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Current limiting the load

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xmen_xwk

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I like IR2121 low side driver, because it has current sensing inside. But the problem is, to limit different currents, I need to keep replace the sense resistor. Which is kind of annoying or tiring. Using voltage divider on sense resistor is not what I want. Instead I want to replicate current sensing of IR2121 and change reference voltage for limiting the current.

I have drawn this schematics, which uses IR2110 driver for now. But the problem is HC08 turns out on as input reaches 2v. While it should be on around 4.8V.

I'm also wondering, is there anything I'm doing wrong ? Any suggestions would be nice.

Thank you.

wu4cxv.png
 

But the problem is HC08 turns out on as input reaches 2v. While it should be on around 4.8V.
Click to expand...
HC series threshold voltage range is specified between 30 and 70 % of Vdd, 1.5 to 3.5 V with 5V supply. 2.2 V is just O.K.

If PulseIn has a higher voltage level than 5V CMOS, a voltage divider may be needed.

An overcurrent limiting circuit should have a kind of latching behavior, e.g. keeping the MOSFET off during the present gate pulse. Otherwise the circuit can result in repeated fast on- and off-switching, destroying the MOSFET by respective switching losses.
 

FvM said:
An overcurrent limiting circuit should have a kind of latching behavior, e.g. keeping the MOSFET off during the present gate pulse. Otherwise the circuit can result in repeated fast on- and off-switching, destroying the MOSFET by respective switching losses.
Click to expand...
I do not understand "keeping the MOSFET off during the present gate pulse". Could you please explain bit more ?
 

Perhaps you should check the circuit behavior with tripping current limiter circuit and decide if it's acceptable as is.
 

FvM said:
Perhaps you should check the circuit behavior with tripping current limiter circuit and decide if it's acceptable as is.
Click to expand...
but to check I will have to put all on breadboard, and I'm not sure about the drawn circuit. Like there should be hysteresis on lm393 to avoid switching mosfet like crazy, but I cant seem to find a way to do that. As if I do the reverse higher voltage might damage the ref voltage source.

Is there anything I should really need to fix ?
 

Hysteresis won't avoid fast on- and off-switching in case of short circuit, only slow down the frequency a bit. Better use a D-FF: asynchronous reset connected to comparator, synchronous set by next gate pulse.
 

FvM said:
Hysteresis won't avoid fast on- and off-switching in case of short circuit, only slow down the frequency a bit. Better use a D-FF: asynchronous reset connected to comparator, synchronous set by next gate pulse.
Click to expand...
yes I want to slow it down, even in case of short circuit. But do not want to switch it off for rest of the pulse.
 

But do not want to switch it off for rest of the pulse.
Click to expand...
Your decision. In this case you should determine the safe maximal switching frequency.

Your circuit shows a resistive load which doesn't limit the current rise and fall time. In a real circuit, parasitic circuit inductance will still achieve finite rise/fall time.
 

FvM said:
Your decision. In this case you should determine the safe maximal switching frequency.

Your circuit shows a resistive load which doesn't limit the current rise and fall time. In a real circuit, parasitic circuit inductance will still achieve finite rise/fall time.
Click to expand...

I think adding a hysteresis would solve fast switching issue ? But since the output of comparator is pulled high, how do I add a hysteresis to lower the ref voltage ?
 

Hi,

As FvM said in post#6:
Hysteresis won't avoid fast on- and off-switching...

I recommend to shut off until new rising PWM input signal. You could do this with a simple flipflop and an AND gate.
There are "tiny logic" or "single logic" ICs.

******
Hysteresis is positive feedback: from output to IN+.

Klaus
 

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