MOSFET as a high side switch? - Page 1
Forum section: electronics / projects

author: kenw232
author_rank: Contributor
timestamp: April 01, 2014, 09:12:12 pm
content: I am trying to use an nchannel mosfet to light a higher voltage lamp. I set it up like this:This works, I guess this is a low side switch because the load sees the 120V constantly even when off. I would now like to convert this to a high side switch. So when the mosfet is "off" there is no 120V seen by the lamp/load until it's switched on again. I put the load after the source but it just shorts out or otherwise does not work. Something like this I believe.My mosfet is this IXTX20N150:http://ca.mouser.com/Search/ProductDetail.aspx?R=IXTX20N150virtualkey65120000virtualkey747-IXTX20N150Does anyone think they can help with this at all? Should I be using a P-Channel MOSFET instead? I don't know what to do.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: April 01, 2014, 09:42:30 pm
content: What does your "Drive Signal" look like? The way you have it drawn, if you were able to turn on the mosfet, the low side of your drive signal would be shorted to 120V, which means the high side of your drive signal would have to be even higher than that.Using an Nch mosfet for high side switching means you need to be able to drive the gate to an even higher voltage than what you're switching. If you don't have a 125+VDC signal available to turn on that mosfet, then you should switch to a Pch.With a Pch, you would pull the gate high with a resistor to turn it off, and your drive signal would pull the gate low to turn it on. If your drive can't handle a pin floating at 120v while the gate is off, then you could use a small Nch mosfet to isolate it. The Pch has the gate pulled high and also connected to the Nch drain. The Nch gate is pulled low and is also connected to the drive. If the drive is low, the Nch is off, the Pch gate gets pulled high, and is off. If the drive is high, the Nch is on, the Pch gate gets pulled low, and the Pch turns on.

author: gregallenwarner
author_rank: Regular Contributor
timestamp: April 01, 2014, 09:43:09 pm
content: You need a P-channel for a high-side switch.This is my go-to resource for using MOSFET's as power switches:http://www.onsemi.com/pub_link/Collateral/AND9093-D.PDF

author: kenw232
author_rank: Contributor
timestamp: April 01, 2014, 10:34:26 pm
content: My signal from my waveform generator switching the mosfet on and off looks like this (Channel 2 in blue). I guess I need a P-Channel Mosfet now.. thanks.

author: kenw232
author_rank: Contributor
timestamp: April 01, 2014, 10:39:43 pm
content: I was eventually hoping to try to switch 1300V with a p-channel mosfet, it appears they don't make anything that high for p-channel at all.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 01, 2014, 10:45:40 pm
content: You also need a high-side referenced drive voltage. Which, if one side of your supply is grounded, means you need a +1200/1205V drive signal. Have fun with that...As I recall from the other forum, you're working with rather high pulse currents too. Is this still the case? MOSFETs aren't a good way to do that, for a number of reasons.Tim

author: max666
author_rank: Frequent Contributor
timestamp: April 01, 2014, 10:48:57 pm
content: A P-channel type is certainly the way to go if you need a high side switch, but if you're hell-bent on using a N-channel MOSFET you have to have a floating drive signal that has no problems with being referenced to your 120VDC.Keeping in mind, that with a N-channel MOSFET on the high side, your drive signal circuit gets pulled to 120VDC, once you switch on the MOSFET.EDIT: Ohhh i see why you get the short now!!! Be careful what you are doing! I think you're shorting out because you are using the signal from a function gen (which is ground referenced!!!) to drive your MOSFET. This is very dangerous and could potentially kill your function gen!

author: dannyf
author_rank: Super Contributor
timestamp: April 01, 2014, 10:52:27 pm
content: QuoteYou can bootstrap the high side to get it to turn on a n-ch switch. That's how most high-side drivers do.

author: kenw232
author_rank: Contributor
timestamp: April 01, 2014, 11:11:41 pm
content: Quote from: max666 on April 01, 2014, 10:48:57 pmThe waveform gen's negative is connected with the negative of the load. I tried to do this but it didn't work either. Keeps shorting out to constant current mode.

author: max666
author_rank: Frequent Contributor
timestamp: April 02, 2014, 02:07:07 am
content: This configuration:has the significant disadvantage that your drive signal has to be >65V for on and <60V for off (very impractical, especially if you want to do it at 1205V/1200V). Furthermore if you switch off that MOSFET, that means R3 pulls the gate to ground, and your load is still retaining some charge (maybe some capacitance), then you could actually get a Gate-Source voltage way below the allowed -40V. Remember the gate of your MOSFET can only take +/-40V V_GSM (or +/-30V V_GSS continuous). So maybe check if your MOSFET is still working, especially since you mentioned you pulled some shorts on him already.This configuration:would be the one i would be using if i had to use a N-type, but as i said, you can't use a function generator that is ground referenced to drive the MOSFET, if your circuit is also ground referenced. However you can use this configuration, if you have a galvanically isolated ~5V signal. Or a galvanically isolated ~5V source that you then switch with an optocoupler to drive the gate of the MOSFET.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 02, 2014, 03:01:25 am
content: In the interests of background, full disclosure, and ethics;http://www.dutchforce.com/~eforum/index.php?showtopic=42250present threadhttp://www.dutchforce.com/~eforum/index.php?showtopic=42232high voltagehttp://www.dutchforce.com/~eforum/index.php?showtopic=41282dangerous circuitsTim

author: diyaudio
author_rank: Frequent Contributor
timestamp: April 02, 2014, 03:28:11 pm
content: Quote from: kenw232 on April 01, 2014, 09:12:12 pmdude, seriously we care about your life and then about your equipment.please used a isolated drive like a optocoupled drive.safety first.!

author: madires
author_rank: Super Contributor
timestamp: April 02, 2014, 03:57:32 pm
content: Quote from: kenw232 on April 01, 2014, 10:39:43 pmYou could use an IGBT instead (supports higher voltage).

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 02, 2014, 04:16:49 pm
content: You can "easily" use a N-channel MOSFET at the high side.Your only problem is that you probably (sorry, definitely) use an isolated power supply to drive the MOSFET gate.The "ground" or more precisely the virtual ground of this isolated supply has to be the source pin of your MOSFET.Then the only thing you need is an isolated driver, look at Analog Devices they have some if I remember correctly.Since you already provide with an isolated power supply, you only need to look for low side drivers with isolation of course.Something like this should do it:http://www.analog.com/en/interface-isolation/isolated-gate-drivers/adum3221/products/product.htmlJust use only one channel or use both connected together, it should work well (Don´t forget the resistor between gate driver and gate!!!! ).If not, you just need an isolator and then a gate driver. (But I think the one above fits pretty well)But what you want is by far totally possible, with the only hard part being the isolated power supply but there areready made modules for that so you shouldn´t have a problem :-)Best Regards, LefterisGreece

author: kenw232
author_rank: Contributor
timestamp: April 03, 2014, 01:08:10 am
content: Quote from: TrickyNekro on April 02, 2014, 04:16:49 pmThanks for the info, its getting a little beyond my expertise though. Would I still be able to control the isolated driver with a waveform generator? I like the idea of controlling the duty cycle of the HV power supply by simply adjusting the waveform generator's duty cycle.

author: kenw232
author_rank: Contributor
timestamp: April 03, 2014, 01:25:02 am
content: Quote from: madires on April 02, 2014, 03:57:32 pmRight, and this would be it I imagine. So the load (M), because its after the Emitter, will not see the 120V DC source voltage when in the off state and therefore the IGBT will provide isolation for the load when off correct? And I still only need a 6V or so signal from my waveform generator to control it...

author: DerekG
author_rank: Frequent Contributor
timestamp: April 03, 2014, 01:27:45 am
content: Quote from: kenw232 on April 01, 2014, 09:12:12 pmN-channel FETs are generally always less expensive than P-channel as the die area (for the same drain-source resistance) is typically half the size (or less). This is to do with the doping of the silicon during manufacture - set by rules of chemistry & physics.N-channel fets are generally always easier to drive (ADDED: for low side driving) than N-channel (ADDED: for high side driving).With N-channel FETs, you simply need to lift the gate above the source voltage (ADDED: for low side driving).For N-channel FETs (ADDED: when used as a high side drive) you will need a high-side-driver to further lift the voltage above the load voltage.Many high-side-drivers are available, but they do cost money & sometimes require further componentry around them.Moral of the story: N-channel FETs cost less & are generally easier to drive (ADDED: for low side drives).ADDED & edited to make my points somewhat clearer.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: April 03, 2014, 02:30:37 am
content: Quote from: DerekG on April 03, 2014, 01:27:45 amEasier for low side switching, sure, but they're certainly not easier for high side switching.Quote from: DerekG on April 03, 2014, 01:27:45 amThat's not a P-channel FET, that's an N-channel FET being used as a high side switch. A P-channel FET is turned on by driving the gate BELOW the source, not above. Since the source is generally connected to your power supply, that means you just bring the gate down to turn it on. Easy peasy.Quote from: DerekG on April 03, 2014, 01:27:45 amDepends on what you're trying to do with it.

author: max666
author_rank: Frequent Contributor
timestamp: April 03, 2014, 07:47:32 am
content: Quote from: T3sl4co1l on April 02, 2014, 03:01:25 amThis is hilarious!Kenw definitely likes to live dangerous.

author: TrickyNekro
author_rank: Frequent Contributor
timestamp: April 03, 2014, 11:14:34 am
content: Quote from: kenw232 on April 03, 2014, 01:08:10 amhmm... eehh... yes and no.... The best would be to use a schmitt trigger before the IC with a 0 - 5V output, not just a wave generator.Simply for protecting the isolator, in case that you accidentally feed another voltage at the input.Also, keep in mind that then you would only be able to feed a square wave in the gate (no matter the generator output the gate of the mosfet willget always a square wave with that config). But yeah, you can use with wave gen. only with some limited functionality.There are lineal isolators, so that you can feed whatever you want in the mosfet, but I think is way far from what you want and is even more complicated :-PSo you can stay with my suggestion when you want no problem.Btw, as all the others mentioned so far. Try to keep safe. Don´t blow yourself up. Anything more that 50V hurts! I speak with personal experience O0Best Regards, LefterisGreece

author: madires
author_rank: Super Contributor
timestamp: April 03, 2014, 11:35:30 am
content: Quote from: kenw232 on April 03, 2014, 01:25:02 amYes, but V_GE is the voltage between Gate and Emitter and has an upper limit of about 20V for most IGBTs. If you're going to use a n-ch IGBT for high side switching you'll need a proper driver. Please search for "IGBT high side switching".QuoteThat's a n-channel depletion mode IGBT without freewheeling diode. It's self-conducting.

author: qno
author_rank: Frequent Contributor
timestamp: April 03, 2014, 12:07:34 pm
content: I think it is better to switch the input of the HV transformer.