The power path
VBUS supplies power; GND provides the return path. A PD agreement can allow a different voltage on VBUS.
USB PD · Start with zero background
“Isn't a USB charger just a plus wire and a minus wire?”
Your charger and phone
are having a conversation.
They do more than carry electricity. They discuss which voltage to use and how much current is allowed. That is the starting point for understanding USB-C charging.
“Plus and minus” is half the picture.
Power needs a supply path and a return path. But supplying power is only one of USB's jobs. For a cable with USB-C at both ends, it helps to separate three roles.[2][3]
Charger
+ Deliver power · VBUS
− Return path · GND
Agree on the conditions · CC
Phone
Connection detection and conversation
CC helps detect attachment and plug orientation. USB-C PD messages also travel along the CC connection. This conversation uses electrical signals, not sound.
The path for photos and files
Negotiating charging and transferring USB data are different jobs. Identical-looking USB-C connectors do not guarantee identical charging or data capabilities.
USB-C is the connector. USB PD is a set of rules for power.
A USB-C port does not by itself promise PD, fast charging or video output. Separate the shape of the connector from the capabilities of the product.[4]
Only three units to get familiar with.
Understanding the difference is enough. You do not need to memorize them.
V
Volts / voltage
The electrical push
Think loosely of water pressure. 5V, 9V and 20V are examples of conditions that can be selected.
A
Amps / current
How much electric charge is flowing
More precisely, charge per second. Current depends on what the device is doing; it does not always reach the allowed maximum.
W
Watts / power
The rate of energy use
For USB's DC supply, voltage × current. At 9V with an actual current of 2A, power is 18W.
Water pressure is only an analogy to help with the idea. Electricity and water do not behave identically.
Try changing V and A
For these numbers
9 V × 2 A
This is a multiplication calculator. It sends no voltage request and does not measure or check the capabilities of any device.
Watch what happens after you plug in.
This is one successful example of moving from 5V to 9V. The charger advertises its options and the phone chooses a supported one. A higher voltage is not applied without agreement.[2]
An illustration, with no connected hardware, captured messages or measurements. Rejections, retries and detailed reception acknowledgments are omitted.
First, detect the connection
Connection detection
Someone is connected. Start at 5V.
This describes CC attachment detection. The sentence is not a literal PD message.
Attach / 5VThe charger offers a menu
Charger → phone
I offer 5V, 9V, 15V or 20V, up to 3A for each.
The charger advertises voltage and current conditions. A PDO is one item in this power menu. This example's menu reaches 60W.
Source_CapabilitiesThe phone chooses its conditions
Phone → charger
Please let me use 9V, with a limit of 2A.
The device requests conditions it supports. Here 2A is the requested current limit, not an instruction to draw 2A continuously.
RequestThe charger agrees and prepares
Charger → phone
Agreed. I am switching to 9V.
Accept agrees to the conditions. Seeing this response alone does not mean 9V charging is already established.
AcceptThe charger announces that power is ready
Charger → phone
9V is ready. You can use it now!
PS_RDY announces that the supply is ready. The phone uses power within the agreed conditions. Actual current and power require separate measurements.
PS_RDYNegotiation does not mean there is no power at first. In a standards-compliant USB-C connection, attachment is detected and a 5V supply starts. PD can then negotiate another voltage if needed. Some devices stay at 5V.[2][3]
“You may use up to 18W” is not “you are using 18W.”
A restaurant can hand you a large menu without making you eat every dish. A charger lists what it can supply. The device's actual use changes with battery level, temperature, screen activity and its internal charging control.[1]
Does that 9V go straight into the battery?
No. Power and charging circuits inside the phone adjust the incoming supply to run the device and charge its battery. Voltage measured on the USB connection is different from the battery's own voltage.
The cable has a role, too.
A capable charger and phone still need a cable that supports the required conditions. Charging speed and available power are not determined by the charger alone.
Charger
Which conditions can it offer?
Cable
Does it support that voltage and current?
Device
What can it receive, and what does it need now?
Some cables carry an identity tag.
A small chip called an e-marker can report a cable's capabilities. Not every cable contains one. For example, using more than 3A or using EPR requires an appropriately rated, electronically marked cable.[2]
Reading that identity is not a test of construction quality or real output under load. Charging capability and data speed are separate specifications to check.
You might be wondering…
Will a 240W charger push 240W into my phone?
Not with a standards-compliant combination. 240W describes the charger's maximum capability under the right conditions. The device negotiates conditions it supports and uses the power it needs. Reaching 240W also requires a compatible device and cable.
Why does charging slow down near a full battery?
The device managing the battery may reduce current as charging progresses. Temperature and activity can also change it. Low recorded power alone does not mean the charger is faulty.
Can a USB-C device charge without USB PD?
Yes. USB-C also supports supplying 5V without PD messages. CC attachment detection and current advertisement are distinct from the exchange of PD messages.
Does an older rectangular USB-A charger have this same conversation?
This page describes modern USB-C-to-USB-C PD charging. USB-A charging can use other mechanisms, including signaling on D+ and D−. It is not necessarily the same process. Older USB connections were not universally limited to two wires, either.
Do I need to learn PPS and EPR already?
The names are enough for now. PPS supports fine voltage adjustments within an advertised range. EPR extends the power range with higher voltages, up to 240W. EPR has its own entry procedure; it is not a matter of simply connecting 48V.
How can a computer charge a phone?
The power provider is called the Source and the receiver the Sink. A wall charger is not the only possible Source. Some devices support both roles and can exchange them when supported.
A small dictionary for reading logs.
- USB PD
- Rules for exchanging power conditions. PD stands for Power Delivery.
- Protocol
- Agreed message formats and procedures that devices can understand.
- Source / Sink
- Power provider / power receiver. In our example: the charger / the phone.
- PDO
- One power-menu item, containing conditions such as supported voltage and current.
- GoodCRC
- An acknowledgment that a message was received, separate from Accept agreeing to its power conditions.
- PS_RDY
- A message saying the supply is ready, not a measurement of actual power consumption.
Scope and further reading
A simplified introduction to common USB-C-to-USB-C PD connections, not an exhaustive description of every message, charging method or specification requirement. The diagrams and conversations were created for this page.