Mosfet's Vgate gets noisy - Page 1
Forum section: electronics / beginners

author: auccapuclla
author_rank: Newbie
timestamp: March 19, 2021, 02:34:40 pm
content: Hello all. So I am working currently in a PID circuit to control an oven. The resistor of this oven is gonna be connected in serie to a N Mosfet, so by changing the values on the Gate of the Mosfet I will be able to control the current trough this resistor.So I tested it out with a normal resistor, like 7 ohm, and I could control the gate furthermore the current trough this resistor. This was done on a breadboard. Since it was working perfectly I proceeded to mount this circuit in a circuit board. Now when I connected the load to my Mosfet I got this noisy in my gate.At first when I connected the load I noticed there were no current being drained so I checked the signal of my Gate and found out this noisy makes me Vgate 0, I need at least 2.2V to turn on my Mosfet.I am looking for some explanation of why when I connected something in my Mosfet my Vgate gets noisy. And if it is possible how to overcome this.Thanks in advance!Cheers!

author: CaptDon
author_rank: Super Contributor
timestamp: March 19, 2021, 03:20:02 pm
content: The gate is basically an open circuit? Changing the resistor is not the way to control current in your heater resistor. In fact the fet may self destruct due to it heating. It looks like the resistor and the Miller effect in your fet has created a bit of a pulse oscillator??

author: auccapuclla
author_rank: Newbie
timestamp: March 19, 2021, 03:23:54 pm
content: Quote from: CaptDon on March 19, 2021, 03:20:02 pmI think I explained myself wrongly, I change the Vgate so I can control the current trough the resistor.Could u explained me how can I get rid of this effect?

author: MikeK
author_rank: Super Contributor
timestamp: March 19, 2021, 03:41:07 pm
content: Shouldn't this be done with PWM control of the gate to minimize heat? Turning the FET partially on is going to generate a lot of heat.

author: CaptDon
author_rank: Super Contributor
timestamp: March 19, 2021, 03:47:51 pm
content: Can you upload a schematic of how your circuit is wired? When the fet is saturated all of the heat will be in your heater resistor. What is the supply voltage and resistance of your heater? At some point as the voltage on the fet rises to half the total supply voltage the fet will have as much heating as the heater resistor although the 'total' heat should be half. Controlling the voltage of the fet's gate to control heater current should only be done with some kind of 'closed loop' control circuit which senses the drain to source voltage. The point where a fet begins to turn on verses the full saturation point is a narrow point on the possible range of voltages on the fet. For instance it begins to turn on at 4vdc gate to source and is fully saturated at 5vdc. Also as the fet heats it will begin to shift its operating point. I hope I understand what you are trying to do with your circuit and my answer makes sense. In 'power control' applications the gate of the fet is usually driven by a low impedance circuit which can push or pull current into the fet gate. There is a huge capacitor effect from gate to source and a smaller 'Miller' capacitance effet from gate to drain which acts like a capacitive voltage divider across the gate of the fet and can lead to instability if the gate drive resistance is high.

author: auccapuclla
author_rank: Newbie
timestamp: March 19, 2021, 04:20:33 pm
content: I don't know if u are asking for a lay out but here it goes.

author: auccapuclla
author_rank: Newbie
timestamp: March 19, 2021, 04:24:13 pm
content: Naively I would say we use a variable DC signal to heat the oven. We don't want to switch on and off.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 19, 2021, 04:38:38 pm
content: For the control there are 2 alternatrive methods:One is a PWM type control to switch the MOSFET on / off for times given by the µC.The other is using control of the current without extra mdulation, like shown in the crude plan.Both methods have there pros and cons. The main ones are possoble high power loss and nonlinear control for the 2nd method, while PWM can cause interference, both electric and maybe also from the fast temperature modulations. Usually the PWM way is preferred for high power and as a simple solution, while the linear control is preferred if ultimate accuracy and low noise is needed. So it depends one the task which way is better.A MOSFET can have a rather steep change in current when the gate voltage changes. So it can be quite sensitive to noise and small effects. One such effect is a thermal effect: with more current the fet gets warmer and thus changes the control voltage to get a given current. So usually one wants some direct feedback around the MOSFET, so that the operational amplifier regulates the current instead of just amplify the gate voltage. Than the DAC output is more accurately controling the current and the exact FET parameters mater less. Usually this uses a small resistor at the FET source and feedback from there. The circuit is like with a controlled current sink.

author: auccapuclla
author_rank: Newbie
timestamp: March 19, 2021, 04:54:58 pm
content: But the circuit no even turn on, I mean the mosfet was all the time off. Before hand I knew it about the mosfet getting hot so I used a heatsink attach to it. Anyways, as no current was drained for minutes I checked what is happening with my Vgate so I found out the noise picture I attached.