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The reason for 5 Volts is obvious, baseline compatibility with USB protocol, lowest common denominator. Higher voltage options were required to deliver the wattage required by modern devices.

Allowing a PD charger to deliver a continuously variable voltage range, for example passing through vehicle supply voltage, or LiPo pack voltage, seems like a simple efficient method of achieving that goal. Yet, my reading of the USB PD spec seems to prohibit that option.

First, is my understanding correct, that such pass through via USB Type C PD 3.0 is a violation of the spec?
Please suggest any way(within PD 3.0 spec, perhaps an alt mode) to pass vehicle supply voltage(always < 20v), current limited to 5A, from a charger/OBDII device I am designing, via USB Type C full featured cable, to an Android device I am also designing.

Second, why are 4 fixed voltage levels required to be provided by charger instead of a continuous range(as in Qualcomm Quick Charge)?

Third, what are the advantages for a charger to supply each of those specific voltages, 9, 15, 20v.

As I understand it, maximum wattage 100W was chosen due to international safety requirements.

Any answer that states the reason for the voltages is to maintain safe current within existing cables must also explain how this is relevant given that PD is only allowed over USB Type C cables and that 5A is allowed at 20V, but only 3A at the lower voltages over the same cable.

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closed as primarily opinion-based by Neil_UK, Bruce Abbott, Marcus Müller, Voltage Spike, winny Oct 24 '17 at 6:27

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ Why this or that standard is never a good question. One can trot out observations, that this or that voltage is used here or there, but without the minutes of the standards setting meetings, and knowledge of the horse-trading that went on over dinners and drinks, the only answer is 'why not?'. The 100W is just as arbitrary. (a) is perhaps more worthwhile to attempt than (b). Perhaps they didn't think of a continuous range, or bottled the complexity, or wanted it 'easier' to use than Qualcomm's (or did Qualcomm release QC second?) who knows? \$\endgroup\$ – Neil_UK Oct 22 '17 at 12:42
  • \$\begingroup\$ @Neil_UK I vehemently disagree. It is important to question standards, that is how they are improved. Someone has "the minutes of the standards setting meetings" and that would be a perfect answer to my question. I am currently designing USB Type C devices and have the option to choose the PD standard, or not. I asked the question to see if there is some purpose behind the voltage choices that I have not yet appreciated. Something that will affect my decision. The standard is so important and so complex that I doubt it was set arbitrarily. I assume I must be missing something. \$\endgroup\$ – slomobile Oct 22 '17 at 14:07
  • \$\begingroup\$ I agree with Neil, @slomobile. There will be no minutes that explain what went through the heads of people participating in that round table meeting or those teleconferences, however the PD standard was achieved. All you'd see would be who was publicly pushing for what, but you'd never see what was being negotiated behind the curtains, nor the motivation of the people. \$\endgroup\$ – Marcus Müller Oct 22 '17 at 15:18
  • \$\begingroup\$ You may be upset by the situation, but that's the way it is. I have participated in many standards setting organisations, and to be frank, in most cases, to call it arbitrary would be to slander the word arbitrary. USB is the result of 'coopertition', the cooperation of several organisations who are in commercial competition with each other. Each has an axe to grind, and they horse-trade this bit of IP for that bit of kudos, and end up with something far more complicated and arbitrary than it needs to be. But, it seems to get some sort of job done. Good luck. \$\endgroup\$ – Neil_UK Oct 22 '17 at 15:20
  • \$\begingroup\$ What kind of difference does it make for you, whether it is 5,9,12,or 20V? Just do what the standard needs. Communicating 2 bits of information to a source power supply is much less expensive than to have the whole multi-bit DAC for arbitrary voltage. \$\endgroup\$ – Ale..chenski Oct 22 '17 at 17:09

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