For any add-on boards such as VR conditioners, optos and OEM interface boards
#44335
JHolland wrote:
Fri Jul 17, 2020 9:17 am
Will you be able to dissipate the heat during current limiting? A StripFET is essentially a trench FET, they can suffer a form of thermal runaway when used in analogue mode.
I was thinking that with RDS(on) the resistor of the MOFFET is max 0.014 Ohm, with a current clamp around 14A: P =R x I^2= 0.014*(14^2) = 2.75W that is 40 times less of the Total dissipation at TC = 25°C =110 W (from datasheet) it should not give enough heat to damage the PCB. Is my thinking wrong?
#44338
filipesbp wrote:
Mon Jul 20, 2020 5:04 pm
I have made o Copy/Past from Speeduino Schematic, that uses the 1k resistor to the Gate pin plus the 100k resistor do GND, I have checked the Megasquirt circuit that use a driver like this and they use a 22R resistor to the Gate and no resistor to GND. So, a better design has you are saying is use a lower resistor and probably the resistor of 100k can be omitted too..
Look at whether the circuit uses a FET driver or not. Your FET driver provides both hard pullup and pulldown, so no pull resistors are necessary, and they are meant to provide high-current for gate discharge performance, so gate resistance is small. Directly-fired FETs use much larger resistors for whatever protections or performance the designer envisions, but that's not your circuit type. Just sayin'.
filipesbp wrote:
Mon Jul 20, 2020 5:17 pm
I was thinking that with RDS(on) the resistor of the MOFFET is max 0.014 Ohm, …
Not speaking for @JHolland, but just random IMO; RDS(ON) is 0.014 at 10V VGS, not typical Speeduino at 4.5 to 5V. Also, I would assume the temperature to be calculated at the operational environment max, which would be around 85°C for other components, not 25°C. I would check performance, resistance, and additive heat at those parameters, and perhaps others. Worst-case. That's me. Do your thing.

David
#44367
The problem with a Trench FET is the trench is made up of many cells and those cells have a negative temperature coefficient and they don't share current 100% evenly. When you run in the analogue region then you can get hot spots that then 'run away'. Trench FETs make very good switches but aren't usually recommended for gate clamping and its also the reason why they generally have very poor avalanche capability compared to a planar FET.
Some manufacturers take this into account when they specify the maximum current, some don't, its a bit of a pig in a poke. If you have good thermal management and a limited clamping duration it may not be an issue anyway. The worst case scenario would be an injector short circuit which would require you to dissipate nearly 200W at 14A - your FET will die with or without the 'protection'.
#44396
JHolland wrote:
Tue Jul 21, 2020 11:17 am
The problem with a Trench FET is the trench is made up of many cells and those cells have a negative temperature coefficient and they don't share current 100% evenly. When you run in the analogue region then you can get hot spots that then 'run away'. Trench FETs make very good switches but aren't usually recommended for gate clamping and its also the reason why they generally have very poor avalanche capability compared to a planar FET.
Some manufacturers take this into account when they specify the maximum current, some don't, its a bit of a pig in a poke. If you have good thermal management and a limited clamping duration it may not be an issue anyway. The worst case scenario would be an injector short circuit which would require you to dissipate nearly 200W at 14A - your FET will die with or without the 'protection'.
Should I use an POWER FET instead of an TRENCH FET? That worst case scenario is almost impossible to handle, what is the solution? Current limiting not controlled by the MOSFET?
#44402
filipesbp wrote:
Wed Jul 22, 2020 6:57 pm
Should I use an POWER FET instead of an TRENCH FET? That worst case scenario is almost impossible to handle, what is the solution? Current limiting not controlled by the MOSFET?

You can use an automotive driver like an NCV7520.
#44415
JHolland wrote:You can use an automotive driver like an NCV7520.
That is really a complete solution, maybe for version 2... The PCB is on the way, lets see what can be done with this circuit, going to update the value of the resistors and put a new image schematic in here.
#44418
JHolland wrote:
Tue Jul 21, 2020 11:17 am
The problem with a Trench FET is the trench is made up of many cells and those cells have a negative temperature coefficient and they don't share current 100% evenly. When you run in the analogue region then you can get hot spots that then 'run away'. Trench FETs make very good switches but aren't usually recommended for gate clamping and its also the reason why they generally have very poor avalanche capability compared to a planar FET...
Found a POWER Mosfet that should be better for this: IRLR8743PbF and uses the same PCB case..

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