The Spinning Wedge Implementation, Part 1

In the previous post, I ended it mentioning there are other concepts that, in theory, would not rely on enormous accelerations and forces. One such concept is that of two spinning disks, each with a hole the size of the aperture on it. They spin at the same speed, but out of phase, so that the holes never line up. When the camera is triggered, one of the disks is slowed down (or sped up), so that at the next pass, the holes line up just as they pass the aperture. I can’t imagine this being easy to accomplish. If we assume that the synchronization of the holes is instantaneous, then the worst case scenario is when the camera is triggered just after one of the holes passed the aperture. In such a case, for the shutter lag to be 4 milliseconds, the disks would have to be spinning at 250 Hz—15,000 RPM.

So in the interest of getting something done, I started working on an actuator suitable for the impulsive shutters.

Continuing from my first tests, I first considered solenoids. But it seemed to me that if a solenoid works because of the ferromagnetism of the plunger, then if the plunger were a permanent magnet, the actuator would be more efficient. As it turns out, that’s what a voice coil is. And the best place to get a rotary voice coil is a hard drive head.

So the idea was simple. Take an old hard drive apart, and wire the head coil to a 300 Volt supply that could be precisely pulsed. Such a supply would need a storage capacitor to source high currents, and the best place to get such a supply is a camera flash. After reading through Sam Goldwasser’s site, it became obvious that the best source for that is a disposable flash camera. I went to two local drug stores; the first refused to give me any of the used ones, but the second one had a kid behind the counter and he gave me my choice of about 10 used disposables with flash.

To enable the head to be actuated in either direction, I decided an H-bridge was the answer. I spent about a month trying to build the circuit, because I had no idea what I was doing. At first, I tried to build it simply with four N-channel MOSFET’s. Although the circuit fired, it behaved very erratically, and, well, it should have never worked in the first place. The problem is that in N-channel MOSFET’s, or IGBT’s, the gate is referenced from the source. For the switches at the bottom of the H-bridge, the source is the ground, or the power supply minus, so driving them is easy. For the ones on top, the source is the voice coil itself, so the gate drives have to have a voltage that is relative to the power supply voltage minus whatever voltage drop there is across the MOSFET or IGBT (or whatever else is doing the switching).

There are several ways of providing this high-side gate voltage. One of the easiest is to use an optoisolator to translate a typical pulse, for example, of 0 to 5 V, to the appropriate level—say, 300 to 320 V. But I figured there should be a chip to do this. Even though I was already using an optoisolator to electrically isolate my pulse generator from any high voltage mistakes I may make, I decided to look for a chip—at the time I didn’t know how to implement the direct-from-optoisolator circuit (and I’m still not sure I do). There are several IGBT drivers, and, not surprisingly, many meant to drive half an H-bridge (2 of the four switches, either on the same side of the H bridge or corresponding high and low sides), or full H-bridges. There is a large variety of these, but the ones from International Rectifier are rated for 600 V.

The whole thing turned out to be extremely complicated, and it took me about 2 weeks to finalize the design. The first problem is that IR makes dozens of different drivers, most of which, at first glance, seem to have the exact same specifications. My choices were limited to half-bridge drivers, of which there are apparently two types. Some are meant to drive an “I” of a bridge—that is, the high-side switch and the low-side switch that should not be conducting at the same time—and some are meant to drive a “direction” of the circuit (the two switches that are supposed to conduct at the same time). The circuit examples at the bottom of the first page of their data sheets is not reliable, and they like to explain the input/output logic by minimalistic timing diagrams that are confusing. The terminology is not clear: “half-bridge drivers” are evidently no the same as “high and low drivers”. In the end, the only way to figure it out is to look at several different ones and examine the timing diagram. The difference is in input/output logic. A half-bridge driver is mean to drive an “I” of the “H” and thus the high-side output is equal to the input pulse while the low-side output is equal to “not” of the input (which is why they designated as “HIN/LIN(bar)”, thus preventing a short circuit condition automatically. A high and low side driver has both the outputs equal to the inputs (“HIN/LIN”) so if you hook it up to an “I” you get a short circuit. It took me a week just to narrow this down, but in the end I chose the IRS2183 (and I don’t even remember why). I didn’t dare venture into the realm of models with advanced protection features.

After that came the adventure of choosing the component values, which their “typical connection” diagrams don’t detail at all. Thankfully, they have a sort of forum where dozens of people ask the same question. Whoever answers the question gives a different response, but the answer is in their design tips and application notes. They give more or less the same answers, with a bit of variation from one to the other. I ended up using DT04-4, which took me about a week to interpret, though at least they tell you where they are pulling the values from so you can copy their example. For the most part, it is poorly written, and some of the more advanced concepts, such as different switch-on and switch-off gate resistors, are completely lost because they don’t explain how to wire it. So I recommend you read some others, too, including AN-978, AN-1123, AN-944 (seems interesting, though I haven’t read it), DT99-7 (haven’t read it), and DT98-2 (all about component values). For the purpose of occasionally pulsing current through the voice coil, most of the issues reviewed in these documents don’t come up—for example, the bootstrap capacitor will always be ready and charged, especially with the IRS2183’s IO logic which keeps the low-side IGBT conducting (thus providing a path to ground for the capacitor to charge).

So then came the time to choose an IGBT. The voice coils apparently tend to be on the order of 10 ohms, and we are pumping 300V into them for 1 millisecond (actually less, since the flash capacitor will discharge quite a bit). Still, IGBT’s are just about the only option here. Although IR offers quite a few, I opted for an ST Micro because it was much cheaper.

Figuring out the SOA (Safe Operating Area) curves was another surprise, and as always, I must thank Sam Goldwasser for getting me through it. I’ll go over the SOA for the STGW30NC60VD, which is the IGBT I chose. First off, I have no idea why in the data sheet it says “Turn-off SOA”.

The first thing to keep in mind is that this SOA is for the full operational range of the transistor—which includes operation as an amplifier (rather than a switch). The other key is that there are two distinct specifications we shouldn’t exceed. First, our flash supply is approximately 300V, so we cannot exceed the voltage rating of the IGBT when it’s not conducting (in this case, 600V). Second, we cannot exceed a maximum current that is specified by the SOA when the IGBT is conducting. When used as a switch, VCE will be the saturation voltage, so the current is defined by the left side of the SOA curve. In this case, the maximum is 100A. The IR SOA’s typically show different limits for pulsed currents, but the same rule applies—only the left side matters. This IGBT is plenty for our purposes, because not only are we pulsing the current, but the capacitor will discharge relatively quickly so the 30A we roughly predict is only instantaneous.