Electronics > Projects, Designs, and Technical Stuff
MOSFET Rds(on) as current shunt - it's not stupid if it works, right?
KerimF:
In my DC to AC inverters, I simply sense the voltage on Rds_on for the overload protection (of the output power MOSFETs).
Fortunately, Rds_on increases with temperature, this lets the overload protection be more effective.
exe:
While I think it might work to certain extend (and I'm curious to see the actual performance), I think just putting a shunt will be better in almost every way possible, except some little drop on it (which can be less than 50mV full-scale). But hey, why not.
If you want to go fancy, put the second MT9700, and use it just a shunt (=always on). This way it will handle properly situation when power is disabled on the original MT9700. But I still think a resistor will be cheaper.
langwadt:
--- Quote from: Siwastaja on January 25, 2025, 07:20:48 am ---Rds_on current sensing for peak overcurrent protection is very usual thing to do on integrated DC/DC converter chips (especially when the power MOSFET itself is integrated, but also on those with external MOSFET, in which case they could have a separate pin for configuring the current limit threshold). In such use case, the fact that current trip level gets lower with increasing temperature might be even considered desirable. Since power dissipation in the MOSFET is relative to this voltage drop, it's really a "dissipation limit" more than a current limit.
In fact I would say 99% of modern DC/DC supplies do that since they are voltage sources and no accurate constant-current mode is needed, just protection.
--- End quote ---
usually called desaturation or desat protection
Siwastaja:
--- Quote from: langwadt on January 25, 2025, 09:30:03 pm ---usually called desaturation or desat protection
--- End quote ---
Yeah, circuit-wise it's the same thing (so worth googling with that term), but the purpose is slightly different: "desaturation" means that there is something wrong with drive voltage and the transistor isn't fully conducting when it should, so voltage across it is larger, dissipation is larger, and you need to turn it off.
Then again, even in desired "full conduction", too much voltage over the transistor again indicates the same problem: too much dissipation, and the remedy is exactly the same, so terms "desat protection" or "Rds_on overcurrent sensing" can be used interchangeably.
You hear "desat detection" more in the context of large IGBTs, and Rds_on current sensing / overcurrent protection in the context of smaller (often integrated-transistors) switch-mode supplies.
HwAoRrDk:
I realised I already had roughly the necessary components to try this out - not the exact components I drafted in my schematic, but functional equivalents. Instead of the INA180A3, I have an ST CS30BL (50V/V gain rather than 100) and some Richtek RT9742 (55mΩ RDS(on) rather than 80).
So I cobbled together the circuit on some old strip board. And... it works! ;D
I'm getting ADC readings about what I would expect from the specified 55mΩ RDS(on). In a test case, current draw in 'idle' state was approx. 55mA, and ADC reading correlates with theoretical calculated value for 60mA, which is close enough for my purposes. Readings from 'active' state were similarly 'close enough'. :-+
As expected, the current sense amplifier output is saturated at nearly 5V when the load switch is off, due to having around 5V across Vsense+ and Vsense-.
I don't have any 5V ESD diodes to hand, so I wasn't able to include that part in the prototype circuit, and so I don't have a handle on how that would influence the performance, but I'm pretty sure now that any impact would be negligible.
One other thing I didn't include was the 100nF capacitor across the load output, as I figure that maybe that's not a good idea when measuring current, as any decoupling may influence the readings (or, at least, introduce some latency to the true current draw). However, maybe I should experiment with that to see.
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