Now that all these parts have been removed from the inverter unit, we can start playing with them a bit and explore their electrical/electronic characteristics. This leads to a deeper understanding of the power engineering that went into this and current best practice in the field -- possibly to the point of being able to re-use this unit either in another Prius or as a component of some completely different project. I've spoken with a couple of colleagues about some fairly scary "Frankenprius" ideas...
There's not much to the boost inductor. Well, except its ability to store and release energy! This is a time-shot of quickly brushing past one of its terminals with a lead from a battery [a Prius module, appropriately enough] whose other terminal is held on the other coil terminal, and the high-voltage "pow" and bits of flying molten metal from that single pulse are pretty impressive. Now, consider that those IGBTs inside the boost switch do this 12,000 times per second and usefully capture all the results! [That figure is real, from watching the CPWM converter-drive lead from the car's ECU.]
How about those big power transistors? What are their characteristics? With gates that massive, how much charge does it take to turn them on? Here we won't get a quantitative idea of that, but a little playing around can give some idea. The Prius battery module is hooked up to emitter and collector through a dropping resistor to limit current to about an amp, and we read the voltage across the transistor. With it off, I see full battery voltage. [Which, by the way, is normal real-life for a 6-cell Prius module even though it's rated at 7.2V]
Using a pin-probe, a couple of fingers, and my body as a large resistor, I can bleed a little charge into the gate lead and watch the voltage across the transistor decrease as it starts going into conduction. Holding long enough, i.e. a few seconds depending on how hard I press the metal, clearly goes all the way into saturation, and then by shifting my other hand to the negative lead I can drain charge and get it to turn off again. Here, though, we've gotten a little way into the linear region...
and by removing the pin-probe to the gate, I can cause the voltage to just freeze right where it is. It just holds there -- doesn't drift up or down for a very long time, until the charge is given a path to dissipate by touching the gate again. Just like a FET should, of course, and the leakage exhibited here is small to nonexistent. Forward voltages appear to be fairly normal -- diodes and saturated transistors at 0.6V when non-trivial currents [~120 mA] are passed through them. The forward voltage of the entire rack's worth of diodes, taken at the supply inputs, reads 0.95 under the same conditions. You can guess what would happen if this were hooked up to the 200V battery as ORNL had it labeled! Playing with higher voltages and currents through the rack will wait until some later phase of testing. The devices in the boost switch are similar, although its diodes appear to have a lower forward voltage of 0.5. Obviously, the lower the better.
The control board has its interface connector nicely documented, albeit at a bit of distance. These connection names are the same as used at the ECU, and described in the "Terminals of ECU" section of the service manual. This is really useful, because it indicates how to hook it up to power and where to find real-life signals in the running car to try and reproduce on the bench.
First, we need to get the best possible pictures, not only of both sides of the board but also attempts to probe *inside* a little by shining light through to show where the vias are and even which direction they might roughly head. This was a quick-n-dirty rig to hold the board at the edges and light from above or below and try to keep the camera exactly positioned the same way. It almost worked as intended; a bit of post-correction was definitely necessary.
Here's a set of four large images that have been stretched around a little bit, and the rear-view ones flipped right-to-left, such that they very closely match up via-for-via when loaded into one's favorite image editor as layers on top of each other. There's a lot of power-plane in there, so this may or may not help with further circuit-tracing but is kept here for reference. The blue tape over the gate connection slots is simply to keep light from blasting up through them from underneath.
With a little more familiarity with how the thing is laid out and what to connect to, it's time to get the *good* module on the bench and start tracing some things. No IGBT rack connections are needed yet, this is just playing on the 12V side and trying to figure out what eventually gets sent to the gates.
To get through the conformal coating to reach solder lands, a very sharp pin probe is needed. It's easy to make one out of a paper clip; the plastic-coated type provided a little more protection against randomly shorting things if, say, it's dropped onto the board.
A little bit of 12V conditioning happens over at the right, and then the entire middle section [red boundary] consists of two mostly independent power supplies. Upon supplying 12V power to IGCT and GINV, the two yellow transformers begin emitting a soft but audible high-frequency hash typical of switching power supplies, and a whole bunch of voltages appear at the small electrolytic caps scattered around them. The two heatsunk power transistors circled by blue dots are tied to ground at the emitter and exhibit the high- frequency [and somewhat jittery] transformer-drive waveform, and the ones circled with yellow dots seem to be linear regulators for 5V that feeds the two processors and some other stuff. The board draws about 0.7A at 12.5 volts, and 0.8A at 10 -- higher draw with less voltage, typical of a switching power supply. At 7.8 volts it draws about an amp while trying vainly to start up. The 5V fails at 5.7 volts input, indicating fairly low drop-out regulators. After the board has been running for a while, the warmest components on it are the little yellow transformers. And not inordinately so -- once again, high efficiency, minimal wasted power.
Strict isolation is maintained between the chassis-grounded 12V system and the entire HV system including battery, motors, and transistor rack. But the inverter is one of the places where they must come close together, so there needs to be a bit of magic here. [The other place, of course, is in the battery ECU for sub-pack voltage monitoring.] There are isolation barriers down the whole length of the board, bridged by pairs of optoisolators to pass signals in various directions, and all the power to the gate-driver circuitry comes from isolated sources. So that's almost TWO layers of isolation if you consider the insulated gates another barrier. Well, you can't really think of it that way because the high-side gate driver circuitry has to operate somewhere above the motor lead's voltage at any given time so that whole section must float around at a fairly high voltage when running. What's not yet clear is exactly how that's done, or [related] how the ground-fault monitoring works.
The isolation gap is even more obvious on the backside of the board, but this also shows how some of the floating power-supply outputs connect to the HV side. The upper and lower edges of the board get back into non-isolated territory, to interface directly with the current-sensors [which are yet another isolation-barrier component!]. In this view, there are twelve fairly distinct denser areas of circuitry near the control-lead slots -- these are the sections that stay isolated and actually control the transistor gates.
With that in mind, a couple of hours of probing around with a scope and meter in "diode check" mode to find where things are hard-connected together has pretty much ferreted out where all the power goes. It is soon apparent that split (+) and (-) supplies are utilized -- they feed several analog components including the current sensors. Those are interesting -- they appear to have two exactly duplicate outputs, which get routed back to the interface connector as signals like "MIVA" and "MIVB" which scope out as exact clones of each other in the running car. Why two?? Not only that, but they swing above AND BELOW ground, right into the hybrid ECU. That means the hybrid ECU must have some negative-supply stuff inside it too. Why not an easy single-supply swing about 2.5V, like the main battery current sensor? A bit of a mystery there. The really fascinating thing is how the gate driver supplies are done. Each power supply section produces four separate 9.5V or so floating outputs, one slightly more heavy-duty [well, at least with a larger filter cap] than the other three. The large one [where "large" simply refers to its capacitor] feeds all the low-side transistor drivers. The three smaller ones each independently feed the three high-side driver sections for MG1 and MG2, which begins to make it clearer how the high-side isolation is done. The answer seems to be six totally separate little supplies. In the assembled module, all the low-side supply negatives are hard-tied to each other, the "E" control leads, and the main (-) supply from the battery. But there's something a little funky about them -- without the transistor rack in place, the negatives [blue] of the low-side driver supplies aren't tied to each other [lightly dotted line] and are only hard-connected to the driver circuitry in the "V" areas. It isn't clear how the negative supply for "U" and "W" gets there but there's some other component in the way. But the three positives per side are definitely tied together as shown. Again, once this is all soldered down to the rack, it's largely irrelevant and all rides wherever the negative of the 200V battery sends it. I can also see that there's an extra optoisolator device from the "V" low-side back into the nonisolated area, which is undoubtedly related somehow to why it's got the "real negative".
Here are some of the early discoveries as to where the switching signals go. I'm sending a simple oscillator into the "GUU" lead, which controls current to the "U" phase of MG1, and trying to find how it eventually gets to the IGBT gates. I've found it up to the processor, which is a dead end until the "GSDN" gate/shutdown lead is brought high too, and then I discover a split-out pair of complementary signals in response that eventually hop through two of the optos and go to the gate driver chips. When GUU is low, the lower transistor of the pair is turned on; when GUU goes high, it turns off and the upper transistor turns on. There is NO both-off state, except for when the overall "gate" lead goes low and then all the transistors turn off [i.e. the definition of "neutral" in the car]. This switching works down to DC and has a little hysteresis, changing state as the input rises above 8 volts and again as it falls below 6.8 or so. No IGBT gate output appears until "xSDN" is brought high, and then the next rising or falling edge at the "xyU" lead starts producing output. In the MG2 section, each transistor gate pair is simply tied together, so the gate driver chips are evidently amply capable of kicking both of those monsters at once.
The last thing anyone wants is both elements of a half-bridge to be on at the same time, which would cause a shoot-through short across the supply and rapid loss of the magic smoke that makes things work. Since you can't instantly turn on one element of a half-bridge and turn off the other and guarantee no transition leakage, there has to be some minimal amount of guard time during which both are solidly OFF, usually enforced in hardware. Here, by sending in a single squarewave input and scoping the corresponding pair of gates, I've found that time to be 6 to 8 microseconds. This appears to be done at the processor outputs; it isn't yet clear if there's a backup system to prevent disaster if the processor should ever wig out and turn on both outputs at once. Given the amount of circuitry around the gates, I would expect an additional level of protection in there someplace -- but that would mean more signals needing to cross the isolation boundaries. Maybe that's part of the feedback path -- remains to be seen. For all its sophistication and apparent complexity, this device is still wholly a slave to the hybrid ECU. It does exactly what it's told via the gates and switching-control lead triples, and only sends a little running data back. It has no sense of 3-phase sequencing and cannot intelligently drive the motors for propulsion by itself. From here, I can try to find some part numbers to help further determine functionality, compare against what other analyses have already found, and start getting into what someone would need to know about switching strategy to usefully control this mess. And I still haven't even touched the cooked DC/DC to try and figure out what the heck happened to it yet.