MOSFET Switching - Short Circuit Protection (need help)

[Mr. Ohm Smith]

Good afternoon,


I recently changed a relay to a MOSFET channel P to reduce space and silence the application. The same pin of microcontroller drives mosfet through a totem pole connection. Mosfet switches a 40W DC lamp. Until this point everything works fine.

The problems is: when the lamp is in a short circuit situation, the mosfet BURNS! My application can't afford a high side drive solution. I can't use fuses too, so I need an automatic solution.

Is there another way to protect Mosfet?


Best Regards,

 

[andre_luis]

You can employ some circuit at power side for sensing the current so that can limit it at a level that fits on MOSFET SOA region. As a first approach, try adopt some solution as that:






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[Mr. Ohm Smith]

Hi, Andre. This is the circuit in details:

 

[dick_freebird]

Perhaps your uC can provide the "smarts" to fo short circuit protection,
if you allow it to sense the lamp voltage or current. For example you could
put a resistor divider from the FET drain to ground with the tap suitably
scaled for an uncommitted digital I/O to "sniff". But this might still have
some issues, like how shorted is the short - i.e. if the "short" is 0.1 ohms,
the FET might still produce >10V across it (many are <10mOhm Rds(on))
and you'd be pushing 100A while still not distinguishing the fault. Current
is a better indicator but requires either that you waste significant
headroom, or add come complexity (gain and thresholding of a lower
amplitude, high side referred sense signal).

But anyway, if you can make a uC-digestible proxy for current then
the short circuit can be realt with algorithmically.

 

[andre_luis]

Perhaps your uC can provide the "smarts" to fo short circuit protection,
if you allow it to sense the lamp voltage or current...

In fact, some uC core families have built-in analog comparator hardware modules with which system can be able to perform such detection with no additional circuitry needed besides above referred resistor divider.



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[vinodstanur]

As dick_freebird suggested at the end, best way is to monitor the current using same MCU.
If you are keeping a series 0.01 ohms resistor of >= 0.5W in between the ground and load (40W bulb), it should drop around 33mV across it. This you can easily read using an ADC and can keep a threshold for 50mv or greater so that you can do over load protection also along with short circuit protection. Ones over load is detected, you can periodically PING the load for very short duration to check if it is still shorted or not.

I have one more suggestion,
Instead of ADC, you can you hardware Analog comparator in MCU which can give you an interrupt on compare match, so that your application code running in the controller will not be disturbed when it is not shorted/over load.

 

[Mr. Ohm Smith]

I already tried some similar solution. I can't use this "shunt" resistor (0.01 ohms), cause the car lamp and its ground are far from my circuit.

So I tryed a uC pin to read the overload, but I think I did it in the wrong point. Could detect overload and act in 80 us but MOSFET didn't turn off. Seems that already was in a short circuit state and just can't turn off.

The far I can read are the MOSFET pins. Does anyone have any idea?

 

[andre_luis]

I already tried some similar solution. I can't use this "shunt" resistor (0.01 ohms), cause the car lamp and its ground are far from my circuit.

Don´t matter how far the load is placed from MOSFET, due measured current flows at the same net. You can use this shunt resistor close to MOSFET device.

So I tryed a uC pin to read the overload, but I think I did it in the wrong point. Could detect overload and act in 80 us

I personally consider Analog comparator a better approach rather than an A/D, which requires some valuable timing for achieve conversion.


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[grittinjames]

Hi,
Please find Maxims Application note,
It will do what you are looking for,
https://www.maximintegrated.com/app-notes/index.mvp/id/2158

There are many similar products available for high side drive with shortcircuit protection, LT also have some chips
PROFET™: Smart High Side Switches from Infeneon also do same purpose

Grittin

 

[andre_luis]

Hi,
Please find Maxims Application note,
It will do what you are looking for,
https://www.maximintegrated.com/app-notes/index.mvp/id/2158

That´s a good solution you suggested :

https://www.maximintegrated.com/images/appnotes/2158/2158Fig02a.gif

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[Mr. Ohm Smith]

That´s it! A low cost solution to avoid a high side drive.

I'll try this and later I'll post a feedback to tell if it works fine.

Thanks everybody!

 

[FvM]

A current limiter doesn't fully protect the switch if it isn't designed to dissipate a large power continuously. You usually need a means that detects and disconnects the short after e.g. a few ms.

 

[Mr. Ohm Smith]

I already tested some high side drives and PROFET™. They are really fantastic devices but my application can't afford its costs. Due to this fact, I had to implement an analog circuit to protect Mosfets.

The circuit suggested by grittinjames: worked just fine after some adjusts.

It takes 200 us from the short circuit until the microcontroller turns off the mosfets. The hardest part is to adjust the shunt resistor power and value to your specific load. Now it is working perfectly.

Thanks everybody!