Paul & Sabrina's cheap DIY 144v motor controller

[DJBecker]

Originally Posted by adamj12b

Which end they stick out is your decision. Some layouts of the car work better 1 way instead of the other. It does not matter.

If you want, you can use a longer B+ bar so you have the B+ and B- on one side, then B+ and M- on the other. I dont think ive seen one like this yet though....

-Adam

That configuration wouldn't be balanced.

The layout should be based around equal path length. The current shouldn't have an easier path to some devices.

This requires that the motor connect to opposite ends of the controller. Otherwise the freewheel current will mostly prefer the "close" diode. When that burns out because it's carrying all of the current, the current will prefer the next device and burn that out in turn.

That is if the device burns out. Sometimes diodes fail by shorting out. When the controller only has a motor current sensor, it can't detect the case when a freewheel device shorts out. Instead it does the opposite of protecting itself. Since most of the battery current flows through the short instead of the motor, it increases the drive PWM in response. And keeps increasing it until every push-side device is destroyed.

(We've replaced all of the push-side MOSFETs only to have them immediately blow up in a spectacular way with the first test. Twice. We now remove and test every diode after any power section failure.)

[sawickm]

Originally Posted by flores

Can you share the model/make of this oven? And at what temperature do you set it? Maybe a picture? Thanks.

Alot of info about Toaster Oven Reflow on the internet !!!

This is a good artical that covers most of that subject well: Openhardware - Toaster Oven SMD

-Mark

[adamj12b]

Originally Posted by DJBecker

That configuration wouldn't be balanced.

The layout should be based around equal path length. The current shouldn't have an easier path to some devices.

This requires that the motor connect to opposite ends of the controller. Otherwise the freewheel current will mostly prefer the "close" diode. When that burns out because it's carrying all of the current, the current will prefer the next device and burn that out in turn.

That is if the device burns out. Sometimes diodes fail by shorting out. When the controller only has a motor current sensor, it can't detect the case when a freewheel device shorts out. Instead it does the opposite of protecting itself. Since most of the battery current flows through the short instead of the motor, it increases the drive PWM in response. And keeps increasing it until every push-side device is destroyed.

(We've replaced all of the push-side MOSFETs only to have them immediately blow up in a spectacular way with the first test. Twice. We now remove and test every diode after any power section failure.)

Good Point.

I was not thinking of that when I rattled off that post.

-Adam

[jackbauer]

So by way of safety the controller should also monitor battery current? And see that they maintain an expected relationship.

[DJBecker]

Originally Posted by jackbauer

So by way of safety the controller should also monitor battery current? And see that they maintain an expected relationship.

Yes. That would avoid this specific type of failure from becoming worse and allow detecting a few more fault types.

An interesting question is the benefit if you are using IGBTs with driver modules. (You are using them instead of MOSFETs, right?) The modules can shutdown on some types of short circuit faults (desaturation "desat" protection). It may be that instead of removing the need for battery current monitoring, it makes it more important. Instead of immediately burning out the "push" side devices, it allows them to keep pushing power into the shorted diodes. Something will eventually melt, and it may not be something safe.

An issue with adding this to the AVR-based Cougar controller design is the A/D converter. Not only is the current chip out of input pins, but the A/D converter is pretty slow. It's already only sampling the motor current every 4th cycle.

That could be addressed by running the A/D converter at a faster clock. The next step up is doubling the converter clock. That would put it only slightly over the rating to get full accuracy. So the LSB would be a little noisier. But given that the controller already has a high level of electrical and magnetic noise, I doubt that it would be significant.

Control theory tells us that doubling the sample rate allows us to accept about double the noise for the same results. (A zillion caveats omitted, the most important of which is a non-correlated noise distribution.) Since we aren't going to be doubling the noise, we should end up with better control plus more available conversions for the battery current monitoring.

But this might not actually work for us. I don't believe that a 16MHz AVR has enough compute cycles to run the existing feedback loop twice as often. I'm sure it could be done, but it would require some really careful code tuning and worst-case analysis.

[jyanof]

Originally Posted by DJBecker

That configuration wouldn't be balanced.

The layout should be based around equal path length. The current shouldn't have an easier path to some devices.

This requires that the motor connect to opposite ends of the controller. Otherwise the freewheel current will mostly prefer the "close" diode. When that burns out because it's carrying all of the current, the current will prefer the next device and burn that out in turn.

Just an FYI, but I've passed 7k miles (in most likely the harshest temperature environment) with this controller and have all 3 bus bars coming out the same end (wouldn't fit otherwise). This could indicate a few things:

-the current may have an easier path through some components, but it's not significant enough to have an effect
-the heat spreader performs adequately enough to distribute the heat and keep the diodes from entering thermal runaway
-there's not enough time on the controller for any problems related to this to surface

I guess we can try to quantify this effect with a model, but I'm not up for it at the moment... I suspect it's ok.

[DJBecker]

Originally Posted by jyanof

... have all 3 bus bars coming out the same end (wouldn't fit otherwise). This could indicate a few things:

-the current may have an easier path through some components, but it's not significant enough to have an effect
-the heat spreader performs adequately enough to distribute the heat and keep the diodes from entering thermal runaway
-there's not enough time on the controller for any problems related to this to surface

I'm guessing that it's a combination of 1+2, combined with the controller not pushing the limits of the components.

The single heat spreader bar might also be the savior here.

MOSFETs spread the load because they get worse when hotter, pushing the current (and heat generation) into the cooler devices. Diodes do the opposite, making them more difficult to parallel. But if you combine the two on a single heat spreader, the MOSFETs will heat up the cool spots and help the load balanced across the diodes.

[harlequin2]

Isn't theory a wonderful thing?
My controller also has the bus bars all out one end, its done about 1500 kms and the temperature has never risen above 60 deg C, even when the air temp is 32 deg C.

[MPaulHolmes]

100% done with 2 of the 4 1000amp controllers. Well, must add the enclosure. 95% done with the 3rd. 30% done with the 4th... I need to test the finished ones out. I got some free samples of precharge resistors. They can handle a crazy amount of surge energy. Like 3000 joules over any amount of time you like. The 22 capacitors are only like 390 or 400 or so joules.

Oh dang it, it's actually like 99% done, since I need to still make the stupid DB-9 connector plug thing for each of them.

[jackbauer]

Paul , any luck with the software?