Electronics > Projects, Designs, and Technical Stuff
N Channel MOSFET Vds safety margin for driving a relais
(1/2) > >>
nemail2:
Hi

the title already tells everything... What safety margin for the N channel MOSFET's Vds should I use when driving a 12V standard G2R Omron relais (about 45mA coil current)? Currently I'm using a FET which has 100V Vds but I'd like to reduce my BOM and maybe use a FET with 20V or 30V Vds which I was using somewhere else in my design already...

Are there any general rules of thumb?

Thanks!

edit: diode across the coil is already present...
Psi:
30V fet for 12V is totally fine.

A 20V fet is getting a bit close but probably still fine. I would use 30V though.

Using the automotive world as an example, all parts in a 12V car should survive 24V without any damage. So a 30V fet would be ok, but 20V fet would be too low.
(Of course in an automotive setting there are lots of voltage spikes, so you would also have some sort of TVS diode to stop spikes over 30V getting to the fet, but that's a separate issue)

For current you probably want to spec something that can handle 200mA continuous. Nice round number with good safety margin, lots of mosfets existing in the 200 - 300mA range

Usually heat production is more of a limiting factor than a chips max current rating.
You should ideally look at the On Resistance (RdsON) at your gate drive current and work out how many watts will be used up as heat from your 45mA.
Then look at the datasheet specs for temp  (θJC/JA) and calculate how hot it will get above ambient temp. Then consider if that is ok.

I doubt you will have any issue for your application because 45mA is tiny, but for bigger power stuff you may notice a mosfet rated for say 100V and 60A but find that it cannot actually run at 60A without water cooling it!
ie, The chip is ok at 60A but its up to you to remove the heat. verses at 61A where the chip may fail as the package is to small to get heat out fast enough no matter how much cooling you have.

Ian.M:
It also has a lot to do with how you snub the back-EMF from the relay coil.   

A simple anti-parallel diode puts the lowest voltage stress on the driver, but significantly slows the flux decay in the relay core slowing down contact opening, which is undesirable if the contacts are in a high energy circuit.

Add a resistor matching the coil DC resistance in series with the anti-parallel diode, and the flux decays as fast as it builds when initially energised, but the driver must withstand double the supply voltage.

Then there are arrangements like snubbing into a Zener, or with a Zener to the gate circuit to provide a controlled rate of turnoff, or using an avalanche rated MOSFET  without any other snubber, all with their own driver voltage rating requirements.
nemail2:
cool bananas!

regarding RDS(on), power dissipation, package limits and so on I'm quite familiar how to choose an appropriate part. My concern was only regarding the voltage spikes from the coil when turning it off.

currently I do have a diode across the coil and I didn't see any issues with that in my prototype (regarding slow contact opening).
The relay carries 4A at max @ 16V (max) when contacts are closed.

Anyway, if I'd put a resistor matching the DC coil resistance in series with the anti-parallel diode, would I then want to use a MOSFET with 48V Vds or higher (due to saying double for margin and double due to the parallel resistor) or still just 30V?

Thanks!
Psi:

--- Quote from: nemail2 on June 18, 2019, 07:22:53 am ---My concern was only regarding the voltage spikes from the coil when turning it off.

--- End quote ---

The relay voltage spike is negative, so gets clamped by the diode to the diodes forward voltage.
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod