Bi-directional High-side switch (Loonking for part!) - Page 1
Forum section: electronics / projects

author: suku
author_rank: Contributor
timestamp: December 19, 2017, 08:09:33 pm
content: I'm looking for a bi-directional switch IC preferably in a small yet easy-to-solder form factor (SOIC or SOT) that does bi-directional switching. Something like the Si3831DV looks good at first glance but I can't seem to find it (or any similar part) on the major suppliers.Thanks for the helpcheers!

author: slugrustle
author_rank: Frequent Contributor
timestamp: December 20, 2017, 03:28:00 am
content: I know this isn't what you're looking for, but it is common to use two Pmos transistors back-to-back so that their body diodes face opposite directions for bidirectional high side switching.You could use something like the Diodes Inc. DMP3085LSD-13, which is just two bare P-channel MOSFETs in one SOIC-8 package. You will still need to wire them together appropriately and drive their gates. Gate drive is usually done with a pullup resistor to the high voltage input and some kind of pulldown, whether an Nmos transistor with a zener to protect the Pmos gate or a constant current sink.

author: suku
author_rank: Contributor
timestamp: December 20, 2017, 12:22:42 pm
content: I put together this test circuit in the simulator to check the on-resistance of the switch.(5V-4.28V) / 2.14A = 0.336 OhmDoes the calculation and the circuit look ok?Is there a reason I couldn't drive the gates of the transistors with an IOPIN of a microcontroller?

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 20, 2017, 02:12:18 pm
content: First question that comes to mind would be, are you sure you need bidirectional switching? If so, why?Looks like there's not a lot of bidirectional power switches available these days, and they are pretty much all in a WCSP package or such.Si3831DV is actually obsolete, that's why you can't find it. (Funny thing is, it's not directly stated on the product page:http://www.vishay.com/ppg?70785, you have to search for this part on their web site and it's stated in the results list:http://www.vishay.com/search?query=Si3831DV&searchChoice=partThe PMOS back-to-back arrangement is an option but you have to be careful about gate driving. Pulling both gates to ground will make both transistors conduct. That's your ON state. But blocking both transistors in all cases is a bit trickier, given than the whole point of a bidirectional switch is that the voltage VB on the "B" side could be greater than the voltage VA on the "A" side. You have to pull up the gates, but to what voltage? What happens if VB is greater than (VA+Vgs)? Just a thought off the top of my head. On the other hand, if VA is guaranteed to always be higher than VB, I don't really see the point of bidirectional?

author: suku
author_rank: Contributor
timestamp: December 20, 2017, 02:33:44 pm
content: In the final product, multiple similar devices will connect to each other, connecting one device to USB will power the whole array. it's not per-determined which of the devices "has" to connect to USB, the user will decided however it's convenient for them...I guess I'm gonna play around a bit more in the simulator to figure out the blocking...

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 20, 2017, 02:53:04 pm
content: Alright.You may find it easier in the end to use a controller like the LTC4416 to drive your PMOS transistors. If you read its datasheet, you'll understand that handling properly all cases is not so trivial. There are probably many other ICs like this, look up "powerpath" controller ICs.

author: suku
author_rank: Contributor
timestamp: December 20, 2017, 04:32:14 pm
content: That is very helpful, thank you, I'll look into that!

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: December 20, 2017, 06:54:56 pm
content: You can take this design as a start

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 20, 2017, 06:59:30 pm
content: Quote from: SiliconWizard on December 20, 2017, 02:12:18 pmYou connect the fets source to source, then pull both gates up to the common source with an Nch fet to drop the gate down when needed.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 20, 2017, 07:00:32 pm
content: Quote from: Leo Bodnar on December 20, 2017, 06:54:56 pmYes that would work, another option that I've used before is the HT0740 driver (wired in exactly the same way), as long as you don't need fast switching.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 20, 2017, 08:10:32 pm
content: Quote from: suicidaleggroll on December 20, 2017, 06:59:30 pmYou mean like so, I suppose (see attached schematics).And that's the body diodes of the PMOS transistors (the diode that actually prevents a single transistor from being used as a bidirectional switch) that actually keep the common source node at the higher of the two voltages (minus the Vf of the diode) even when both transistors are turned off. Hadn't thought of them! They are often more of an annoyance than anything elseIt should work indeed. May not be completely fool-proof but pretty simple. It has a minimum working voltage of approximately 0.8V (will depend on the transistors).Just a thought, but from the short description of your product, wouldn't you benefit more from some kind of power OR'ing scheme rather than bidirectional switches?

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2017, 12:29:08 am
content: Maxim sells them but good luck finding them on their obtuse web site.

author: slugrustle
author_rank: Frequent Contributor
timestamp: December 23, 2017, 09:23:11 pm
content: The SiP32431 has reverse blocking capability. See especially the block diagram on page 6 of the datasheet, the paragraph "Protection Against Reverse Voltage Condition" on page 7, and the "Reverse Blocking Current" spec on page 3.Datasheet:http://www.vishay.com/docs/66597/sip32431.pdfDigi-Key:https://www.digikey.com/product-detail/en/vishay-siliconix/SIP32431DR3-T1GE3/SIP32431DR3-T1GE3CT-ND/I stumbled across this part looking for a nice load switch for a personal project and thought of you.