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MyCircuit

I need some advice on the circuit I plan on building.

This circuit is to run any high current (Max. 100amps) device in my car. I want to use a high side MOSFET to supply power to any field device. An Arduino will be used to provide a PWM signal (PWM_Out1). The Arduino will also be used to read the current through the shunt R23. INA169 is used to amplify the shunt signal and provide it to the Arduino via AI1. DI1 is also run back to the Arduino and it provides the status of the fuse F1. D4 is just used as an indicator if the fet is on. D10 id my flyback diode. TP1 Jumper is used just to trigger the fet in the field for testing purposes. I used two NPN transistors to have the same logic from the fet that the Arduino provides. D7 is used to limit Vgs voltage to 12v The 24v supply is being provided by a buck/boost circuit (still working on that).

I am definitely not an engineer but I have done a lot of reading over the last few days.

Any comments are welcome.

circuit circuit

OK, I changed the circuit to fix some of the issues. Now it seems like there is an issue when the fuses blow, current id drawn through the Vgs zener. In the case below, if the fet is being told to conduct and the fuse blows, my blown fuse indicator still remains high. Any idea how to resolve this? My blown fuse indicator would get connected between the 1k resistor and the 5.1v zener diode.

Circuit

Here is the revised circuit now. Just trying to figure out the feedback through the fet when its told to conduct and the fuse is blown. My blown fuse indicator @D1 still remains high in this condition.

Anton Menshov
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jonfarrugia
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  • The INA169 high side current sense amplifier ground pin (2) is disconnected from ground. The blown fuse detector circuit is a bit unnecessary, why not use a plain old voltage divider to get 0 - 5 V? D10 isn't going to protect your MOSFET from inductive transients generated by the load, if that's your aim. It needs to be in parallel with the load (not the switch) to act as a freewheeling diode. The gate driver is fine for just holding the gate high but likely way too slow for PWM at any reasonable frequency. – jms Mar 10 '16 at 05:09
  • What's the point of the 24V supply? If you want to drive that Pmos correctly, just use a transistor to pull it low to conduct. Your 24V supply will only just exceed the +-20V rating and fry your fet. Other questions worth asking youself: Do you want inrush limiting? Do you really need high side control or would low side be sufficient (low side is cheaper so bother to look into it)? Why frequency are you going to drive this at? Will you need a MOSFET driving circuit to turn the gate on and off quick enough? etc. etc. – Dave Mar 10 '16 at 05:19
  • Good catch on the INA169 Grounding issue. – jonfarrugia Mar 10 '16 at 05:29
  • @Dave, IRF2204 is an NMOS. – The Photon Mar 10 '16 at 05:36
  • I wanted to use a blown fuse detector with transistors because the plan is to have the 12v supply adjustable to 24v. using the transistor i would just switch the 5v supply on/off to the arduino input. So I'll move D10 from ground to OUT1. The mosfet will need to be cycled at a frequency that is outside the audioable range. So I was thinking around 30khz. I am using the 24v supply to drive the nchannel mosfet as a high side switch. Pfets with this amperage rating are very expensive. – jonfarrugia Mar 10 '16 at 05:40
  • @ThePhoton, that literally changes none of my comments except that you now pull it high to turn on. It's still wasting 24V for no reason. Plus there's still all the design questions to answer. And now he has the added difficulty of using an NMOS as a High side controller (admittedly this isn't a problem due to the low voltage, but with a truck [24V] this could be a bit more of a pain). – Dave Mar 10 '16 at 05:46
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    @Dave, you want to pull it well above 12 V to ensure it's fully switched on, particularly if you're going to run 100 A through it. Rds(on) is specified with 10 V Vgs, so you need at least 22 V to reach the datasheet operating point. 24 V gives you some margin of safety and isn't at all out of line for this kind of design. – The Photon Mar 10 '16 at 05:50
  • @ThePhoton, The data sheet clearly states a Vgs of +-20V, 22V means fried, 24V means fried faster. I mean if you really want to turn it on I could see 18V with good clamp for safety. Or am I looking at the wrong datasheet? – Dave Mar 10 '16 at 05:56
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    @Dave, The source isn't tied to ground. Vgs will be 24 - 12 = 12 V, and 12 is less than 20 in my neighborhood. – The Photon Mar 10 '16 at 05:57
  • Obviously when you're working with 100 A if something gets messed up (load shorted to ground, for example) things can go pear-shaped pretty quickly. – The Photon Mar 10 '16 at 05:58
  • @ThePhoton, It shall be a race! Which will blow first? The fuse or the Mosfet on short circuit? Or both? – Dave Mar 10 '16 at 06:01
  • @Dave, I'm not betting on the fuse...But it's still a 3-way race between the FET oxide breakdown, the channel burning up and the shunt resistor catching fire. – The Photon Mar 10 '16 at 06:15
  • @Dave the zener from gate to source wouldn't allow for exessive gate voltage, especially considering the weak 10k pull-up. – jms Mar 10 '16 at 06:32
  • @jms, the irony being that the 10k resistor is why his circuit won't work at 30Khz. That 10K resistor limits his turn on time to something like 31.7us (using 24V 0->10Vgs, 10k, 5890pF[from datasheet]) (which is longer than a 30khz cycle). This assumes that there is no change in the voltage level of the source (the source doesn't rise, slowing down the turn on time), so this is a **best case** number. Any resistor he picks that would let him have faster turn on would then ruin power consumption in the "off" state and would challenge the abilities of many Zener diodes thermal dissipation limits. – Dave Mar 10 '16 at 13:15
  • @Dave I acknowledged in the very first comment that "The gate driver is fine for just holding the gate high but likely way too slow for PWM at any reasonable frequency", I was just wondering why you kept writing that Vgs would reach excessive values. – jms Mar 10 '16 at 13:52
  • @jms, I was thinking about the negative voltage possibility on turn off (Q4 brings gate to ground quite quickly). But looking at the design more, I decided that even that was unlikely to blow the mosfet from excess Vgs. That's kind of why I called it ironic instead of showing you that you were wrong, cause you weren't. Now, on the odd possibility that the load is highly inductive, he might need another zener in series facing the opposite ways to keep the negative Vgs limit. – Dave Mar 10 '16 at 14:09
  • I am going to guess that you want a gate driver that can provide 1A or more to have a more reasonable switching time for 30KHz PWM. Drivers with internal charge pump can usually operate at all duty cycle, but the internal charge pump is usually weak and therefore the switching speed is slow. Driver using bootstrap are restricted certain duty cycle. But if you are to have a higher than main supply voltage, then the bootstrap voltage can be supplied easily with a regulator and not rely on the a bootstrap boost capacitor from switching. – rioraxe Mar 11 '16 at 05:42
  • To fix some of your indicators, one way is to add zener diodes to get the thresholds to where you want them to be. Since the thresholds you need don't need to be very precise, another way is to use a resistor voltage divider and the Vbe of a transistor as the threshold to generate indication that you need. – rioraxe Mar 11 '16 at 05:48

1 Answers1

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Even if R23 is 0.001 ohm, it will dissipate 10 W at 100 A. If you don't know what Kelvin sensing is, you won't be able to get this circuit to work.

You don't need D10.

jp314
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  • Yes. I see your point about my shunt. How else would I measure the current? – jonfarrugia Mar 10 '16 at 05:49
  • @jonfarrugia A 0.1 milliohm shunt would dissipate 1W. A hall effect current sensor would allow for lossless, isolated current measurement. – jms Mar 10 '16 at 06:29
  • OK I'll be using the ACS758 IC capable of 150amp using hall effect. It was mentioned that the mosfet driver may not handle the pwm frequency. What can I do so that it will handle 30khz? – jonfarrugia Mar 10 '16 at 07:12
  • @jonfarrugia, the reason your circuit won't work at 30KHz has to do with your 10k pull up resistor. It can't turn on your gate fast enough. Here is a post about some circuits that can be used to improve turn on/ turn off time. http://electronics.stackexchange.com/questions/137090/bootstrap-circuit-for-high-side-mosfet-driver – Dave Mar 10 '16 at 13:27
  • I looked at that posting and those IC drivers are great but in this application I need the entire PWM range of 0 to 100%. So I cant use an IC with an internal charge pump. I was looking for a High side driver Didn't contain a charge pump and allowed for an input pwm signal of 0-100%. However, I wasn't able to locate one. Do you know of one? – jonfarrugia Mar 10 '16 at 15:41