Hi, this is my first post in arduino forum.
I'm currently trying to build a coilgun. I've try the circuit and basically it's working when using the light bulb as load and with 300V 100uF cap. But when using the coil and a large cap like 220uF cap, it burnt my mosfet or IGBT every time. So now I have no idea where is the problem. Perhaps I need a higher current Mosfet or IGBT?
The schematic is attached.
"Perhaps I need a higher current Mosfet or IGBT?".
Didn't you try this?
Paul
The IPP60R099CP continuous drain current is 31A and the peak current is 93A. I'm not sure if this is high enough for my circuit.
Please correct me if I'm wrong. So, my cap is 300V 220uF there will be around 9.9J energy stored in the cap. The power dissipation in 20ms will be 10J/20ms = 495W, current flow through the load and the mosfet will be 500W/300V = 1.65A. Base on the calculation, the mosfet is quite sustainable. So, I just wanna know is there any circuit problem other than trying higher current mosfet?
Thx
Have you watched the circuit performance with an oscilloscope?
Paul
Actually, I don't have a oscilloscope. Maybe I have to get one. Just want some more advice before getting the oscilloscope.
Ah, the R2 in the schematic should be 1k ohm, my mistake. I use 1k ohm when I test the circuit.
Still thank you.
Why is there even a gate resistor when using a gate driver? I notice it's indeed in the datasheet's typical application circuit, but can't find a value for it.
Why are there no decoupling caps on the gate driver's pin 6, as recommended by the data sheet?
wvmarle:
Why is there even a gate resistor when using a gate driver? I notice it's indeed in the datasheet's typical application circuit, but can't find a value for it.Why are there no decoupling caps on the gate driver's pin 6, as recommended by the data sheet?
I've often wondered why a gate resistor is used with a gate driver IC - it is not well documented, but it is there to limit the current that the gate driver supplies.
The IR4427 can source 1.5A (minimum) / 2.3A (typical). The resistor is used to limit this current to the rated value, so a resistance of around (12V/2.3A) ≈ 5Ω would be used.
The decoupling capacitors should be fitted for correct operation.
johnny2arduino:
The IPP60R099CP continuous drain current is 31A and the peak current is 93A. I'm not sure if this is high enough for my circuit.
Please correct me if I'm wrong. So, my cap is 300V 220uF there will be around 9.9J energy stored in the cap. The power dissipation in 20ms will be 10J/20ms = 495W, current flow through the load and the mosfet will be 500W/300V = 1.65A. Base on the calculation, the mosfet is quite sustainable. So, I just wanna know is there any circuit problem other than trying higher current mosfet?Thx
Your method of calculating the current is seriously flawed.
You have assumed that the capacitor will supply a constant current for the whole duration of the 20ms input pulse, and only then will the charge in the capacitor become depleted.
This is not the case, the current will be much higher and the capacitor is likely to be fully discharged long before the end of the 20ms pulse.
In practice the peak current will be much higher and only limited by the resistance/inductance of the coil (plus any losses in the wiring and MOSFET).
Have you measured the resistance of the coil?
My guess is that it is only a few ohms, and that you may have hundreds of amps flowing.
Also you need to consider the fact that you have a parallel LCR circuit, and that the current is likely to be oscillatory.
JohnLincoln:
This is not the case, the current will be much higher and the capacitor is likely to be fully discharged long before the end of the 20ms pulse.In practice the peak current will be much higher and only limited by the resistance/inductance of the coil (plus any losses in the wiring and MOSFET).
Have you measured the resistance of the coil?
My guess is that it is only a few ohms, and that you may have hundreds of amps flowing.Also you need to consider the fact that you have a parallel LCR circuit, and that the current may be oscillatory.
You are right. I've thought the discharging time question yesterday. The coil resistance is just 1 ohm, maybe I should add a resistor to limit the current discharging from the cap.
Parallel LCR circuit looks interesting. Thank you for the advice.
johnny2arduino:
You are right. I've thought the discharging time question yesterday. The coil resistance is just 1 ohm, maybe I should add a resistor to limit the current discharging from the cap.Parallel LCR circuit looks interesting. Thank you for the advice.
I don't think that adding a resistor to limit the current is the right way to go - it will only make the circuit less efficient, due to the power dissipated by the resistor.
Surely the whole object of the exercise is to get the current as high as possible?
The peak current will be way lower than the 300A the 1 Ohm resistance and 300V suggests due to the inductance of the coil: this strongly resists the current jumping from zero to 300A.
If you know the inductance of the coil itshould be possible to calculate the current profile as the cap discharges.
JohnLincoln:
I don't think that adding a resistor to limit the current is the right way to go - it will only make the circuit less efficient, due to the power dissipated by the resistor.Surely the whole object of the exercise is to get the current as high as possible?
Yeah, that's the problem. I want to keep the circuit efficient but don't want to burn the mosfet.
Does it make sense if I use multiple mosfets in parallel to divide the current flowing through each one?
wvmarle:
If you know the inductance of the coil itshould be possible to calculate the current profile as the cap discharges.
Maybe this is another way.
johnny2arduino:
Does it make sense if I use multiple mosfets in parallel to divide the current flowing through each one?
Yes.
You can easily add a second one to the other output of your MOSFET driver.
Do remember to reduce the resistor value! That 1k resistor is still 200 times too big, 5Ω sounds much more sensible to me, using a 1k resistor renders the whole gate driver rather useless. You want to get that MOSFET to switch on really really fast, as otherwise you probably lose a lot of your capacitor's energy there, and that may indeed be what burns your MOSFET.
Definitely reduce the value of gate stopper resistor as reccomended above. Large values will increase the rise and fall times and losses/heat. Could be even more important when driving devces in parallel.
There is good information to be found googling mosfet driving, gate reistors etc, but as a head start these two are very informative, and one has a footnote regarding high supply voltages.
Also it would pay to look at the mosfet data sheet and see if you are within safe operating area for your device - Vds, on time, Id, It could be surprising what you find.
A "snubber" may also be needed.
Layout/wiring is also important, not only is the voltage dangerous, there is likely more than enough energy to kill if in contact so all wiring must be safe; wiring lengths must be kept as short as possible as any wires add extra inductance which can cause parasitics and unwanted coupling. Follow all reccommendations that can be found for similar circuits.
just an observation....
we typically warn people of the near-death experience with mains voltages of 120 volts at 20 amps.
so, 300 volts and 300 amps is not getting eyebrows to lift ?
the kid in me want to see the working video on youtube !
dave-in-nj:
so, 300 volts and 300 amps is not getting eyebrows to lift ?
As opposed to the projectile he is launching /smile.
OP has been working on it for a while and is still alive, so I do have the feeling OP knows what they're doing, including whatever they use for charge pump (300V is not a direct mains connection).
With many posters that are asking about mains connected projects I don't get that feeling.
I appreciate all the advice. I understand high voltage and current are very dangerous. So I will be very careful when implementing the project.
Thank you all 
