Current limiter using MCU

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gauravkothari23

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Hi all.
I have a project where i have to drop the current from 0.7A to 5A as per the user selection. (Circuit Attached).
As per the circuit. i have interfaced 3 buttons, SET, UP and DOWN for the user to select the amount of current to be grounded.
Am using 220V to 12V SMPS and 7805 Regulator to power the MCU (Not show in circuit diagram)
110V DC INPUT is an external input from where the voltage input would be 110V DC and 25 to 50 Amps of Current.
what my purpose is as soon as the i receive 110V DC input, 0.7A to 5A as per the user selection has to be straight away grounded, for which i am using Mosfet Q1. and shunt resistor. and rest of the current will be sent to 110V DC LOAD OUTPUT.

I am using U3(PC817) to detect 110V DC Input.
I Will control the current to be grounded using PWM and will monitor the current using shunt resistor using ADC pin of MCU.
What my problem is, when the mosfet gets ON during PWM, huge amount of current will flow through the mosfet which will OFF the 110V DC LOAD OUTPUT for a specific period of time.
Can any body please recommened me any idea for how can i do this. or what kind of circuit has to be implemented.

 

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PWM filter calculations, see attached. And a cancellation technique.


Regards, Dana.
 

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  • Cancel PWM DAC ripple with analog subtraction _ EDN.pdf
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Hi All,
have tested the system as per the circuit. it works well where i can adjust the current across mosfets from 0 to 5A as per the requirement.
but again i am facing a small issue. instead of 110V DC, i am using 12V DC as INPUT (just for testing) which is being drived straight to GND using 4 Mosfet. Total current across mosfet is 1 Amps. so around 300mA across each mosfet. but within a few minutes around 2 to 3 minutes, the mosfet gets extremely HOT around 110 Degree Celsius.
why is it that even at such a small current and low watts across each mosfet, the mosfet is getting extreme HOT.
 

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KlausST said:
Hi,

Quite expectable.

Did you use a big heatsink and a fan? .. as recommended already in post#4

Klaus
Click to expand...
not yet.
Because currently for testing purpose i am using 12V 1 Amps only across 4 mosfets, which makes 300mA per mosfet at 12V which is approx 3.6W per mosfet. so as far as i think, for 3.6W i dont nead a FAN for cooling.
Later ON, once the things works perfectly, i will use to with 110V DC.
 

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Hi,

What do you want to hear?
I can only repeat: because of the dissipated power.

So it seems you don´t believe in my posts, don´t believe in the datasheet, don´t believe in what you see.
It´s difficult to give good advice.

--> Install a suitable heatsink, maybe with fan.

Klaus
 

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crutschow said:
Where do you think that 3.6W of power is going to go?
Look up the thermal resistance to air, without a heat sink, of the MOSFETs you are using,
Click to expand...
The datasheet shows
Thermal Resistance, Junction-to-Ambient -- 40 °CW.

does it mean that per watts dissipated through mosfet, the temperature would rise to 40 degree C
--- Updated ---

i have used the same circuit in 2 of my other applications which are motor drivers.
one is 12V 25 Amps motor where i am using IRF3205 4 in parallel. which i am using without heatsink and even at full load, the mosfet does not get hot greater than 45 degree.
other i am using is 180V 3 Amps motor where i am using 20N60 mosfet 6 in parallel. even here i am using without heatsink where the temperature does not rises above 60 degree even at full load.
the circuit that i am using for both motor driver is the same as i am using in current circuit. but op amp is not used. driving the mosfets straight using microcontroller using PWM.

so why in this case the mosfets are getting extremely hot even at 3.6W of consumption. i agree that datasheet says Thermal Resistance, Junction-to-Ambient -- 40 °CW.
Then why are my mosfets using in motor drivers does not get extreme hot even at arround 100W of dissipation.
 
Last edited:

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gauravkothari23 said:
does it mean that per watts dissipated through mosfet, the temperature would rise to 40 degree C
Click to expand...
Not exactly.
It means that each watt dissipated will raise the junction temperature 40°C above ambient.
So if it's dissipating 3.6W and the ambient temperature is 25°C, the junction temperature would be 3.6*40+25 = 169°C (a little toasty ).
 

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12V maybe is the motor voltage, maybe the power supply voltage ... but it can't be the voltage across the transistor while the transistor current is 25A.
It's impossible. You can't fool physics.
You tell this, but you miss to give a detailed schematics and the according operation conditions.
So indeed we can't verify your informations ... making detailed discussion impossible.

P (heat) = V x I is always true and it does not matter whether you do this with a resistor, diode, BJT, MOSFET...

In post#4 I wrote:
Mind: the MOSFET will generate 550W of heat. This is a lot. It may be enough to heat your room.
Click to expand...
And this exactly is how I meant it. It will heat your room significantly. It also will increase your electric bill.

gauravkothari23 said:
Then why are my mosfets using in motor drivers does not get extreme hot even at arround 100W of dissipation.
Click to expand...
This simply is impossible. Provide complete operation conditions.

Klaus
 

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gauravkothari23 said:
Then why are my mosfets using in motor drivers does not get extreme hot even at arround 100W of dissipation.
Click to expand...
It's rarely 100W power dissipation in the transistor. You are apparently ignoring the difference between linear and switched mode operation.
 

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OK, got it, Sorry it was my mistake.
i got confused with dissipation in the load and across transistor. now i clearly understood.
the 12V motor driver circuit what i am using has a motor as a load and has a voltage drop approx 12V across motor, so the voltage across mosfet would be very low.
for Eg.
Power supply i am using is 12V
Voltage drop across Motor = 11.50V
Voltage across Mosfet would be 0.5V
so power dissipation across mosfet would be 0.5V x 25Amps = 12.5W
and the same in my existing circuit voltage across mosfet itself is 12V so power disspiation would be 12V x 1 Amps = 12W
Am i right..??
 

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