Can a power MOSFET be turned on in this scenario? - Page 1
Forum section: electronics / general

author: onemilimeter
author_rank: Frequent Contributor
timestamp: August 11, 2011, 08:55:05 pm
content: FDP036N10AN-Channel PowerTrench® MOSFET100V, 176A, 3.6m?If the Vgs of a power MOSFET (specs as given above) is left open (e.g. not connected to any gate drive circuit), will current flow from drain to source when a DC voltage (e.g. 20V) is applied across the drain and source terminals (+Vdc at drain terminal and -Vdc at source terminal)?Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: August 11, 2011, 09:39:44 pm
content: Your question is too vague.Assuming you mean what I think you do, yes, if the get is left floating a MOSFET can turn on due to noise but it's unpredictable.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: August 11, 2011, 10:28:54 pm
content: Quote from: Hero999 on August 11, 2011, 09:39:44 pmThanks. You're right... I should mention "... the gate is leftfloating...".

author: Zero999
author_rank: Super Contributor
timestamp: August 11, 2011, 10:40:43 pm
content: Then should should roll a die, the MOSFET might be on, off, anywhere in between or randomly oscillating in response to the noise induced at the gate; I think the latter is most likely and it'll probably be at the mains frequency or a harmonic.

author: Time
author_rank: Frequent Contributor
timestamp: August 11, 2011, 10:58:09 pm
content: Seems more or less unpredictable.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: August 11, 2011, 11:44:38 pm
content: As the mentioned earlier, the specifications of the power MOSFET are as follows:FDP036N10AN-Channel PowerTrench® MOSFET100V, 176A, 3.6mOhmThe power MOSFET is attached to a heatsink cooled by a fan. An electrolytic capacitor (2200uF) is added as shown in the figure below. When Switch-1 is ON and if the current limit of the DC power supply is set to 5A, do you think the power MOSFET will blow if it's turned on (e.g. due to noise acting on the floating gate)? Do you think the current from the capacitor can cause any failure?Cheers

author: PeterG
author_rank: Frequent Contributor
timestamp: August 12, 2011, 02:26:35 am
content: I am interested in why such a setup would be used. I believe the outcome will be random with the Gate floating the way it is.Regards

author: DavidDLC
author_rank: Frequent Contributor
timestamp: August 12, 2011, 03:47:35 am
content: Why you don't use simulation and post the results later ?

author: onemilimeter
author_rank: Frequent Contributor
timestamp: August 12, 2011, 08:48:23 am
content: Quote from: PeterG on August 12, 2011, 02:26:35 amI accidentally "setup" the arrangement and blew several power MOSFETs. I wish to understand the failure mechanism. Thanks.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 12, 2011, 09:29:55 am
content: Rule #1 of MOSFETs is that you never leave the gate floating. Because they are essentially infinite impedance at DC, there is no bias current to pull it into a well defined state. Both stray electric fields and the parasitic capacitance to the gate and source can easily push it around. The most common result is oscillation: if the gate picks up a bit of charge the channel starts to conduct. The current draw pulls the drain voltage down towards the source, and the drain-gate (miller) capacitance drags the gate down with it. This causes the transistor to shut off, and the drain voltage rise. Again, the capacitive coupling causes the gate to try to follow the increasing voltage of the drain and the FET turns on again.The dynamics of what actually happens depend on the series resistance on the supply, the parasitic capacitance, and the circuit environment in which it is connected. MOSFETs in integrated circuits usually have protection diodes connected to the supply rails. The reverse leakage current of these diodes also affects the biasing. Logic inputs have a complimentary pair NMOS and PMOS with their gates tied together. Here, the common result is for the gate to float to halfway between the supply voltages so that both transistors are turned half-on, causing a large class-A current to flow, possibly overheating the device. In any case, free floating mosfet gates are extremely susceptible to pickup of stray fields, so if nothing else you are likely to see oscillation at 50/60 Hz.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: August 12, 2011, 10:57:23 am
content: Can the circuit configuration (connecting a resistor between the gate-source terminals) in attached figure prevent failure due to open/floating gate?If yes, is there any rule-of-thumb for the value of the resistor that should be used? Large value e.g. 20kohm?Do all the power MOSFETs or only the upper power MOSFETs need such resistor?How would such resistor affect the switching dynamic (e.g. rise/fall times) of the power MOSFET?Cheers.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 12, 2011, 11:17:38 am
content: Yes, that will work. Both the high and low side MOSFETs need a pull-down resistor: when the top MOSFETs are driven on, the bottom MOSFETs become vulnerable and vice versa.The resistors can be quite big in general. If high frequency signals/pulses will be applied to the drain you have to make sure that the gate current flowing through the gate-drain capacitance will not turn the FET on. Normally the pull-downs will not affect the turn-on time. It will load the gate driver slightly, but for high speed switching the junction capacitance dominated the gate drive.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: August 12, 2011, 11:38:14 am
content: Quote from: ejeffrey on August 12, 2011, 11:17:38 amIf I replace the resistor with zener diode, will the zener diode do the same job?Cheers

author: ejeffrey
author_rank: Super Contributor
timestamp: August 12, 2011, 12:13:30 pm
content: What are you trying to do? Why can't you just connect something to the gates? As soon as you put a gate driver that applies a definite voltage to the gate these problems go away.A zener will probably work, but not like you think or want. The turn-on voltage of the zener is probably too high to be of use -- the MOSFET will already be partially on by the time the zener does anything zener-like. The leakage current of the diode is probably enough to keep the MOSFET from turning on, but then as soon as you try to actually drive the FET you will have to contend with the zener turning on and taking your gate current. The sharp turn-on of a zener is actually the opposite of what you want.What is wrong with a simple resistor?

author: onemilimeter
author_rank: Frequent Contributor
timestamp: August 12, 2011, 12:23:23 pm
content: Quote from: ejeffrey on August 12, 2011, 12:13:30 pmI agree that a resistor is a simple and great solution. I'm trying to figure out the possible causes that blew the power MOSFETs in my inverter. Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: August 12, 2011, 05:07:13 pm
content: Quote from: DavidDLC on August 12, 2011, 03:47:35 amWhat good will that do? A simulator won't give you the correct answer here.The simulator may generate an error or assume the MOSFET is off.The problem with simulators when they fall into the hands of nubes is they don't simulate things like the ambient electromagnetic noise which will be picked up by the MOSFET gate.Quote from: onemilimeter on August 12, 2011, 08:48:23 amIt's difficult to say.If there's nothing limiting the current then the MOSFET will turn on, causing a large current to flow.It there's a load in series with the MOSFET, it might die from switching losses as it will switch on/off so slowly that the losses will cause it to dissipate too much power.