USB Power Delivery (USB PD) is a charging and power negotiation protocol defined by the USB Implementers Forum that lets a power source and a device agree on voltage and current before higher power flows. In hardware connectivity, it is the standard that allows one USB-C charger to power phones, laptops, monitors and docking stations, from a few watts up to 240 W in its Extended Power Range.
Without negotiation, a USB port supplies only 5 V at a limited current. USB PD adds a digital conversation that raises the voltage only when both sides and the cable can handle it, which is what makes fast charging over a universal connector both possible and safe.
How USB Power Delivery negotiates power
USB PD communication runs over the configuration channel (CC) wire of the USB-C connector, separate from the data lines. When a cable is attached, the source first provides the default 5 V. It then sends a list of Source Capabilities, called power data objects, describing each voltage and maximum current it can supply.
The device, called the sink, selects one option and sends a Request message. The source replies with Accept, adjusts its output and finally sends PS_RDY to signal that the new voltage is stable. The whole exchange takes a fraction of a second, and it can be repeated at any time, for example when a laptop battery is full and needs less power.
The cable takes part in the decision. Standard USB-C cables are rated for 3 A. Cables carrying 5 A must contain an electronic marker (e-marker) chip that reports their rating, and the source queries it before offering higher currents. This prevents a thin cable from overheating.
| Step | Message or action | Purpose |
|---|---|---|
| Attach | CC detection, default 5 V | Establish a safe baseline |
| Cable check | E-marker query | Learn the cable’s current and voltage rating |
| Advertise | Source Capabilities | List available power levels |
| Request | Sink selects a level | Ask for the needed voltage and current |
| Confirm | Accept and PS_RDY | Switch output and confirm stability |
Power levels in the SPR and EPR ranges
USB PD defines fixed voltage levels grouped into two ranges. Standard Power Range (SPR) reaches 100 W, while Extended Power Range (EPR), introduced in USB PD 3.1, raises the ceiling to 240 W by using higher voltages rather than more current.
| Voltage | Maximum current | Maximum power | Range |
|---|---|---|---|
| 5 V | 3 A | 15 W | SPR |
| 9 V | 3 A | 27 W | SPR |
| 15 V | 3 A | 45 W | SPR |
| 20 V | 3 A or 5 A | 60 W or 100 W | SPR |
| 28 V | 5 A | 140 W | EPR |
| 36 V | 5 A | 180 W | EPR |
| 48 V | 5 A | 240 W | EPR |
Besides fixed levels, USB PD 3.0 added the Programmable Power Supply (PPS) mode, in which the sink requests voltage in 20 mV steps and sets a current limit. PPS lets a phone move part of the charging regulation into the charger, which reduces heat inside the phone. EPR adds a similar Adjustable Voltage Supply mode for higher voltages.
USB PD compared with other USB charging methods
USB PD is often confused with USB-C itself and with older charging schemes. USB-C is the connector; USB PD is one protocol that can run over it. The table places the main methods side by side.
| Method | Voltage | Typical maximum power | Negotiation |
|---|---|---|---|
| Default USB 2.0 port | 5 V | 2.5 W | None |
| USB Battery Charging 1.2 | 5 V | 7.5 W | Simple line detection |
| USB Type-C Current | 5 V | 15 W | Resistor signaling on CC |
| USB PD SPR | 5 to 20 V | 100 W | Digital messages on CC |
| USB PD EPR | Up to 48 V | 240 W | Digital messages plus EPR entry |
| Proprietary fast charging | Varies | Varies | Vendor-specific signaling |
Proprietary schemes work only with matching chargers and devices. USB PD is vendor-neutral, which is why many regions now favor it, and why the European common charger rules require it for devices that support fast charging above 15 W.
Roles, swaps and multi-port chargers
USB PD separates the power role from the data role. A laptop can be a sink while charging and a source when powering a phone. Dual-role power devices support a power role swap, which lets two laptops or a laptop and a monitor agree on which one supplies power without unplugging.
Multi-port chargers share a total power budget between ports. A 100 W two-port charger might offer 100 W on one port but split to, for example, 65 W and 30 W when both are in use, and it renegotiates power with the first device when a second one is connected. This brief renegotiation can cause a short charging interruption.
Practical pitfalls with USB Power Delivery
| Symptom | Likely cause | Remedy |
|---|---|---|
| Laptop charges slowly or reports a low-power charger | Charger wattage below laptop requirement | Use a charger matching the laptop rating |
| Charging stops above 60 W | Cable without 5 A e-marker | Use a cable rated for 100 W or 240 W |
| Phone charges slower than advertised | Charger lacks PPS | Choose a charger supporting PPS |
| Power drops when a second device connects | Shared power budget | Check per-port allocation of the charger |
| No charging at all | USB-A to USB-C cable | USB PD requires a USB-C to USB-C path |
A reliable rule is that power is limited by the weakest link: charger, cable or device. All three must support the desired level, and the cable rating is the element most often overlooked.
Stripped to the essentials
USB Power Delivery is a short conversation between a charger and a device before charging starts. The charger says which power levels it can provide, the device picks the one it needs and the charger switches to it.
Because of this agreement, one USB-C charger can safely charge a phone, a tablet and a laptop, each at the right speed. Nothing gets too much power, because nothing is sent until both sides and the cable agree.
Where it is applied
A consultant travels with a laptop that needs 65 W, a phone and wireless earbuds. Previously she carried three different chargers. She replaces them with a single 100 W two-port USB PD charger and two USB-C cables, one of them rated for 5 A.
When she plugs in only the laptop, it negotiates 20 V at 3.25 A and charges at full speed. When she adds the phone, the charger reallocates its budget, giving the laptop 65 W and the phone 30 W through PPS, which keeps the phone cooler during charging.
The setup cuts her travel kit to one charger and two cables, and every device charges at its designed rate because each one negotiates its own power level.