Gate Driving your Way to Victory

Friday, May 14th, 2010 @ 4:47 | LOLrio Kart, Project Build Reports, Reference Posts

As previously illustrated on this site, LOLrioKart’s motor controller is one big lesson in power electronics that I’m learning as I build it (…and blow it up…and rebuild it). While I must admit that it’s frustrating to have a controller that never really works, and which just sits on the metastable edge between functional and explosive, it’s a valuable learning experience to figure out exactly how to steer this edge towards the functional and reliable side.

You know, like finding out that your oscilloscope inputs are grounded so you absolutely shouldn’t probe a floating ground with it because it’ll dead short your power supply through itself and everything else in the path. Many times.

I found out that’s reason #1 why Segfault’s dual custom H-bridges all exploded. Live and learn, I suppose.

Cross-conduction

Another dominant cause of power amplifier failure is shoot-through, or cross conduction. In your average half-bridge circuit, the worst thing that can happen is your top and bottom side switches on the same leg being turned on at once. That usually results in very high pulsed current draw from the power supply, then…

Alright, so it’s not that bad. But shoot-through will generally blow fuses, melt wires, or destroy an output component (in the worst case causing it to fail short) if you‘re not a pussy don’t use fuses like me.  3 phase block commutation generally avoids this shoot-through phenomenon because it ensures the top and bottom sides of one phase are never switched consecutively.

However, DC motor drivers do not have that luxury, because there is essentially only one phase to switch. Therefore, whether in software or through hardware, provisions are generally made for shoot-through protection. It usually takes the form of a section of delay-circuitry, combinatory logic, a delay state in software, or if you’re Shane, driving optocouplers in inverse-parallel because they form a convenient delay circuit already.

During the times I have experimented with full-bridge or half bridge DC motor drivers, I’ve generally used a variant of the delay circuit presented in 6.131, the same one that is documented on Segfault’s progress report post. It takes a while to make, is nonintuitive, and more complex than it needs to be, taking up 5 of 6 inverters in a 7414 hex inverter.

For a little while I’ve been searching for a FET driver chip that has this function built in. After some digging on Digikey and places like FindChips, I found the IR21844.

Like the Ixys IX6R11P7 chips I favor now, it has a standard low side driver and an isolated, bootstrappable high side. The difference between the 2184(4) and the 6R11 is the input. The 6R11 takes two individual inputs, one high and once low. They do not affect eachother, and you could build in a self-destruct mode for your controller if you wanted because it can command both switches to turn on at once.

The IR21844, however, takes a single input and splits it internally into an inverted and noninverted output. So that means if your input is logical low, then the low side is on and high side is off – and vice versa. Not only that, but it also has internal shoot-through protection (IR calls it “deadtime”). Even better is that the 21844 variant allows you to tune the deadtime to the taste of your favorite mutant semiconductors.

It basically tells me that I wasted a year fumbling with this crap when someone already made something I can just buy – like everything else in life. The 21844 comes in 8 and 14 pin DIP through-hole components (+1) as well as SOIC (ewww… but handy if I ever come out of the course VI closet). And so, I gave a few quarters to the Internet semiconductor gumball machine for some units to experiment with.

Here’s the test circuit I rigged together using the schematic found in IR’s datasheet. I’m reading the two outputs using two channels of the MITERscope. The input is a 50% duty cycle square wave, 5Vpp and 10Khz. The single potentiometer is a 100Kohm unit that I’m using for deadtime adjustment.

See? I wasn’t lying. One input to the 21844 is split into two complementary outputs.

This scope screenshot is actually a little deceiving. The vertical scale is 5 volts per division and the zero levels are located on the X axis (channel 1) and 2 major divisions down (channel 2). What looks like the two square waves not meeting in the center is actually them overlapping. I should have used a wider division setting.

What’s really interesting – and what sold me on these – is the built-in deadtime.

Check this zoomed shot of the above square waves. What you see is an approx. 1us delay between when the low side turns off (channel 1 high to low) and the high side turns on (channel 2 low to high).

This is good. This break-before-make behavior ensures that your +V and GND rails never see eachother in the middle.

Even better is the fact that it does the exact opposite thing on the other side. On a low-to-high transition of the output, the high side turns off before the low side turns on again. Again, break before make. This proves that one output isn’t just a time delayed version of the other, it’s legitimately being trimmed to fit neatly into the other.

One implication of this is that the high side will completely shut off at a point before 0% input duty cycle. This seems to be inherent in the nature of delay stages and I conjecture is part of the reason why control deadbands exist. Complementary to this, the low side will completely shut off at a point before 100%, which must be considered if your drive circuitry is bootstrapped on the high side i.e. does not have its own isolated power supply at all times.

The deadtime adjustment is internally grounded on the 2184 but brought out to a pin on the 21844 variant. You connect it to ground through a variable resistance of your choice – datasheet page 10 gives a rundown on deadtime vs. resistance.

As advertised, the deadtime is adjustable from about 400ns (0 ohms)…

…To roughly 2.5us when my 100Kohm pot was pegged. The 21844 appears to support deadtimes of up to 5 microseconds.

Indeed, when I just straight yanked the pot out of the circuit, all the output shut off. I suppose an infinite deadtime is just like being permanently off.

With this in mind, I went ahead and made a little buck converter just like I did in 6.131. I used two IRF2807 FETs that I had leftover, running in bootstrapped high side mode. It was still running on the 50/50 square wave input. Will it work? Will everything explode?!

Well, what happened first was that I shorted the high side through the scope probe, killing half of the 21844.

Sad.

That’s why I bought a rail of 30.

But then it worked! On the (real) first try, even! That is, well enough to totally smoke a power resistor of too small value while I wasn’t watching.

hey guys i made a power electronic widget that worked on the first try do i win a prize do i do i do i

The next stage was to replace the resistor with something more meaningful. Something that actually does some useful work besides making a small smoke column… I mean, it can do that too, but it should be designed to do otherwise.

I have an idea – let’s try a DC motor!

HO REGEN BRAKING YAY

I probably played with this thing for half an hour – just gunning the motor, then quickly dropping the duty cycle and watching the power supply’s voltage spike up. This half bridge circuit brings me one step closer to a 4 quadrant synchronous regenerative drive for LOLrioKart, and who knows what else. To add the other two quadrants, I just need to make another half-bridge and connect the motor up between them.

Speaking of LOLrioKart, can these little driver chips actually handle semiconductors the size of those I have an excessive amount of? One way to find out is to try driving the gate. This is an IXYS VMM65001F, good to 100 volts (same as a Swapfet) and 680 amps, which is at least three Swapfets. This is a bigger transistor than LOLrioKart can ever hope to push. Just because of that, it’s a good example to test the 2184s on.

Also because it is a convenient half-bridge module, and two of them will easily make an oversized H-bridge for said LOLrioKart.

I drove the gigaFET using the bootstrapped high side.

It took a full 1.7us to turn on, where I defined turn-on as the gate voltage reaching approx. 67% of the drive voltage. There are many ways of defining “turn-on”, this is just one of them.You can even see a little bit of the “Miller Plateau“.

This is not bad given that the on period is very long compared to the switching period. During this switch is when the FET acts like a resistor, dissipating power. LOLrioKart’s PWM runs at 4Khz, which is a half-period of .125 milliseconds, still about 70 times longer than a switching cycle. Not the best, but most likely better than anything else I have on the kart at this moment.

And the turnoff occurs a little faster, since the gate voltage only has to reach 4 volts before it falls under the threshold voltage. The turnoff itself is mostly clean.

So where does that leave me? I now have a method of switching a half-bridge in an orderly manner, without external glue circuitry. I also happen to have enough large semiconductors to make a 600 amp peak capacity H-bridge. I have this dumb shopping cart with a motor on it.

Will there be a kartroller version 6?!

 

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