In order to understand what is USB PD , let's learn some basic knowledge:

Firstly, P（Power)=V (Voltage) * I (Current).

Secondly,   Battery power voltage is usually between 3V (empty) to 4.2V (fully charged).

According to the law of physics, to increase power you can either 

1. Increase electricity current 
2. Increase power voltage
3. Dynamically promote either and both electricity current and power voltage

If battery was a traditional waterwheel, the process of charging is using ongoing water current to propel waterwheel to power electricity; in this case power voltage is the height of water stream, and electrify current is the size of the water stream. 

Proposal 1: increase electricity current 

Both OPPO VOOC / OnePlus Dash / Huawei Super Charge Protocol all opt to increase their electricity current, this would also minimize the heat produced during production of power, the downside of this option is it require specialized cable and charger, it requires higher cost production hence decrease availability. 

Proposal 2: increase powering voltage 

Qualcomm QC 2.0 and some alternative regulations based on Qualcomm QC 2.0 has evolved itself into Huawei FCP and Samsung AFC. 

Proposal 3：Dynamically alternate increase in both power voltage and electricity current 

As we stated previously, battery power voltage is usually within 3V - 4.2V, alternating dynamically increase in voltage and current charges battery at highest efficiency. For example, there’s nil to very little heat produce at start of battery charging, it increase power voltage, when battery started to released waste by product as heat energy, it automatically alternate to increase in electricity current to retain highest efficiency of battery charging.

Both Qualcomm QC 3.0 / QC 4.0 has adapted alternate charging solution. Qualcomm as a mobile chip supplier has been limited itself to maximum of 27 watts due to current on market battery power; whereas laptop USB PD supports up to 100W as well as QC 4.0. USB IF is a non profitable organization, it does not holds any conflict of interest between all manufacturer, we see USB PD is a future trend for charging solution.