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For a project, I need a means of switching a high-current circuit between two inductors. The purpose is, each inductor serves as an electromagnet, and they are switched every 15s or so in order for one of the two to cool before starting again. The system I've designed uses arduino as a controller and a couple of MOSFETs wired inline in order to electronically switch the circuit. The current delivered to each inductor should be around 75A at 12VDC. In this specific example, I used three 35A rated MOSFETS in each 'bank' which would function similar to a single relay. Here is the schematic:

Designed Schematic under review

Bank B of MOSFETs is a splice from green line to Bank C. The idea is that Bank A will act as a supply switch (Open = NO Power), Bank B (Closed = IND1 ON, IND2 OFF), Bank C (Closed = IND1 OFF, IND2 ON). The supply switch acts to isolate IND2 from IND1. To the contrary, with not having the supply switch and splice, the only way I could supply power to the second inductor is if I were also supplying it to the first. That is why I have included the leftmost third bank instead of a simple 2, since I need to bypass power past the first inductor.

I am asking for a few aspects:

1) I feel I need more diodes in the circuit to protect against kickback. Have the diodes been added in the right places?

2) Are all the connections correct for the switches and the circuit to function? (Specifically the MOSFET banks and Source/Drain)

Here is an image i've found of an example NCH circuit:

Example N-Channel MOSFET Circuit

It would appear from this image, that I do in fact have the connections of source and drain for each MOSFET wired correctly, with source on the positive end and drain on negative end. Please clarify if it is wrong.

3) Is there a solution to the VGS issue described, of the MOSFETS not switching simultaneously?

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  • \$\begingroup\$ Both inductors are in parallel. You are not switching between them. Also you schematic would benefit from shorter wires so we can see the components better. \$\endgroup\$ – Oldfart Aug 6 '18 at 20:20
  • \$\begingroup\$ And use ground symbols to eradicate all the 0 V lines. You could also tie three gates together and use one gate resistor for each set. Draw the transistors the right way up so that current flows from top of the schematic to the bottom. \$\endgroup\$ – Transistor Aug 6 '18 at 20:26
  • \$\begingroup\$ I suspect that your cool time will be much longer than your heating time unless you have some sort of forced cooling. Is there a reason you haven't designed one decent inductor? You are also operating right on the 25 A limit of the MOSFETs. I'd go for four or five in parallel and make sure you've done your thermal calculations. \$\endgroup\$ – Transistor Aug 6 '18 at 20:29
  • \$\begingroup\$ Posted Edits and updated schematic. Thoughts now? \$\endgroup\$ – user193912 Aug 6 '18 at 21:25
  • \$\begingroup\$ Sorry, the schematic got worse. The transistor sources need to connect to ground, but your green wire is floating. You have also flipped the source and drain for some of your transistors. Forgive me, but I'm not sure you are ready for this project. \$\endgroup\$ – Elliot Alderson Aug 6 '18 at 21:36
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The red "wire" that connects from the negative end of the 12V supply and runs horizontally to the drains of the MOSFETs should not be there. The drains of the three MOSFETs should be connected together and to a solenoid, but not to the 12V supply.

If the MOSFETs are rated for a maximum current of 25A I would recommend using at least four in parallel to drive 75A.

There may be many other problems with your circuit but you haven't provided enough information to be any more specific. And please, please, clean up your schematic to reduce the excessive white space.

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I am asking for a few aspects: I feel I need more diodes in the circuit to protect against kickback. Please describe where they should be added?

No, the place where you need the diodes is on the flyback for the inductor as shown. Make sure the diodes can handle the current/voltage from the coils which can get quite large. You may need to parallel diodes if they get hot or burn out. (without knowing the flyback current\voltage it will be hard to say how many you need)

I am wondering if I have all the connections correct for it to function. (Specifically the MOSFET banks)

Aside from what's aready been said, the mosefets connected to the red wire are not necessary and are redundant. You can already switch the loads, and a switch for the switches seems redundant and only adds complexity

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  • \$\begingroup\$ And you are positive the only diodes needed are the ones on the inductors as shown? I would assume there might be diodes on the MOSFETs or the negative line of the circuit. \$\endgroup\$ – user193912 Aug 6 '18 at 23:13

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