Mosfet - Gate protection from voltage spikes - Page 1
Forum section: electronics / projects

author: 2X
author_rank: Frequent Contributor
timestamp: July 10, 2016, 04:41:34 pm
content: Hello,I want to on/off a P-Channel mosfet with the car's central switch. Because of the transient phenomenons (like the moment that the switch is closing) the voltage of the car when the machine is ON it contains noise (voltage spikes). Which is the more reliable way for the protection of the mosfet's gate? Is it with a TVS diode and with which Vbr voltage (design 1) or with a low pass filter (design 2)?IRF4905 Datasheet:http://www.infineon.com/dgdl/irf4905.pdf?fileId=5546d462533600a4015355e32165197c

author: rx8pilot
author_rank: Super Contributor
timestamp: July 10, 2016, 04:57:25 pm
content: Your TVS config is in the wrong place - it should go to ground after a current limiting resistor.

author: 2X
author_rank: Frequent Contributor
timestamp: July 10, 2016, 05:08:02 pm
content: Because it is P-Channel Mosfet I placed the TVS between Gate and Source. According to some designs that I saw on the web, they have placed the diode between Gate and Source. The R3 resistor I placed to minimize the current that draws instantaneously the capacitance of the mosfet's gate and consequently to minimize the instantaneously heat at the mosfet's gate (am I wrong to this?).

author: rx8pilot
author_rank: Super Contributor
timestamp: July 10, 2016, 05:18:39 pm
content: If you put the TVS between R3 and the gate, you can use a rather small TVS or even a Zener diode since it would only see the current allowed by R3 (which could be bigger value if needed). An inbound spike that rises above the TVS threshold would hold the voltage at that point even if it is still rising. So the FET should turn on but without the gate being potentially damaged.

author: 2X
author_rank: Frequent Contributor
timestamp: July 10, 2016, 05:26:05 pm
content: So the protection with TVS diode (which is more fast than zener diodes) is more reliable than the low pass filter?

author: rx8pilot
author_rank: Super Contributor
timestamp: July 10, 2016, 05:29:55 pm
content: For design 2 with the filter scheme. That would help with fast noise, but an event that lasts longer than the time constant of the filter would pass any high voltage right through. A zener / TVS through a current limiting resistor can deal with longer periods of over voltage on the gate.Obviously, this does not account for a big spike on the input voltage that would make the Vgs too big - is that what you were trying to accomplish?

author: 2X
author_rank: Frequent Contributor
timestamp: July 10, 2016, 05:41:10 pm
content: I want to manage the voltage spikes that come up from switch on/of and maybe from the car's altenator. If I leave the TVS diode without limit resistor the below TVS (with vbr=16,7V) would appropriate for that use? I have read (maybe it is not right) that if you place a resistor in series with a TVS, then the response time of the TVS is decreased.P.S. rx8pilot thanks a lot for your time and your replies.

author: rx8pilot
author_rank: Super Contributor
timestamp: July 10, 2016, 06:02:30 pm
content: If you limit the gate voltage through any method and the there is a voltage spike at the source pin - the Vgs max of 20V could be exceeded. If you protect the whole circuit from something like a load dump or other source of voltage spike, it would have to be a much larger TVS to provide enough of a short to ground to hold off the voltage rise.If you are just trying to protect the gate only, the resistor should not be in series with the TVS. The TVS anode would be between the R3 and gate and the cathode direct to ground. R3 would provide current limiting so that when the TVS sees a voltage above it's threshold and starts conducting, it only has to pass the current allowed by R3.

author: 2X
author_rank: Frequent Contributor
timestamp: July 10, 2016, 06:19:07 pm
content: On the datasheet there is Vgs that is 20Volt. If I connect the TVScathodebetween the R3 and gate and theanodedirect to ground then does the Vgs kept below 20V?TVS anode would be between the R3 and gate and the cathode direct to ground-If i connect the TVS like the way that you describe I think that the TVS will work like simple diode.Transient voltage protection of N-Mosfets - PDF file

author: Gyro
author_rank: Super Contributor
timestamp: July 10, 2016, 06:52:15 pm
content: I agree. In the case of a P channel mosfet (or any mosfet come to that) the protection TVS should go between the gate and source. In this case that means between gate and +14V, with anode to the gate, cathode to +14V as you have drawn in your first attachment. I would personally add a second series resistor, directly at the gate for stability (but that's probably just me).I think the confusion is possibly because you have put the switch contact driving the gate to +14V to turn off, with a pulldown resistor to GND turning it on. It would probably be more conventional to have a pullup resistor to +14V turning the fet off and the switch contact going to GND via a series resistor closing to turn it on. It of course will work either way though, and your way uses one less part.Edit: I just realised that you DO have a series resistor directly at the mosfet gate... I would add the second resistor (1- 10k) in series with the switch contacts regardless to improve the effectiveness of the TVS in respect of any low impedance spikes entering via the lead to the switch. With this series resistor you could just use a suitable zener rather than a TVS.

author: 2X
author_rank: Frequent Contributor
timestamp: July 10, 2016, 07:15:21 pm
content: The below TVS (with vbr=16,7V and with Vr=15V) would be appropriate for that use?SMBJ15A:http://gr.mouser.com/ProductDetail/Littelfuse/SMBJ15A/?qs=sGAEpiMZZMssaSaMl4au6EPHikxJjQ61

author: Gyro
author_rank: Super Contributor
timestamp: July 10, 2016, 07:22:57 pm
content: You do need the resistor in series with the switch contact - that TVS breaks down at 16.7V but doesn't actually clamp until 24.4V. You need the resistor to limit the current it can see under worst case spike conditions.I'd go for a lower voltage TVS or Zener in combination with the series resistor, pretty much any mosfet will be fully turned on by 10V.

author: 2X
author_rank: Frequent Contributor
timestamp: July 10, 2016, 07:32:18 pm
content: Ok. One more question. Can I remove the R3 resistor (design 4) now that I placed Rlimit resistor?I'd go for a lower voltage TVS or Zener in combination with the series resistor, pretty much any mosfet will be fully turned on by 10V.- The car voltage when the engine is ON goes at 14V approximately and consequently a TVS with Vbr=10V will be active all the time when the switch will be close.

author: Gyro
author_rank: Super Contributor
timestamp: July 10, 2016, 07:38:10 pm
content: Glad you managed to decipher my rambling description. Personally I'd be tempted to keep R3 (close to the gate) to prevent any possibility of parasitic oscillation of the mosfet. I'd probably also drop R6 to 10k to reduce the effect of noise pickup.

author: uncle_bob
author_rank: Unknown
timestamp: July 10, 2016, 07:39:23 pm
content: Quote from: 2X on July 10, 2016, 07:32:18 pmHiFor automotive use you have a *very* large pull down resistor. That will pretty much guarantee the MOSFET switches very slowly and dissipates maximum power while it switches. It will stay in the active region long enough that odd things like oscillation are a real possibility. Taking out the 100 ohm resistor simply increases the likelihood of the circuit going into oscillation.Bob

author: 2X
author_rank: Frequent Contributor
timestamp: July 10, 2016, 07:46:12 pm
content: The final deisign. Thanks all of you for the replies.

author: uncle_bob
author_rank: Unknown
timestamp: July 10, 2016, 07:55:49 pm
content: Quote from: 2X on July 10, 2016, 07:46:12 pmHiWith a lower value resistor in after the 1K, your "off" voltage is something other than zero. If you move the 10K over by the switch that issue goes away.Bob

author: 2X
author_rank: Frequent Contributor
timestamp: July 10, 2016, 08:10:59 pm
content: Please can you explain why the off voltage will be not 0V? When the switch is open the "RLimit" resistor doesn't exist.The PMOS is OFF (open circuit) when the voltage on the gate is higer than the Vgs_threshold and ON (short circuit) when the voltage on the gate is lowe than the Vgs_threshold.

author: uncle_bob
author_rank: Unknown
timestamp: July 10, 2016, 08:14:55 pm
content: Quote from: 2X on July 10, 2016, 08:10:59 pmHiWith the two resistors in series, the voltage presented to the gate when the circuit is "off" is something other than zero. It's the voltage from gate to source that matters in this case, not the voltage to ground.You very much want the Vgs to be zero when the MOSFET is supposed to be off. The threshold voltage has a tolerance on it and a temperature dependance.Bob

author: rx8pilot
author_rank: Super Contributor
timestamp: July 10, 2016, 11:04:51 pm
content: I was definitely looking at this 'upside down' as an N-FET. The workable answer seems to have surfaced with some guidance from uncle_bob.

author: uncle_bob
author_rank: Unknown
timestamp: July 10, 2016, 11:12:36 pm
content: Quote from: rx8pilot on July 10, 2016, 11:04:51 pmHiThat's the gotcha. It is sort of an upside down and backwards way to do this. That gets you into some *really* easy traps looking at the circuit. Sometimes you have to do things an odd way, that's ok. When you do, you spend almost as much time re-sorting things out as you do actually working on the circuit.Bob