N-Channel MOSFET as a high side switch (no PWM) - Page 1
Forum section: electronics / beginners

author: T_guttata
author_rank: Regular Contributor
timestamp: March 16, 2022, 07:28:31 pm
content: HiHow could you use a N-Channel MOSFET as a high side switch (just simple on/off, no PWM)? Gate voltage must be higher than Drain to turn the MOSFET on. Because VCC might be variable, I thought to use a zener diode to clamp the voltage between source and gate. I tried to figure out a schematic. Could it work? If no, why?Simplest solution would be to use a P-Channel MOSFET, but Rdson is significantly higher and there are much more N-Channel options around.Thanks for your help.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 16, 2022, 07:52:12 pm
content: I think you meant gate must be greater than source, not drain. Have you tried breadboarding it?Looks to me like it might work with the clamp on Vgs. R1 will need to be 2W or more to keep its temperature rise down. (I calculate about 1.25W dissipation with Q6 on.) You may want to decrease R8 so voltage at A can be lower to drive Q6. Not an expert, though.

author: Benta
author_rank: Super Contributor
timestamp: March 16, 2022, 08:21:54 pm
content: What switching speed (on/off time)?

author: T_guttata
author_rank: Regular Contributor
timestamp: March 16, 2022, 08:46:51 pm
content: Quote from: jpanhalt on March 16, 2022, 07:52:12 pmYes and no. Source and Drain are close if the MOSFET has low Rdson. Gate voltage must be higher than VCC. I have not yet tried breadbording it.Quote from: jpanhalt on March 16, 2022, 07:52:12 pmI have not calculated power dissipation in detail, I might need to change some resistors. I could lower R8, but it should work with 5V. I wanted to get a feedback on the concept first, before I do the values.Quote from: Benta on March 16, 2022, 08:21:54 pmFor simple on/off switching speed should not be critical. How can you estimate the switching speed? If I use resistors with lower values switching speed increases but also power disspiation, right?What switching speed (on/off time)?

author: Benta
author_rank: Super Contributor
timestamp: March 16, 2022, 09:00:32 pm
content: Quote from: T_guttata on March 16, 2022, 08:46:51 pmThat was not my question: ns, us, ms? Which range? If you feed us bad info, you get bad answers.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 16, 2022, 09:02:02 pm
content: Quote from: T_guttata on March 16, 2022, 08:46:51 pmThe specification is still Vgs.QuoteNot knowing the NPN transistor, I used a gain of 10. That's why with a low control voltage (e.g., 5V), I suggested you might want a smaller base resistor.

author: Marco
author_rank: Super Contributor
timestamp: March 16, 2022, 11:13:11 pm
content: Quote from: T_guttata on March 16, 2022, 07:28:31 pmGenerally with a charge pump and a level shifter, some high side MOSFET drivers have charge pumps build in, but a pin on a micro putting out a square wave works fine too.

author: Benta
author_rank: Super Contributor
timestamp: March 16, 2022, 11:31:58 pm
content: Can be done a lot simpler with photovoltaic couplers. That's why I asked about switching speed. But the OP apparently thought he/she needed to outsmart me instead of answering my question.These are the best around right now:https://www.vishay.com/solid-state-relays/list/product-84639/These are kind of industry standard:https://toshiba.semicon-storage.com/eu/semiconductor/product/isolators-solid-state-relays/detail.TLP591B.htmlBut then we're in the ms range of switching.

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 17, 2022, 12:23:17 am
content: As said. Photovoltaic driver (Not for fast switching), charge pump and such.If PWM was possible, between 1 and 99% duty cycle would enable you to use a simple bootstrap circuit.You can never run at 100% or the capacitor would eventually discharge.But if Rds is the only problem, just parallel 2 of them, it might be a cheaper solution than adding the driver.That's the good about them, just remember the gate capacitance doubles!

author: Terry Bites
author_rank: Super Contributor
timestamp: March 17, 2022, 05:06:26 pm
content: Its not uncommon to see a charge pump arrangement to obtain the higher Vgs eg MIC5019 or LTC7001.I'd just go for the PMOS and save myself the head-ache.

author: CMTan
author_rank: Contributor
timestamp: March 20, 2022, 04:53:57 pm
content: Typically you use a charge pump to raise the voltage above Vcc so that there is enough voltage to drive the gate to be above the source.If you are able to use a low side switch instead of a high side switch then using the NMOS as a low side switch will be easier.

author: T_guttata
author_rank: Regular Contributor
timestamp: March 20, 2022, 05:06:00 pm
content: Quote from: Benta on March 16, 2022, 09:00:32 pmCool down buddy, it was not my intention to refuse to give information.The time lag would not be critical, ms would be fine. But: if it takes ms for the main MOSFET to turn on completely, it might be a problem. There should be a sharp edge in the current graph for the main MOSFET.Quote from: Marco on March 16, 2022, 11:13:11 pmA charge pump does not work with 100% duty cycle? As I wrote in the title, I'm looking for a solution which works without PWM.Quote from: DavidAlfa on March 17, 2022, 12:23:17 amThe point is: if current matters, you will need much more than 2 P-channel MOSFETs in parallel to replace 1 N-channel. That's why I ask. You will find N-Channel MOSFETs which carry 5x the amount of current of the best P-channel MOSFETs.Quote from: CMTan on March 20, 2022, 04:53:57 pmIs there any solution for 100% duty cycle?

author: Zero999
author_rank: Super Contributor
timestamp: March 20, 2022, 06:18:06 pm
content: The charge pump circuit can't turn the MOSFET on with a 100% duty cycle.How about using a photovoltaic opto-coupler?https://toshiba.semicon-storage.com/info/docget.jsp?did=65308https://www.mouser.co.uk/datasheet/2/427/VISH_S_A0009803440_1-2570692.pdfhttps://www.mouser.co.uk/datasheet/2/408/TLP3905_datasheet_en_20200929-1916319.pdf

author: Benta
author_rank: Super Contributor
timestamp: March 20, 2022, 07:33:52 pm
content: Quote from: Zero999 on March 20, 2022, 06:18:06 pmSee reply #7.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 20, 2022, 08:10:56 pm
content: Actually you can make a voltage doubler based on capacitors that gives DC after filtering. The ICL7660 is a classic, low voltage IC (https://www.renesas.com/eu/en/www/doc/datasheet/icl7660s-a.pdf).It is also common practice for generating a negative bias for LCD screens with an MCU (attachment). Source:http://ww1.microchip.com/downloads/en/DeviceDoc/01146B.pdfI have used that design with every LCD display I have used. It was either built into the display or was done with the MCU. And of course a different class of voltage boosting are switching regulators.Frankly, I would like the see the OP explore his DIY version, as presumably the 50V DC is readily available to him.

author: Ian.M
author_rank: Super Contributor
timestamp: March 20, 2022, 08:34:43 pm
content: You can also get high side N-MOSFET gate drivers with internal charge pumps for the 'over the top' gate drive required. They can do 100% duty cycle and DC switching. The usual search term within high side gate drivers is "static switch". Here are some at Mouser:https://mou.sr/3wnpdes

author: T_guttata
author_rank: Regular Contributor
timestamp: March 21, 2022, 08:43:39 pm
content: Quote from: Zero999 on March 20, 2022, 06:18:06 pmWould a photovoltaic opto-coupler work with fast transients for VCC? How fast can it held the voltage difference between gate and source?Quote from: jpanhalt on March 20, 2022, 08:10:56 pmWell, those 50V would approximately be required if VCC shall be around 35V. If both voltage levels are available, great. But often this will not be the case.Quote from: Ian.M on March 20, 2022, 08:34:43 pmThanks, did not know that. Unfortunately theye are very expensive :-( For that price I can easily get several P-Channel MOSFETs and put them in parallel.

author: Zero999
author_rank: Super Contributor
timestamp: March 21, 2022, 09:30:48 pm
content: Quote from: T_guttata on March 21, 2022, 08:43:39 pmWhat do you mean?Transients on VCC are no problem for the opto-coupler.The main downside is it's slow, so no good for PWM or fast switching. The temperature rise due to the energy dissipated by the MOSFET during switching safe operating need to be considered. Use a MOSFET with as smaller gate charge as possible and a turn off speed up circuit, if it's not built in to the photo-coupler.

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 21, 2022, 09:45:21 pm
content: Maybe he means the switching time.Optocouplers usually work in the range of 10-100us, photovoltaic couplers are often slower as they take the energy from light itself, also heavily depends on the mosfet gate capacitance, the datasheets normally show very optimistic values (ex for 200pf, not usual, they're commonly around 1-2nF or more)These devices are for low frequencies or simple on/off switches, but there're fast options if you search a bit.

author: T_guttata
author_rank: Regular Contributor
timestamp: March 21, 2022, 10:00:15 pm
content: Let's say the main MOSFET is turned off first. Therefore, Source should be @ 0V?Now, what happens if the MOSFET shall be turned on by the photovoltaic coupler? Gate voltage must increase, but then as the MOSFET starts conducting Source voltage will rise because the MOSFET is on the high side?What will happen with the current trough the main MOSFET? Will it rise continuously until the gate is fully charged?

author: Zero999
author_rank: Super Contributor
timestamp: March 21, 2022, 10:21:21 pm
content: Quote from: DavidAlfa on March 21, 2022, 09:45:21 pmA photovoltaic coupler is a type of opto-coupler. It just has a string of photodiodes on the output, rather than a singe photodiode, or phototransistor., to hive a much higher voltage to turn on a MOSFET.Quote from: T_guttata on March 21, 2022, 10:00:15 pmWhat do you mean by main MOSFET? Post a schematic.Remember the photo-coupler can only switch the MOSFET slowly. Although it's possible to reduce the off time, with external components, it's much slower than a propper MOSFEET driver.QuoteYes. The photo-coupler acts like a low current power supply, connected between the gate and source.

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 22, 2022, 09:52:25 am
content: Quote from: Zero999 on March 21, 2022, 10:21:21 pmI know. Where did I ask? I just said they're slowerThe opto has secondary VCC, with the power always available, the rising/falling times depend mainly on the output drive transistor strength.But the photovoltaic has to scavenge the energy from the led, which generates just a couple of microamps, they're much slower than plain optocouplers.

author: CMTan
author_rank: Contributor
timestamp: March 22, 2022, 11:31:44 am
content: The key to the question is to be able to have a voltage a least 5 to 10V above Vcc.There are many solutions1. Use a charge pump (if this works for you. Some charge pump requires active clock which may not work for you)2. Use a capacitor switch voltage doubler (like ICL7660 mentioned earlier or any other voltage doubler. This uses an internal clock)3. Use another voltage source which is higher than Vcc.If this voltage can be created either by the 3 ways or any other ways, the problem is solved to use NMOS as a high side switch

author: Zero999
author_rank: Super Contributor
timestamp: March 22, 2022, 11:57:08 am
content: Quote from: DavidAlfa on March 22, 2022, 09:52:25 amYou're right. I'm correcting your use of terminology. A photovoltaic coupler is a type of optocoupler. Not all optocouplers have a transistor ouputAn opto-coupler or photo-coupler is just a name for a device consisting of a device which emits light optically coupled to a light detector. The emitter is nearly always an LED, but incandescent lamps have been used in the past. Examples of photo detectors are a: photodiode, phototransistor, photovoltaic cells (basically a string of photodiodes), an LDR, photo-TRIAC etc.

author: freda
author_rank: Regular Contributor
timestamp: March 22, 2022, 01:59:24 pm
content: opto-couplers are generally slow. My suggestion is use a isolated DC/DC converter to supply Vgs. see circuit fragment...." alt="" class="bbc_img" />Of course i would only recommend this circuit if the high side voltage is not too high, say 50V or less.

author: T_guttata
author_rank: Regular Contributor
timestamp: March 23, 2022, 08:38:54 pm
content: Quote from: CMTan on March 22, 2022, 11:31:44 amThere are many ways to create the required voltage, but obviously it should be at reasonnable complexity and cost.I could use a SMPS, but it will cost probably $5-10 and will require >20 parts...A photovoltaic coupler + N-Channel MOSFET (High Side) would look like attached, right?Which voltage difference is created by the photovoltaic coupler? Is that called "open circuit voltage" in the data sheet?

author: Zero999
author_rank: Super Contributor
timestamp: March 23, 2022, 08:47:03 pm
content: Quote from: T_guttata on March 23, 2022, 08:38:54 pmYes, it's the open circuit voltage, since the MOSFET's gate draws no current, when charged.Your circuit contains one error: a series resistor is required to limit the current through the LED.

author: Benta
author_rank: Super Contributor
timestamp: March 23, 2022, 11:13:25 pm
content: Use the VO1263 instead if you can source it.It's probably the best on the market right now, costs the same as the 1271 and is dual, meaning you can double the output current.Expect switching times in the 1...10 ms range, depending on your MOSFET.

author: CMTan
author_rank: Contributor
timestamp: March 24, 2022, 07:35:47 am
content: Yes, that would work too.

author: Zero999
author_rank: Super Contributor
timestamp: March 24, 2022, 08:38:12 am
content: Quote from: Benta on March 23, 2022, 11:13:25 pmThe VO1263 is twice the price of the VOM1271 on Mouser and Digi-Key.https://www.mouser.co.uk/ProductDetail/Vishay-Semiconductors/VOM1271T?qs=9vrc9xEJKyxSd3E39M%252BFqQ%3D%3Dhttps://www.mouser.co.uk/c/optoelectronics/optocouplers-photocouplers/photodiode-output-optocouplers/?series=VO1263https://www.digikey.co.uk/en/products/filter/optoisolators-transistor-photovoltaic-output/903?s=N4IgTCBcDaIGoHkCMYBsBmABCAugXyAhttps://www.digikey.co.uk/en/products/detail/vishay-semiconductor-opto-division/VOM1271T/3588700?s=N4IgTCBcDaIGoHkCyBGMB2FIC6BfIAIt's a decent enough suggestion. The LEDs can be connected in series, so no extra drive current is required. The downside is it lacks the built-in fast turn-off circuit of the VOM1271. Also beware that the open circuit voltage can go as high as 16.5V, which is too high for some logic level MOSFETs, although this is better for driving some non-logic level devices. A resistor can be put across the output to limit the voltage, if it's an issue. This will speed up the turn off time and might make an additional turn off circuit unnecessary.https://www.vishay.com/docs/84639/vo1263aa.pdfhttps://www.vishay.com/docs/83469/vom1271.pdf

author: T_guttata
author_rank: Regular Contributor
timestamp: March 24, 2022, 05:59:40 pm
content: Unfortunately, none of those parts is currently available.What's about ASSR-V622-302E,https://docs.broadcom.com/doc/AV02-0259EN?

author: Benta
author_rank: Super Contributor
timestamp: March 24, 2022, 07:53:28 pm
content: Quote from: T_guttata on March 24, 2022, 05:59:40 pmWould work, but you'll need a logic-level MOSFET.

author: Zero999
author_rank: Super Contributor
timestamp: March 24, 2022, 08:59:25 pm
content: The APV1121SX is available from Digi-Key. It will also require a logic level MOSFET.https://www.digikey.co.uk/en/products/detail/panasonic-electric-works/APV1121SX/2804730

author: T_guttata
author_rank: Regular Contributor
timestamp: March 24, 2022, 09:56:46 pm
content: Nice :-) There are nice logic level MOSFEts from Nexperia, for example: BUK9Y6R5-40HI might order some to test the switching behaviour.For which current do I have to set the limiting resistor at the input? In the datasheet it says: typical LED operate current 0.85mA, maximum 3mA. Absolute maximum rating LED forward current: 50mA.

author: CMTan
author_rank: Contributor
timestamp: March 25, 2022, 07:13:55 am
content: Hi T_guttata,I am wondering why to take so much trouble to use a photovoltaic coupler just to drive the NMOS high side switch.If you really need a NMOS part for the high side switch it will be much easier to use a smart high side switch that takes care of the problem we have discussed about.Please take a look at ITS4140N.This is Smart High-Side Power Switch that allows direct logic interface and also have several protection mechanism built-in. No need to worry about the gate voltage for the NMOS at all.

author: Zero999
author_rank: Super Contributor
timestamp: March 25, 2022, 09:08:04 am
content: Quote from: CMTan on March 25, 2022, 07:13:55 amThat's a decent suggestion. I doubt the ITS4140N is suitable. It has an on resistance of 1Ω, whereas the MOSFET has an RONof 6.5mΩ. A big problem is availability.Quote from: T_guttata on March 24, 2022, 09:56:46 pmWhich opto-coupler are you talking about?The higher the LED current, the higher the output current and the faster it will turn the MOSFET on. The output voltage is weakly affected by the LED current, whice has to increase exponentially, for the open circuit voltage to increase linearly.The turn off time is independent of the LED current. If there's a turn off circuit, it will be faster, that a basic pull-down resistor. I had a play in LTSpice. The opto-coupler is modelled as a behavioural current source, with the current set to 1/714 of the emitter current, mating 14µA out, with 10ma in given on the APV1121SX datasheet and there are a load of diodes in series to limit the open circuit voltage.Q1 and D1 form the fast turn-off circuit. If they're removed, the MOSFET takes more than one cycle to turn off.

author: T_guttata
author_rank: Regular Contributor
timestamp: March 25, 2022, 09:41:05 pm
content: Quote from: CMTan on March 25, 2022, 07:13:55 amThanks for the idea. Unfortunately it will not work.The main point is to use N-channel MOSFETs instead of P-Channel because Rdson is lower and they can candle much higher current. So if you choose a N-channel with 5 mOhm Rdson because 25 mOhm for the P-Type counter part is too high, the Rdson of 1 Ohm (ITS4140N) is obviously not an option.Quote from: Zero999 on March 25, 2022, 09:08:04 amAPV2121SWhat does it mean "takes more than one cycle"?

author: freda
author_rank: Regular Contributor
timestamp: March 26, 2022, 01:03:01 am
content: Hi T_guttata,did you miss my prior post ?with it you can choose whatever low RdsON MOSFET you can afford (and get), not worry whether its a logic threshold or not, orbe too concerned with Gate charge, the dc/dc converters small enough to put on your PCB, can drive many mosfets.Whats more you can switch it as slow as you want as its not a boostrap circuit.No exotic parts required.The only nuance if your switching a hi voltage hi side, then the isolation potential of the dc/dc needs considerationand the direct coupled pull down should instead be a opto-coupled switch.I just rechecked your OP, and my circuit should be fine to re-configure to yours.Please..pleaseQuoteOh look, No Stock!!Maybe those posters work for Vishay or Infineon Marketing department?

author: T_guttata
author_rank: Regular Contributor
timestamp: March 26, 2022, 08:42:58 am
content: @ fredaI did not really understand your circuit. You would suggest to use the B0509S-1WR3 to create a voltage difference of 9V to switch the N-Channel MOSFET on the high side? What else would I need?

author: CMTan
author_rank: Contributor
timestamp: March 26, 2022, 10:04:35 am
content: Yes, it would work too. Like what I said earlier, you need to create a voltage at leave 5 to 10V higher than Vcc.Any isolated DC-DC will work.You just need to connect the output reference ground of DC-DC to Vcc. Since it is isolated, this connection is not a problem as long as the isolation (Vin to Vout) is < 1500V (for the one mentioned).With this higher voltage you can drive the gate and use whatever NMOS of your choice.

author: T_guttata
author_rank: Regular Contributor
timestamp: March 26, 2022, 11:01:42 am
content: So basically this B0509S-1WR3 is just a "one component solution" for a SMPS circuit, right?

author: Zero999
author_rank: Super Contributor
timestamp: March 27, 2022, 07:54:44 am
content: Quote from: freda on March 22, 2022, 01:59:24 pm