Mosfet, what am I doing wrong? - Page 1
Forum section: electronics / beginners

author: Szewczykm
author_rank: Regular Contributor
timestamp: December 18, 2012, 04:32:54 am
content: I'm using a FQP3N30 mosfet. (Datasheethttp://www.fairchildsemi.com/ds/FQ/FQP3N30.pdf) I'm driving the gate with an atmel microcontroller. When using it as a switch, running 12v and 60ma or so from drain to source (ground). It works great, no problem.The load is LED's, and it works.Now, changing the load to a 12v DC motor drawing 3 amps (with diode for protection), the motor doesn't budge. If I put the power directly to the motor from the power source, it runs great.I know that using a transistor, you need a certain amount of current on the base to get to saturation with a high current load.I didn't think the same was true with a mosfet. Am I missing something?

author: Jeff1946
author_rank: Regular Contributor
timestamp: December 18, 2012, 04:48:34 am
content: Look at figure 2 on the spec sheet, see if you are putting enough voltage on the gate for the current the motor needs.

author: Szewczykm
author_rank: Regular Contributor
timestamp: December 18, 2012, 05:06:49 am
content: Quote from: Jeff1946 on December 18, 2012, 04:48:34 amSo if the motor needs 3 amps, it looks like it's saying that there would need to be a little over 7 volts on the gate. But the max Vgs shows 5 volts. Does that just mean that this is a poor choice for a mosfet to drive 3 amps? Even though the max Is shows as 3.2 amps?

author: Monkeh
author_rank: Super Contributor
timestamp: December 18, 2012, 05:10:31 am
content: Quote from: Szewczykm on December 18, 2012, 05:06:49 amThat is Vgs(th), the threshold voltage. This is aminimumvoltage for operation. Vgss is 30V.The FET is adequate. The driver is not. Get a logic level FET.

author: ptricks
author_rank: Frequent Contributor
timestamp: December 18, 2012, 05:12:12 am
content: Quote from: Szewczykm on December 18, 2012, 05:06:49 amRead over this pdf, it explains why you are seeing what you are and how to correct it. Sorry if I am not just giving an answer but this is something that you really should go ahead and learn because it is so important in electronics.http://www.irf.com/technical-info/appnotes/mosfet.pdf

author: Szewczykm
author_rank: Regular Contributor
timestamp: December 18, 2012, 05:17:09 am
content: Quote from: ptricks on December 18, 2012, 05:12:12 amNo need to apologize, I absolutely want to learn it. Thank you!

author: BravoV
author_rank: Super Contributor
timestamp: December 18, 2012, 05:18:56 am
content: Beginner's mistake when handling power mosfet for the 1st time, I did experienced similar problem when I started tinkering with them.My own rule of thumb whenever dealing with non logic power mosfet, the voltage supply at the Vgs (voltage between Gate and Source pin) at least 10 volts to turn it on "properly".Also its recommended to put a pair of current limiting resistor and a 15 volts zener in series between G and S pin, that will beefy up the mosfet.

author: Szewczykm
author_rank: Regular Contributor
timestamp: December 18, 2012, 05:23:12 am
content: I picked the mosfet because of it's form factor and it's max current rating. It looks like I made a rookie mistake.Are there some "run of the mill" logic level FETs? I don't want to pick any weirdo parts. It's just so tough when you start sifting through options on Mouser.Thanks!Mike

author: TerraHertz
author_rank: Super Contributor
timestamp: December 18, 2012, 05:28:34 am
content: Actually, for driving a motor from a CPU logic signal, it's a poor choice of MSOFET.See the RDS(on) of 2.2 ohms? That's really high for a power switching MOSFET. At 3 amps, that's going to be 6.6V source to drain. More than half your available supply to the motor. Actually the current will be less - some value resulting from the summed resistance of motor plus MOSFET.This MOSFET is optimized for high speed switching in high voltage circuits, where a few volts drop don't matter much. Hence the low gate charge and fast switching times. Did you salvage it from a mains switch-mode supply?There are plenty of lower speed, lower voltage logic-drive MOSFETS with on-resistances well under 0.1 ohm.You can also find ones with internal clamp diodes to absorb the motor inductance flyback on turn-off.

author: ptricks
author_rank: Frequent Contributor
timestamp: December 18, 2012, 05:33:50 am
content: Quote from: Szewczykm on December 18, 2012, 05:23:12 amYou can use the ones you already have you just need to use a mosfet driver chip instead of using the ports on the micro , it is better to do that anyway since it can offer other benefits.The MCP1406 /MCP1407 should work and cost is only about $1

author: MrsR
author_rank: Regular Contributor
timestamp: December 18, 2012, 07:29:42 am
content: Quote from: Szewczykm on December 18, 2012, 04:32:54 amAre you driving the MOSFET direct from the microcontroler??I am using Transistors to drive the gates on a project using a ATMEGA48V as the MOSFET I am using is a P,N Type I have 2.2k resistors on the P v- side and 3.3k on the N v+ side. You do need to load the transistors. My problem is I am changing to Gate Drivers and can't get it to work. The Data sheet should give you the load but check to see if it requires a capacitive load too. It took me a lot of time to get mine to work properly but I will be asking for HELP soon with these Gate Drivers. I have a component circuit PDF if you think it may help.NEW TO ALL THIS ALSO

author: Szewczykm
author_rank: Regular Contributor
timestamp: December 21, 2012, 01:13:58 am
content: Quote from: ptricks on December 18, 2012, 05:33:50 amI decided to try RFP12N10L -http://www.fairchildsemi.com/ds/RF/RFP12N10L.pdfIt passed a quick breadboard test without using a MC to control the gate. (Just touching it with 5v from the power supply) It worked great.However, when I use it in circuit with the MC, as soon as the gate goes high, the motor I'm controlling gives a quick little shake, and then the MC reboots.The microcontroller does share ground with the motor. I don't know if that is causing the problem. I'll start poking around with the scope, but I was hoping for some advice. Thanks!Mike

author: Szewczykm
author_rank: Regular Contributor
timestamp: December 21, 2012, 03:10:57 am
content: Quote from: Szewczykm on December 21, 2012, 01:13:58 amOK, though the MC was running on 5v and the motor on 12v, they were bothing being powered by the same power supply. The initial draw of the motor was enough to strain the supply so 12v and 5v were interrupted. A 470uf cap at the power input across the 12v power input solved the reboot problem. There's still a flicker on the 12v in when the motor starts so I'm going to up the cap size until I don't get the flicker anymore.

author: McMonster
author_rank: Frequent Contributor
timestamp: December 21, 2012, 05:38:13 am
content: Quote from: Szewczykm on December 21, 2012, 03:10:57 amGet a low ESR/ESL capacitor and put it across the supply rails of the motor as close to it as possible. This thread reminds me of how I tried to use MOSFETs in my microcontroller project the first time, looks like all the mistakes are common in this case.One more thing, even if use use a logic level FET always remember to put a resistor between microcontrollers IO pin and FET's gate! Charging the gate without it can cause very short but very high peak current, way outside of what any micro can handle safely. Look at the micro's maximum IO pin current and put an appriopriate resistor in series to limit it.

author: lewis
author_rank: Frequent Contributor
timestamp: December 21, 2012, 09:35:22 am
content: STP36NF06L is a suitable logic level MOSFET. You might want to consider adding inrush protection to the motor to make life easier for the MOSFET and the PSU. You can PWM the MOSFET with the micro at several kHz (20-30kHz works well for smaller motors) for a few hundred milliseconds, ramping up the duty cycle, and hence motor speed, 'slowly'.