And, speaking as a EE who actually plays with DC->DC conversion for a living..
It's actually a bit interesting and has to do with frequency. AC power runs at 60/50 Hz. In Ye Olden Days, if one wanted to get, say, 600V DC to play with, one would run the wall socket power into a boat anchor of a transformer. 120 or 220 VAC in; 600 VAC out. One would then rectify the 600 VAC to DC with diodes of one flavor or another, and there one was.
And I wasn't kidding about the transformer being a boat anchor: The ones used in TV sets were 20-30 lbs, and that was only for a couple of hundred watts of power. If one wanted to run 10kW to 20 kW in this fashion, then we're talking about using a forklift to get the transformer moved around.
Thing is, to make a transformer work, one has to make a magnetic field. One does this by winding copper around iron, iron having the characteristic of multiplying the magnetic field strength. The more power one wants to move, the stronger the magnetic field one needs. But, there's a problem: Run too big a magnetic field and the magnetic field in the iron stops increasing. This is called, "saturation" and, when it happens, Bad Things Happen. (At least when it doesn't happen by design; there are things like magnetic amplifiers where saturation happens on purpose.)
Thing is, though, that at higher frequencies, magnetic materials can be formulated to handle higher power with less weight. Which is why, if one has ever been in an airplane, one will hear a 400 Hz humming below the floorboards, rather than a 60 Hz hum: Generators, transformers, and what-all weight a
lot less when they run at 400 Hz for the same power.
Now, in modern hoo-ha like TVs, computers, and Teslas, the usual trick is to forgo the boat anchors and do the conversions at much, much higher frequencies. The usual trick is to:
- Rectify the incoming AC power to some form of DC. This requires Big Transistors with Serious Heat Sinks.
- Using these switching transistors, run the current back and forth inside a small transformer at frequencies of 500 kHz and up. 1 MHz isn't unusual. Seeing as the frequencies are 'way the heck up there, the transformers can be tiny: What used to require a 20 lbs boat anchor to move 300W now requires a 6 or 10 oz module.
- Take the high-frequency output of the secondary of the transformer and rectify it to the DC voltage level of one's choice.
Interestingly, this arrangement gets one power conversion efficiencies in the 85%-95% range, which is actually better than the old boat anchor arrangement. Fundamentally, less losses in the iron. Further, by playing with how much current to run through the primary of the transformer (pulse width modulation on the switching transistors) one can vary both the input and output voltages. In case you were wondering, this is why the power brick on your laptop can take in anything from 120 VAC to 240 VAC, without flipping switches. Now, a Tesla does accept 120 VAC to 240 VAC, but it's possible in the interests of efficiency that they're also playing with relays in there.
Point is, though: Converting from 60Hz/50Hz city power to DC, at least before the DC-DC conversion step, takes serious hardware: big capacitors, big switching transistors, and so on. In large part, all because it
has to run at low, city power frequencies. On most Teslas these days, each city-power to high voltage DC step is done with 18A modules: Three for M3 LR and P, two for SR.
Now, this business about "bypassing" the rectifiers: Technically, true. Instead of going through the AC->DC first stage conversion step, the incoming high-voltage DC goes to the
second DC->DC step, where the input voltage is around 480 VDC. The output voltage of that DC-DC stage is designed to
vary. If one has a pretty-much discharged battery, its voltage is low and the second DC-DC stage is actively controlled to have a given current at some volts above the current battery voltage. As the battery charges up, the rate of charge (i.e., current into the battery) is varied by actively controlling the output voltage of the second DC-DC conversion stage.
In a strict sense, then, it's this second DC-DC stage and its control hardware that's the actual entity that doing the charging on a Tesla. Switching frequencies in this block are, I'm sure, as high as Tesla can make them, in order to reduce weight and get good efficiency.
So, once again:
1. The charger is in the car. It accepts around 480 DC or some such and generates an output voltage and current that
actually charges the battery.
2. There's two or three City Power to 480 VDC blocks in the Tesla that lets it charge from city power.
3. The Wall Connector and/or Mobile Connector simply provides another way to get City Power into the City Power to 480 VDC blocks.
4. Superchargers bypass the City Power to 480 VDC blocks and apply 480 VDC directly to the second bunch of DC-DC converters.
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