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Old 09-09-2010, 12:09 AM   #3741 (permalink)
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This is a very interesting thread. I guess I've been working in a vacuum too long. I always have to learn the hard way anyway. I doubt anybody's interested in an oversimplified and feature-free controller but I thought I'd post a short clip of the waveform of the pre-amp stage (this feeds into the power mosfets). It is running at 20khz. I wish I could show you the final mosfet stage but I burned my test mosfet up and I'm waiting on replacements.


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Old 09-09-2010, 12:23 AM   #3742 (permalink)
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ok read the whole thing!!! yep took a few days. I think you should get a prize for reading the whole thing!! Great work to all that have added to the mix. I wanted to post so many times while reading but then I would look at the date and it was like a year ago. Thanks for taking on the controller project. It will be a great help to many of us who are building our own EV's
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Old 09-10-2010, 10:19 AM   #3743 (permalink)
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I apologize for my ignorance. I tried reading quite a bit of this thread. I still can't figure out why the diodes are necessary between the drain and V+ ??
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Old 09-10-2010, 11:40 AM   #3744 (permalink)
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to Princeton: As I understand it, you don't need the diodes if you are using a resistive load such as a light bulb; but when using a large inductive load such as a big motor, you need the diodes. As Paul says, it's like a water hammer... when the mosfets shut off, there is a large spike/lump of current that wants to keep going, and the diodes let it loop back to the motor instead of pounding through your mosfets.
Paul, if this is not right, please correct me!
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Old 09-10-2010, 12:54 PM   #3745 (permalink)
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OK, I will make this the simplest explanation there is. lol

Have you ever turned off the faucet really fast after running the water full blast? When you do this the pipes bang. Same thing. Its the water abruptly coming to a stop.

This the just what happens with the voltage when you turn off during each cycle of the pwm. The voltage spikes and smash's into the closed gate of the mosfet. The diode is basically a pressure release that allows the voltage to flow back into the battery and not pound the mosfet to death.

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Old 09-10-2010, 02:13 PM   #3746 (permalink)
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Quote:
Originally Posted by princeton View Post
I apologize for my ignorance. I tried reading quite a bit of this thread. I still can't figure out why the diodes are necessary between the drain and V+ ??
edown has it down pretty good. hehe. here's more info:
Flyback diode - Wikipedia, the free encyclopedia

Should try it: get a motor (or maybe a relay solenoid, something that's inductive) and a battery. Maybe an automotive relay and your car battery. Connect and disconnect the load to the battery and you'll see a spark when you remove the load. That spark is caused by a voltage that is high enough to break down the air and conduct electricity. the high voltage is induced by the inductance of the load and the changing current when the battery is disconnected.

I think you also asked about placing the diodes near the motor vs in the controller. while the motor may have the bulk of the inductance, the cables themselves that go to the motor from the controller have a finite inductance too (often called parasitic inductance). You can probably search the 'net to find equations for inductance of a wire. If the diodes are placed at the motor, the current in the cable will still experience a sudden change in current that is unclamped by the diodes. Coupled with the inductance of the wire, a large voltage spike will be produced and possibly take out the mosfets.

it's vitally important that the mosfets, diodes, and input caps are close together. draw your circuit and highlight the wires (or pcb traces) that are conducting current when the mosfet is on. draw it again, but this time highlight the wires that are carrying current when the mosfet is off. compare the two. there should be wires that carry current in one but not the other. the length of these wires should be minimized.
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Old 09-10-2010, 02:27 PM   #3747 (permalink)
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Quote:
Originally Posted by adamj12b View Post
This the just what happens with the voltage when you turn off during each cycle of the pwm. The voltage spikes and smash's into the closed gate of the mosfet. The diode is basically a pressure release that allows the voltage to flow back into the battery and not pound the mosfet to death.
Sorry adam, but have to correct a couple things :

Quote:
This the just what happens with the current when you turn off during each cycle of the pwm. The voltage spikes are created by the rapid change in current and inductance and smash into the closed gate of the mosfet. The diode is basically a pressure release that allows the currentto flow through the motor and not pound the mosfet to death.
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Old 09-10-2010, 09:28 PM   #3748 (permalink)
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From the information you guys have given me, I typed up a picture of what I assume is happening when a diode is added to the circuit. The circuit shows a P-channel mosfet switch.

It appears that when the positive voltage output is stopped abruptly by the P channel mosfet turning off, the induction from the motor produces a negative voltage on the wire that was previously receiving a positive voltage from the mosfet. The voltage across the mosfet (source to drain) increases ABOVE the battery voltage because the drain wire (motor's positive wire) is now NEGATIVE. The excess voltage across the mosfet will likely blow it without protection. The diode shunts any NEGATIVE voltage on the drain wire (motor's positive wire) to ground. By shunting the negative voltage on the motor's positive wire (mosfet drain wire) to ground, the mosfet is protected.

Let me know if I'm thinking correctly on this.
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Old 09-10-2010, 09:35 PM   #3749 (permalink)
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An N-channel mosfet would be used though.
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Old 09-10-2010, 10:11 PM   #3750 (permalink)
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You are correct. I revised the drawing for an N-channel.

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Last edited by princeton; 09-10-2010 at 10:17 PM..
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