Why is my power board still dividing a lot of voltage when the mosfet is driven? - Page 1
Forum section: electronics / beginners

author: man1
author_rank: Contributor
timestamp: April 19, 2025, 02:09:15 am
content: I added 5V to my power board with a 10 ohm load. But the voltage across my load is only 1.5v, and I measure 3.5v from GND to port G. Why can't my load be fully voltage divisible, and it doesn't work with a 1ohm

author: pqass
author_rank: Super Contributor
timestamp: April 19, 2025, 02:35:20 am
content: According to thedatasheet, your chosen device has a VGS(th)between 2V and 4V (where it just begins to conduct).Also, the lowest RDS(on)of 80milliohms is achieved when the gate is driven with 20V VGS.You measured 3.5V. There's your problem; apply more gate voltage (try 12V but no more than 20V).What's your current requirement?

author: Konkedout
author_rank: Frequent Contributor
timestamp: April 19, 2025, 03:09:06 am
content: Wow your FET is a 1200V SiC device. Unless your supply voltage is over 500V this does not make sense. You will be making little use of a big transistor. You will get better operation from a lower voltage silicon MOSFET.Generally choose a transistor voltage rating that is 110% to 200% of the highest voltage to which it will be subjected. If your supply voltage is 5V and you are not boosting it, use a 20V - 30V MOSFET.But the other answer is also correct. Depending upon what FET you use, the gate voltage should be at least up to the rating at which Rds(on) is specified on the datasheet.

author: man1
author_rank: Contributor
timestamp: April 19, 2025, 05:53:31 am
content: Quote from: pqass on April 19, 2025, 02:35:20 amThe drive voltage I use is 12v .3.5v measured with a multimeter because the duty cycle is 30%

author: pqass
author_rank: Super Contributor
timestamp: April 19, 2025, 06:41:51 am
content: Quote from: man1 on April 19, 2025, 05:53:31 am"Why can't my load be fully voltage divisible..."You're PWM-ing your load and it appears that you're getting 1.5V (of 5V) which is 30%, as expected.10R across 5V is 500mA so "... it doesn't work with a 1ohm" means you can't switch 5A?Big MOSFETs have high gate capacitance so need higher current to drive the gate.Usually, this is best done with a gate driver IC. What are you using to drive the gate?Also, what's the PWM frequency?

author: man1
author_rank: Contributor
timestamp: April 19, 2025, 06:55:39 am
content: Quote from: pqass on April 19, 2025, 06:41:51 amMaybe I don't fully understand what a mosfet does, in my understanding it is a switch that should be voltage independent. If I give 5v, shouldn't the current through 10 ohms be 0.5A and the voltage be 5V, why does the mos split 3.5V.I used the IR2127 driver chip to drive it.

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 19, 2025, 07:14:34 am
content: A mulitimeter is a very poor tool for checking out a mosfet you're using for PWM. You'd want to use a scope for that...

author: man1
author_rank: Contributor
timestamp: April 19, 2025, 07:45:36 am
content: Quote from: pqass on April 19, 2025, 06:41:51 ampwm frequency is 5khz

author: SteveThackery
author_rank: Super Contributor
timestamp: April 19, 2025, 10:37:48 am
content: Quote from: man1 on April 19, 2025, 06:55:39 amHave you read the replies?

author: mikerj
author_rank: Super Contributor
timestamp: April 19, 2025, 10:59:16 am
content: Quote from: man1 on April 19, 2025, 06:55:39 amYou are driving the MOSFET with a 30% PWM load, so the average voltage across your load will be 30% of the supply 5v * (30/100) =1.5v

author: xavier60
author_rank: Super Contributor
timestamp: April 19, 2025, 12:07:25 pm
content: He had a driver problem back on this thread,https://www.eevblog.com/forum/beginners/why-does-the-pwm-drive-signal-from-my-mosfet-become-constant-voltage-at-the-outp/msg5881134/#msg5881134I'm assuming it's fixed, he doesn't always reply.

author: man1
author_rank: Contributor
timestamp: April 19, 2025, 12:20:43 pm
content: Quote from: xavier60 on April 19, 2025, 12:07:25 pmThis D3 didn't work, I've removed the D3 and now have the waveform, but ran into other problems

author: man1
author_rank: Contributor
timestamp: April 19, 2025, 12:31:24 pm
content: Quote from: SteveThackery on April 19, 2025, 10:37:48 amWhose not read？

author: CaptDon
author_rank: Super Contributor
timestamp: April 19, 2025, 01:13:18 pm
content: If your load is pure resistance you don't even need C3 and D2. Why did you chose a MOSFET with such a huge high voltage rating? They generally have higher on resistance which is not helping your somewhat low voltage circuit.

author: man1
author_rank: Contributor
timestamp: April 19, 2025, 03:01:16 pm
content: Quote from: CaptDon on April 19, 2025, 01:13:18 pmI would like to get the junction temperature using the thermo-electrical parametric method and measure the VDS, vds is easier to measure with a high RDS

author: pqass
author_rank: Super Contributor
timestamp: April 19, 2025, 04:12:25 pm
content: TheIR2127is very sophisticated.It can provide it's own boosted gate supply (Vb) and has an isolated ground (Vs) including current sensing, shutdown, and fault reporting.You need to provide a more complete schematic including how you drive the gate and provide all voltages used (logic and load, not just write Vcc).Maybe you should first prototype switching on/off old MOSFETs (like the IRF540) on modest loads (<100mA).Do simple switching and then try PWM-ing LEDs or small motors using a 555 (seehere).MOSFETs usually need a minimum of 10V on the gate to turn them fully on.Larger ones like you're using need to have higher current drivers because they have more gate capacitance.But driver ICs have many features that may introduce complexity for beginners.Multimeters will average the voltage received or report lower than you think if the frequency is too high (>> multimeter bandwidth).__________________IR2127 application noteto calculate the resistor values in the attached schematic.

author: man1
author_rank: Contributor
timestamp: April 21, 2025, 10:33:50 am
content: Quote from: pqass on April 19, 2025, 04:12:25 pmextremely grateful

author: man1
author_rank: Contributor
timestamp: April 22, 2025, 03:00:14 am
content: Quote from: pqass on April 19, 2025, 04:12:25 pmHi, I also want to ask, I need to measure vds, now I add 2a of constant current and the measured vds is 2.6v, is there any problem with this value?

author: pqass
author_rank: Super Contributor
timestamp: April 22, 2025, 03:33:34 pm
content: Quote from: man1 on April 22, 2025, 03:00:14 amThat seems too high for just a 2A load with your MOSFET (GC2M0080120D). That's 2.5W (2.6V*2A*50%(PWM))!It should be closer to 160mW (RDS(on)=80mA so P=I2R=2A2*0.080ohms*50%(PWM)).Nevermind the PWM or the IR2127 driver for now. You need to walk before you can run.Start with a simple switcher (see attached) and measure VDSwith a small load first (100mA) then try for 2A.It should only take 10 minutes to configure with alligator clip wiring. Use a heatsink.Then you can add the IR2127 driver.Only after you've confirmed that you still have a similar VDSwith the driver, then apply PWM to the driver input._________________________Simulation of the attached schematichere.

author: man1
author_rank: Contributor
timestamp: April 23, 2025, 06:21:30 am
content: Quote from: pqass on April 22, 2025, 03:33:34 pmI followed what you said and put 12v directly to the mos, the load is 1Ω, pass 1v through and the current only goes to about 0.6a. The measured vds is about 230mv, why can't I reach 1a by passing constant voltage current, is this vds reasonable?

author: xvr
author_rank: Frequent Contributor
timestamp: April 23, 2025, 01:05:14 pm
content: Quote1V on load + 0.23V on MOSFET is 1.23V. Power supply provides 12V. Where is the rest (10.77V)? Seems that your PSU switched to CC mode.

author: man1
author_rank: Contributor
timestamp: April 23, 2025, 01:11:11 pm
content: Quote from: xvr on April 23, 2025, 01:05:14 pmYou're missing the point, my drive voltage is already 12v. But my Vds is 230mv, the supply voltage added to my power circuit is 1v, and the current flowing through the load is 0.7, of which 0.3v has disappeared somehow.

author: man1
author_rank: Contributor
timestamp: April 25, 2025, 03:28:52 pm
content: Quote from: pqass on April 19, 2025, 04:12:25 pmMay I ask if it's the number 1 or 7 that's labeled in your picture, I'm a little confused