-AC/DC 450 mA

author: lewis
author_rank: Frequent Contributor
timestamp: February 10, 2015, 10:02:43 pm
content: There are photovoltaic optocouplers available designed precisely for this purpose:https://www.google.co.uk/search?q=photovoltaic%20optocouplerHaving the MOSFET sources and system ground at the same potential significantly limits the usefulness of this circuit, isolated is the way to go. Especially if you want to switch AC.The circuit you'll need is on the last page of this PDF:http://www.irf.com/technical-info/designtp/dt94-5.pdf