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Comment by rzwitserloot | Hacker News Reader

Especially considering the fact that this is harping on about Ampere. Which is _not_ the number to be looking at here; that'd be watts. That fridge is chugging down 6A at 110 or 220V (assuming it's a new fridge, unless its absolutely gigantic or incredulously inefficient, sounds like that'd be a 110V model) - not at 1 to 2 volts.

If someone can build a fridge that is so efficient, it can make do with 6A @ 2V, dang. Where can I buy me one of those? That's 12W total, I can power one of these for a full hour with 4 AA batteries.

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I thought the same initially, and do think the analogy is bad, but a few seconds later I wondered if the point that they were making was that the interconnects carry the same amperage: the required gauge for a connector (i.e. wire) is determined by amps, not watts. As a result you can send more power down smaller cables at higher voltages.

Yeah the ohmic losses in the power delivery networks are the killer and the topic of this article.

That is the point they are making, but comparing it with "three refrigerators" is seemingly invoking the power of a fridge.

To understand what they're saying, you need to understand current as distinct from power regardless of the scale. If you do understand this distinction, but you don't have a feel for what 100 amps is, perhaps a good comparison is starting a car.

As someone who has designed many chips, amperage absolutely matters because it is not DC, it has very rapid transients based on workloads and that, combined with inductance, can make power delivery very difficult. Additionally, the high current requires careful design of the package and routing because of resistance and electromigration even in the DC case.

I think the point is that the text makes it seem like you have something consuming the same amount of power as 3 refrigerators in your pocket.

Can you comment on why is there always a drive towards lower voltage? And how this impacts design. A nieve take would have thought higher voltage would equate to less loss to heat. But then with a higher voltage I guess those transients become relatively bigger?

A large part of modern chip design is accomplishing what you are trying to do inside a given power budget. Thus, anything that lowers power is usually a huge win. Switching power in CMOS is roughly proportional to V^2 (basic textbook, real design includes a vastly larger number of issues), so lowering V is a huge win. Also, reducing the parasitic cap with a better process is a big win for a multitude of reasons. But shrinking the process generally would require the voltage to scale also because the gate oxide was getting thinner. That is not really true any more as we reach supply voltages that are so close the Von of the devices, but used to be a clear trend.

High voltages are good when you want to deliver power because that is what you need (motor HP, space heater, etc.). But we don't use an IC for power, we use it to do logic, and in that case, logic is just much more efficient at low voltages.

> this is harping on about Ampere. Which is _not_ the number to be looking at here; that'd be watts.

You say that like it's obvious. I don't see why.

Agreed.

In my electrical engineering classes, we use amps to determine the gauge of wires.

The important calculation here is watts = I^2 * R

Where R is the resistance of your wire, and watts is the power wasted in your wires.

And I is amps. That little squared sign is a bit intimidating. Under normal circumstances, you want to increase voltage to reduce wire loss. But computer chips only operate at low voltage.

And the reason they operate at low voltages has to do with the micro-scale these SoC transistors are separated from each other. Increase the voltage and you get a shortcut, which will render your chip useless.

This is true but also wrong. The transistors themselves have a specific operating voltage.

On current technology, you are absolutely correct. But if you go and say something "let's do instead those high voltage transistors, so we can deliver to them more power" you can't, exactly because at this scale you'll get shortcuts. It's a physics limitation.

> Increase the voltage and you get a shortcut

It's possible to fill them up with goo that has a much higher dielectric breakdown than air, so I don't believe this is the reason

A transistor it has in its composition a semiconductor material. That's the whole idea behind the transistor as a programmable switch, using a gate(drain) variable intensity to drive how much it gets opened. And if you increase the voltage that semiconductor becomes a conductor instead, hence the shortcut I was talking about - goo or no goo.

Amps is a measure of current. Then they switch to talking about power which is measured in watts, but use amps as the unit. Power measured in Watts is apples to apples. Amps are only one half of the equation for watts. So comparing a fridge to a phone is like.... comparing a fridge to a phone.

Watts are a function of Volts * Amps. So them talking about a fridge using 6 amps at 120 volts is really quite a silly comparison to using 200 amps at 1 volt. The fridge is using 720 watts of power compared to 200 watts from the chip.

In no way is it like having three fridges in your pocket.

They don't "switch to talking about power", they discuss both the power and current to phone chips as opposed to other chips. For fridges, they only mention amps. They're not using the best wording but they're not doing what you claim there.

It isn’t, the example is confusing. It is about amps, as this is due to very low voltage, very high amperage with power movement. As the amperage is the interesting part here (due to conductor sizing, losses to resistance, etc.)

Actually for power delivery networks current (amperes) is mostly what you care about

No, amperes is the number to look at. If you just want to deliver more power on the SAME wire, you can do so by increasing the voltage. This is why a pretty thin wire can deliver power to a high speed train, or even an entire town. Trains often use voltages in the 16-50 kV range, and power lines that power entire towns can be upwards of 100 kV; at that voltage the entire town's average power might be only a handful of amps. (Heat dissipation of a resistive wire is I^2*R, not dependent on voltage.)

In the case of a CPU, higher voltages don't work with the semiconductors, so they have no choice but to use high amperes, and that becomes a problem.

>>a fridge that is so efficient, it can make do with 6A @ 2V, dang

That's 12watts. You just need some great insulation and enough time. If you are willing to never open the door and can wait days for your beer to chill, a 12-watt fridge is very doable.

"[..] Austin Wilde held up the source of power that had enabled a Disinto to chew up a mountain in half a second - two flashlight batteries!"

It's amazing how well Asimov's robot stories have aged in these A.I. times.

A modern fridge compressor uses less than 300W (for a big one). They're actually surprisingly efficient.

Even ancient ones don't go over 300W, but 6A at 120V is 720VA which even with a terrible power factor of 0.5 is still 360W. Fridge compressors are usually 2-3A, or 100-200W.

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