Preferable way to generate higher potential to switch NFETs on the high side - Page 1
Forum section: electronics / beginners

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: September 17, 2023, 03:20:35 pm
content: Hello guys, I have a rather basic question but one which I haven't found an "industry" standard answer for.If I want to use an NFET on the high side, what would be the best way to generate a high voltage for the gate? Assume an MCU based system, and for now let's also assume there's going to be no switching action happening (aka just turning the load on and off, the higher voltage generation itself can have switching involved).EDIT: My apologies if I wasn't clear. Duty cycle is 100%, that is the FET is turned on all the time, no switching is involved.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 17, 2023, 03:48:26 pm
content: There's no real standard answer, because it depends on the requirements for switching speed, power consumption, current/voltage being switched, isolation requirements, FET RDS-ON specs, etc...That said, there's probably a highside driver IC for almost every situation.

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2023, 03:59:43 pm
content: What's the duty cycle?If it doesn't have to reach 100% duty, use a bootstrapped driver.https://www.ti.com/lit/an/slua887a/slua887a.pdf

author: Benta
author_rank: Super Contributor
timestamp: September 17, 2023, 04:24:19 pm
content: Photovoltaic optocouplers.I can recommend the Vishay VO1263.

author: tggzzz
author_rank: Super Contributor
timestamp: September 17, 2023, 04:28:32 pm
content: Several companies produce dedicated "high side driver" ics; they can be found via gurgle.If those aren't suitable, indicate why and we may be able to help.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: September 17, 2023, 04:40:30 pm
content: Quote from: tggzzz on September 17, 2023, 04:28:32 pmI guess I am looking at the problem from a more "hobbyist" point of view. Specialized ICs are abundant and do the job well but what if I can't get them for whatever reason.Jellybean charge pumps, DC-to-DC boost ICs, 555s I have them in stock. I have tons of M7 diodes so I wouldn't mind building an MCU GPIO controlled voltage doubler with many many stagesThink of it as a thought experiment perhaps?

author: Terry Bites
author_rank: Super Contributor
timestamp: September 17, 2023, 04:48:05 pm
content: Use a p-fet!There's no standard situation so there's no standard solution.Have a look at Infineon's wide selection for an idea of how diverse it can get:www.infineon.com/cms/en/product/power/gate-driver-ics/

author: MasterT
author_rank: Frequent Contributor
timestamp: September 17, 2023, 04:48:20 pm
content: Two options:

author: Zero999
author_rank: Super Contributor
timestamp: September 17, 2023, 04:58:36 pm
content: Refer to the thread linked below for a few discrete designs which use bootstrapping to switch a high side MOSFET.https://www.eevblog.com/forum/beginners/how-to-drive-a-low-rdson-mosfet-for-pwm/msg4178833/#msg4178833

author: ledtester
author_rank: Super Contributor
timestamp: September 17, 2023, 04:59:03 pm
content: Quote from: BadeBhaiya on September 17, 2023, 04:40:30 pmA recent post in another thread:https://www.eevblog.com/forum/beginners/ssd1306-oled-breakout-on-portable-atmega328-project/msg5062651/#msg5062651

author: tggzzz
author_rank: Super Contributor
timestamp: September 17, 2023, 05:17:32 pm
content: Quote from: BadeBhaiya on September 17, 2023, 04:40:30 pmIt would have been polite to mention that.Mentioning that kind of thing will help us give more suitable responses - and help us avoid thinking homework and/or ChatGPT is involved!

author: ejeffrey
author_rank: Super Contributor
timestamp: September 17, 2023, 09:57:44 pm
content: I think bootstrapping is as close as you can get to "standard" whether it's done by a dedicated high side driver or by hand. As long as you can live with a maximum on time they work great. That's usually fine, most of the time 99% PWM is as good as 100%.For pure static switching applications, you can either use a pfet or a charge pump.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 17, 2023, 10:31:00 pm
content: A lot of people automatically reject PFETs because they are worse than N channel devices. That's true but for simple on/off that's not always a big deal. You do need more silicon to get the same Rds-on but you aren't concerned about the gate charge and switching characteristics. There are relatively high current PFETs available for use in things like soft power switches and reverse polarity protection circuits where a simple solution is desirable. You weight the cost of the extra silicon against the circuitry needed to bias the N channel device.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 17, 2023, 11:13:33 pm
content: Quote from: BadeBhaiya on September 17, 2023, 03:20:35 pmIf your MCU is fed from a voltage regulator, and you just want to switch Vdd feeding some other circuitry, you may already have a source of sufficiently high voltage (input of the regulator) to drive the N-FET gate.Post a drawing of what you actually want to switch. Maybe there's another solution that we haven't guessed at.

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2023, 01:44:46 am
content: This has worked for me.

author: DavidAlfa
author_rank: Super Contributor
timestamp: September 18, 2023, 04:07:31 am
content: Quote from: xavier60 on September 18, 2023, 01:44:46 amInteresting circuit. So it pumps about 2x VDD (-3x diode drops) in top of the fet output, right?For 5VDD, about 8.3V?Sort-of bootstrapping but allowing DC switching.

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2023, 04:31:39 am
content: Quote from: DavidAlfa on September 18, 2023, 04:07:31 amThat's right. One possible flaw is that rail transients could stress the MOSFET and MCU. The Gate could be protected with a Zener diode.

author: Zero999
author_rank: Super Contributor
timestamp: September 18, 2023, 07:32:23 am
content: Quote from: xavier60 on September 18, 2023, 04:31:39 amThe main disadvantage is its slow, so only suitable for switching static loads.

author: Psi
author_rank: Super Contributor
timestamp: September 18, 2023, 07:52:15 am
content: Quote from: Benta on September 17, 2023, 04:24:19 pmPretty expensive though, triple the price of little 5V to 12V DCDC module and a cheap dual opto gate driver.I'd love something like the VO1263 if it only cost ~$1Or price was the same but it had 8 channels on one chip.

author: Zero999
author_rank: Super Contributor
timestamp: September 18, 2023, 08:06:41 am
content: Quote from: Psi on September 18, 2023, 07:52:15 amHow about the TLP3905?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 18, 2023, 08:29:17 am
content: Quote from: Zero999 on September 18, 2023, 08:06:41 amAnother option is the Skyworks Si8751 - much faster than a PV isolator

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2023, 08:56:42 am
content: Q2 will pull the Gate through the turn off plateau quicker and make the Gate drive less sensitive to leakage currents. C3 might need some tweaking.

author: Zero999
author_rank: Super Contributor
timestamp: September 18, 2023, 09:22:25 am
content: Quote from: xavier60 on September 18, 2023, 08:56:42 amUnfortunately that won't make any difference because C3 and R1 are responsible for the tardy off time, not the MOSFET's gate capacitance.C3 should be made much smaller, perhaps even omited and the turn-off transistor connected so it pulls the gate down to 0V, when required.

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2023, 10:12:04 am
content: Quote from: Zero999 on September 18, 2023, 09:22:25 amYes, the transistor didn't make much difference. I tested with an IRFZ44 switching 1A at 12V. The turn off time is about 1mS with a 100K and 1nF between G-S. Good enough for static switching.

author: tggzzz
author_rank: Super Contributor
timestamp: September 18, 2023, 12:07:55 pm
content: Quote from: xavier60 on September 18, 2023, 10:12:04 amMeasuring time in units of conductance (i.e. S, Siemens) is suboptimal.Mind you, some people are happy to quote a nuclear power station output as 2000mW, and quote a backup battery as being 1kW.

author: xavier60
author_rank: Super Contributor
timestamp: September 18, 2023, 12:12:56 pm
content: That was supposed to be "one millisecond".

author: tggzzz
author_rank: Super Contributor
timestamp: September 18, 2023, 12:14:39 pm
content: Quote from: xavier60 on September 18, 2023, 12:12:56 pmNo doubt. But you wrote "one milliSiemen". You should have written 1ms

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 18, 2023, 01:57:03 pm
content: For some DIY ideas...It start with consideration of the power supply. Is there any higher voltage available from which you can derive a gate voltage?If you are working with a 5V microcontroller, then making a charge pump from just a few diodes and capacitors may be enough. Then also add a bleed resistor between gate and source of your FET to be able to turn it off. Then connect this to a PWM pin to switch the FET, and turn off PWM to turn the FET off again.If you only have 3V3, then a charge pump may need more stages, and combined with the loss in each stage it gets doubtful. But you can make a simple step-up SMPS with just some inductor and a switch (BJT or FET) on a uC pin. Short circuit the inductor to GND for a while to get some current through it, then open the switch. so the current goes into something to drive the gate with. You probably need an extra capacitor on the Gate of the fet, and a diode from preventing that capacitor from discharging through the inductor again.

author: Psi
author_rank: Super Contributor
timestamp: September 18, 2023, 10:58:06 pm
content: Quote from: Zero999 on September 18, 2023, 08:06:41 amInteresting, I wonder if 50uA from a TLP3905 would be enough current to turn my 1mR 4xfet bank on fast enough (~100A DC load switch).Might need some external logic to charge up an external cap before dumping that into the gates a few 10's of ms later. Or might be fine as is. Though it's probably not intended to drive 4 gates at once.Actually, could probably work around the issue and have the high side switch on first, slowly using TLP3905, then the low side can switch on fast, due to not needing isolated power or TLP3905.

author: xavier60
author_rank: Super Contributor
timestamp: September 19, 2023, 12:23:20 am
content: In case it has been missed, the op only needs static switching. From post #1,"EDIT: My apologies if I wasn't clear. Duty cycle is 100%, that is the FET is turned on all the time, no switching is involved."

author: Zero999
author_rank: Super Contributor
timestamp: September 19, 2023, 09:33:52 am
content: Quote from: Psi on September 18, 2023, 10:58:06 pmI wonder if the could tolerate higher current pulses for tens of ms, just to speed up the turn on time? The absolute maximum forward current is 30mA, but it might take 100mA or so fo 10ms.Quote from: xavier60 on September 19, 2023, 12:23:20 amNote to original poster: after someone's responsed, it's better to post an update, rather than editing, as most people will miss it as they won't go back and read the first post again.

author: Psi
author_rank: Super Contributor
timestamp: September 20, 2023, 09:12:32 am
content: Quote from: Zero999 on September 19, 2023, 09:33:52 amProbably could do since it's just an LED, but you are holding it on after the larger pulse is over, so it's not as good as when you are PWM'ing LEDs and have off time immediately after for cool down.Would have to test a 30 or so too destruction to find where the limits are and see how much safety margin there is.I think a cap that has to charge up before the output is coupled to the gate sounds like the best plan. 1uF cap, p-fet and zener diode on pfet gate maybe. but getting it to latch rather than turn the p-fet on gradually would be the trick.

author: Zero999
author_rank: Super Contributor
timestamp: September 20, 2023, 11:35:43 am
content: Quote from: Psi on September 20, 2023, 09:12:32 amThe steady state current can be much lower.QuoteWhat do you mean? How will that work?

author: Psi
author_rank: Super Contributor
timestamp: September 20, 2023, 12:11:03 pm
content: Quote from: Zero999 on September 20, 2023, 11:35:43 amNot sure, but if you don't need the fet to turn on immediately and can wait 10, 20, 100ms, Then there must be a way to store up the charge from the photo cell array so you can dump it all into the gate at once a few ms later and turn it on faster.Just need a series fet that is normally disconnected but controlled but the voltage output. So when the LED turns on the photo array has to start charging a cap from 0V and not until the cap gets to 10V does the fet connect it to the gate.Hence the cap can dump amps into the fet to turn it on fast rather than trying to turn it on with 50uA directly out of the photocell array.