Morgan LaMoore wrote:

> Except that in a PFC boost converter, the power drawn is proportional

> to the square of the input voltage, so there is low current at low

> voltage.

That is a consequence of the power factor control algorithm, which

forces input current to follow input voltage. But all the PFC converters

I've tested will run on a straight DC input. In this case, the input

current they draw is *inversely* proportional to the input voltage. I.e.

a 100 watt 90-264vac PFC power supply draws:

- 100v at 1a

- 200v at 0.5a

- 300v at 0.33a etc.

> For an EV, you want constant power, so current is inversely

> proportional to voltage. I suspect that this will have a big effect on

> efficiency.

It just means that the voltage and current ratings of the semiconductors

and boost inductor need to take this into account.

> Also, we're talking much higher power here. How many 20kW wall

> adapters do you see?

As a rule, cost per KW goes down as the power goes up. I.e. one 20kw

supply costs less than ten 1kw supplies.

>> Wide-range DC/DC step-down converters are also common.

> Most of these are low current

Of course; that's where the demand is. The market uses a million times

more little DC/DC's than big ones.

> an EV would need 100 times more current, which means 10000 times

> more resistive losses. You don't really want to use 10000 times

> more copper to make up for that.

No; it doesn't work like this. On a crude basis, you could get 100 times

the current by just using 100 little DC/DC converters; thus 100 times

the semiconductors, copper, etc.

But in practice, you'd use a much smaller number of semiconductors and

other parts. When you double the power of a part its size, cost, and

weight will *less* than double. A 0.3w transistor is $0.10; a 3w

transistor around $0.30; a 30w transistor $1; a 300w transistor is $3 etc.

At some point this breaks down, because nobody makes 30,000 watt

transistors. So you have to use many smaller parts in parallel. But even

then, the size/cost/weight go up linearly, not as a squared function.

--

Ring the bells that still can ring

Forget the perfect offering

There is a crack in everything

That's how the light gets in -- Leonard Cohen

--

Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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