On a barrel-jack laptop, adapter power enters the board through a jack, a fuse, a protection diode and a pair of MOSFETs, and reaches the charger. On a USB-C laptop the path is longer and smarter: power enters only after a negotiation, passes through switches controlled by the PD controller, may meet power from another port or a barrel jack, and reaches a charger that must cope with input voltages from 5 V to 20 V or more. Knowing each stage of that path tells you where to measure when a USB-C laptop will not charge.
The stages of the path
| Stage | Parts | Job |
|---|---|---|
| Port | USB-C connector, VBUS pins | Power enters |
| Protection | TVS diodes, overvoltage protection switch | Clamp spikes, block overvoltage |
| Sink switches | Back-to-back MOSFETs or a switch IC, driven by the PD controller | Connect VBUS to the board only after a valid contract |
| Combining | Ideal-diode controllers or switches | Merge multiple ports or a barrel jack |
| Charger input | Input sense resistor, input capacitors | Measure and filter input |
| Charger | Buck-boost charger IC | Make the system rail from any input voltage |
VBUS is described under vbus.
Stage 1: the port and protection
VBUS enters through several connector pins. Close to the connector sit protection parts:
- a TVS diode to clamp spikes;
- on many boards, an overvoltage protection switch that disconnects VBUS if it rises above a safe level;
- capacitors to filter the input.
Damaged ports and shorted TVS diodes are the most common faults at this stage.
Stage 2: the sink switches
The PD controller keeps the sink switches off until it has negotiated a contract with the charger. Then it turns them on, and VBUS reaches the board. The switches are usually two N-channel MOSFETs back to back, so that no current flows in either direction while they are off. Their gates are driven by the PD controller or a dedicated driver.
Measurements:
- with a charger attached and a contract made — VBUS present before the switches and after them;
- gate drive — present when the switches should be on;
- with power removed — each MOSFET's drain–source in both directions.
The controller side is covered in USB-C Laptop Not Charging: PD Controller Diagnosis, and the schematic pages in Reading the USB-C Section of a Laptop Schematic.
Stage 3: combining inputs
Laptops with two USB-C ports, or a USB-C port and a barrel jack, must combine their inputs without letting one feed back into the other. They use:
- ideal-diode controllers — a MOSFET driven to behave like a diode with almost no voltage drop, described under ideal diode controller;
- switches selected by the PD controller or EC, so only one source is connected at a time.
A fault here can make one port charge and the other not, or make the laptop charge only from the barrel jack.
Stage 4: the charger input
After combining, the input passes an input sense resistor — used by the charger to measure and limit input current — and input capacitors. Then it reaches the charger IC.
Stage 5: the buck-boost charger
Because a USB-C charger may supply anything from 5 V to 20 V or more, the laptop's charger must produce the system rail from inputs both below and above the battery voltage. Buck-boost chargers do this with four switches around one inductor. The charger's general behaviour is covered in The Laptop Charging IC: How It Works.
The dead-battery start
When the battery is flat and nothing on the board has power, the PD controller must still be able to negotiate. Designs handle this by:
- powering the PD controller from VBUS through a small regulator when nothing else is available;
- allowing the sink switches to turn on at the default 5 V before a full negotiation, so the board can wake.
If this path fails, the laptop charges normally when the battery has some charge but cannot revive a flat battery — a distinctive symptom.
The reverse direction
USB-C ports can also supply power to accessories. A separate source switch connects a 5 V rail to VBUS when a device that needs power is attached. A short on an accessory or cable trips that switch's current limit; a failed source switch leaves accessories unpowered while charging still works.
Measuring along the path
With a charger connected through a USB-C power meter and a contract made, work from the port inwards and note the voltage at each stage: at the connector's VBUS capacitors, after the overvoltage protection switch, after the sink switches, after any combining stage, and at the charger's input capacitors. Each point should show essentially the contract voltage. The first point where it disappears marks the faulty stage. With power removed, the same points measured to ground find a short that stops the charger from completing the contract in the first place.
Laptops with a barrel jack too
Some laptops have both a USB-C port and a barrel jack. The two inputs meet in the combining stage, and the firmware may prefer one over the other when both are connected. A fault in the combining parts can block one input entirely while the other works, or let the barrel input back-feed the USB-C port. When only one input works, compare the voltages on each side of the combining switches with each source connected in turn.
Where faults usually hide
| Symptom | Likely stage |
|---|---|
| Charger sees nothing, no 5 V | Port, CC lines, PD controller |
| 5 V only, no contract | PD controller supply or firmware |
| Contract made, no power on the board | Sink switches or their drive |
| One port charges, the other does not | That port's protection, switches or combining path |
| Charges with battery present, not when flat | Dead-battery path |
| Power on the board, battery not charging | Charger, battery, EC settings |
A USB-C power meter between charger and laptop tells you the contract state quickly; the available profiles for a charger are listed by USB-C Power Delivery Calculator.
Example: one port of two
A laptop with two USB-C ports charges from the right port only. On the left port, a USB-C meter shows a 20 V contract, so the PD controller and negotiation work. With the charger attached, VBUS is present before the left port's sink switches and absent after them; their gate drive is present. With power removed, one of the back-to-back MOSFETs reads open in both directions — its gate drive arrives but it never conducts. A new MOSFET restores charging from the left port.
FAQ
Why does a USB-C laptop need sink switches?
So power enters only after a valid contract, and never flows back out of the port while the laptop powers itself.
What is an ideal-diode controller?
A circuit that makes a MOSFET behave like a diode with very little voltage drop, used to combine power inputs.
Why can my laptop charge with a battery but not from flat?
The path that powers the PD controller from VBUS alone, or the default sink path, is faulty.
Why does only one port charge?
That port's protection, switches or combining path has a fault, while the shared parts work.
Why does the charger need to be buck-boost?
USB-C inputs can be below or above the battery voltage, so the charger must step both down and up.
Bottom line
The USB-C power path runs from VBUS through protection and sink switches, combining circuits and an input sense resistor to a buck-boost charger. Measure stage by stage with a contract in place, check gate drives and switches, and remember the dead-battery path and the combining circuits when only some ports or some situations fail.