Current limiter. Would this design work? LTspice simulation

Thread Starter

vggrek

Joined Aug 5, 2018
10
Hi, I made a circuit based on a circuit of the member @crutschow The goal was to have an adjustable, low drop voltage, current limiter independent from the sense resistor value. A ZXCT1010 current sensor and a transistor replaced the trigger in the original circuit. I had to add a cap because of the oscilation. The current limiter has to not blow up on a short circuit and keep limiting the current for max 5s (in fact I will add a circuit to shutdown after max 100ms). The current limit would be up to 7A. Input voltage max 20V. The LTspice files, datasheets, attached.
Would this work? Is the simulation correct or it's a gigo situation?
The current spikes are dangerous? Can they be avoided, suppressed, flattened?
The power supply has any other limitations except the max power, (max output voltage and max output current)? Can this circuit be placed after a non isolated buck converter?
Is the FQPF27P06 a good choice?
Have a happy new year.zxct1010_281220_current_limiter_the best till now_other mosfet.png
 

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crutschow

Joined Mar 14, 2008
27,220
Why do you have R3 and R4 limiting the MOSFET Vgs ON voltage to 40% of the supply voltage?
The minimum Vgs for the FQPF27P06 to be fully on is 10V (see below).
Just select their value to keep the maximum Vgs below 20V.

1609427396462.png
The current spikes are dangerous?
I didn't simulate the circuit but current spikes are normal due to the finite operate time of the circuit.
I don't see that as a problem, assuming the circuit purpose is to protect the voltage source from long term over-current.
Can this circuit be placed after a non isolated buck converter?
Yes.
Is the FQPF27P06 a good choice?
Looks okay to me.
 

Thread Starter

vggrek

Joined Aug 5, 2018
10
Why do you have R3 and R4 limiting the MOSFET Vgs ON voltage to 40% of the supply voltage?
The minimum Vgs for the FQPF27P06 to be fully on is 10V (see below).
Just select their value to keep the maximum Vgs below 20V.

View attachment 226382
Thank you for the immediate response. My fault, wrong calculation. R3>R4*Vgs(on)/(Vs-Vgs(on)), so I chose R3=20k, R4=5k.
(Vsource=13V, Vgs=-10.3V; Vsource=20V, Vgs=-15.9V)
I replaced the Rpot with a potentiometer Rtot=1k, w=0.26 and I ended with an "LTspice error: time step too small", when the short circuit happens. I ignore it :).
I want to feed an mcu analog port from the ZXCT output, before the pot. Should I use an opamp, as a buffer, to not drain current?
 

Alec_t

Joined Sep 17, 2013
12,071
I haven't simulated your circuit either, but it looks to me as though when Q1 turns on there will be excessive Q2 base current unless a current-limiting resistor is added.
 

Thread Starter

vggrek

Joined Aug 5, 2018
10
I haven't simulated your circuit either, but it looks to me as though when Q1 turns on there will be excessive Q2 base current unless a current-limiting resistor is added.
Thank you, yes exceeds. I googled and I found maxIb 5ma, peak 10ma. A 3k base resistor gives a peak Ib(Q2) 4.5mA. Is it good?
I would appreciate a lot if you find time to run the simulation.
 

Alec_t

Joined Sep 17, 2013
12,071
I ran your sim and the circuit inherently has a maximum Q2 base current of ~33uA. I'm surprised it's that low, but it seems no additional resistor in series with the base is required.
 

Thread Starter

vggrek

Joined Aug 5, 2018
10
@crutschow @Alec_t
Attached the circuit with the changes till now. (R3,R4 to have the mosfet fully on, R5 added to limit the Ib of the Q2)
One image has the simulation output with the R5=10k base resistor and one without base resistor. Values of the Ib are included.
The red plot is the variable load. When the Isense goes up smoothly the Ib is safely low. The problem appears when the short circuit happens. Have I to use a bigger base resistor?
The Isense plot is cropped. At the short circuit moments at 100ms and 200ms the Isense goes up to 220A! Hope the reality would be more merciful.
circuit_revised.PNGNO base resistor.PNG10k base resistor.PNG
 
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