A laptop motherboard contains dozens of MOSFETs, and they are among the parts most often found shorted after a power fault. But they do very different jobs in different places, and the right way to test one depends on where it sits. A MOSFET in a regulator's power stage behaves differently from one switching a USB port or protecting the adapter input. This guide maps the MOSFETs you will find on a laptop board and gives a practical test method for each group.
The MOSFET families on a laptop board
| Location | Typical type | Job | Common failure |
|---|---|---|---|
| Regulator power stages | Dual N-channel, or integrated power stage | Switch the system rail into the inductor | Drain–source short |
| Charger section | N-channel or P-channel pairs | Input switching, charging, battery connection | Short after adapter faults |
| Input protection | Back-to-back pair | Block reverse current, disconnect bad adapters | Short or open after surges |
| Load switches for rails | P-channel, or N-channel with a driver | Turn run rails and port power on and off | Short, or stuck partly on |
| Signal switching | Small N-channel | Level shifting, inverting enables, driving LEDs | Gate damage from static or liquid |
The basics of the device are in the glossary under MOSFET; the general multimeter technique is covered in How to Test a Transistor with a Multimeter: BJT and MOSFET.
Power-stage MOSFETs
In a buck regulator, a high-side MOSFET connects the system rail to the switch node, and a low-side MOSFET connects the switch node to ground. The two are often in one package, or replaced by an integrated power stage that also contains the gate driver. The arrangement is explained under high side and low side and in Buck Converters on Laptop Boards: How a Rail Is Made.
Unpowered tests
Measure at the inductor's switch-node pad:
- Switch node to ground — near zero means a shorted low-side MOSFET, or something else on the node.
- Switch node to the system rail — near zero means a shorted high-side MOSFET.
- System rail to ground — a short here may be a high-side and low-side MOSFET both failed, or an input capacitor.
Then compare with an identical phase or regulator elsewhere on the board in diode mode. Integrated power stages have more pins — supply, enable, PWM input, current monitor — and each should read like the same pin on a sibling stage.
Confirming before replacement
Lift the inductor to separate the switch node from the output. If the short stays on the switch-node side, the power stage is the suspect. Remove it and measure the pads again: if the short is gone, the stage was at fault.
Load-switch MOSFETs
Load switches connect an always-on rail to a switched rail. A P-channel MOSFET is common: its source sits on the input rail, its drain on the switched rail, and a resistor keeps its gate high (off) until a driver pulls it low.
Powered tests
- Gate voltage — when the rail should be off, the gate should be at the source voltage. When it should be on, the gate should be several volts below the source.
- Drain voltage — when the rail should be on, drain and source should be almost equal. A noticeable drop under load means a damaged MOSFET or one that is not fully on.
- Rail present when it should be off — the MOSFET is shorted, or its gate is not being pulled up.
Unpowered tests
Compare drain–source in both directions with a sibling switch. A P-channel MOSFET shows its body diode from drain to source (red probe on drain). A reading near zero both ways is a short.
Input and battery switches
The charger section uses MOSFETs to connect the adapter, to block reverse current and to connect the battery. These see the highest voltages and the largest surges.
- With power removed, measure each MOSFET's drain–source in both directions and compare with the datasheet's body-diode direction.
- With the adapter connected, measure the voltage on both sides of the input MOSFETs. If the adapter voltage stops at the MOSFETs, check their gate drive from the charger IC before replacing them — a charger that has not accepted the adapter will not turn them on.
- A shorted input MOSFET pair often comes with a shorted TVS diode or a damaged charger IC after a bad adapter.
Signal MOSFETs
Small SOT-23 MOSFETs invert enables, shift logic levels and drive LEDs. Liquid and static damage their gates. Test them in diode mode against a sibling and on a powered board by checking that the output follows the gate as the schematic says it should.
Testing under power: the voltage drop method
On a working board, a MOSFET that is fully on drops very little voltage: the drop equals the current times its on-resistance, described under rds on. Measuring drain-to-source voltage on a powered, loaded circuit is a sensitive test:
- A drop of a few millivolts is normal.
- A drop of hundreds of millivolts means the MOSFET is not fully on — weak gate drive or a damaged device — and it will run hot.
Heat as a clue
A MOSFET that runs much hotter than its siblings is either carrying more current, not fully on, or partly shorted. A thermal camera makes this obvious; a finger on a current-limited board can confirm it. Duty cycle, ripple and peak current for a regulator phase can be estimated with Buck Converter Calculator (Duty Cycle and Inductor Ripple) to judge whether its heat is plausible.
Replacement
Match the channel type, package and pinout, voltage rating, current rating, on-resistance and gate threshold. For power stages, use the same part or a documented equivalent — gate charge and switching behaviour affect efficiency and the controller's stability. Always find why a MOSFET failed before powering a board with a new one: a shorted load or a driver that switched both MOSFETs on at once will destroy the replacement.
Example: a load switch stuck half on
A laptop's battery drains in a day while switched off. With the adapter removed and the laptop off, a 3.3 V run rail that should be at zero reads about 1.4 V. Its load switch is a P-channel MOSFET; its gate measures about 2 V instead of the full 3.3 V of its source. The gate's pull-up resistor is intact, but the small N-channel MOSFET that pulls the gate down reads leaky in diode mode compared with an identical part next to it. Replacing that small MOSFET lets the gate rise fully, the switched rail drops to zero in standby, and the battery drain stops.
FAQ
Can I test MOSFETs without removing them?
Yes, for most faults: compare with sibling parts in diode mode and check gate and drain voltages on a powered board. Remove the part only to confirm.
Why did my replacement power-stage MOSFET fail immediately?
The cause remained: a shorted output, a faulty controller, or a gate driver turning both switches on together.
How do I find a MOSFET's pinout?
From its datasheet, or from the schematic symbol and the boardview pin numbers.
Are integrated power stages tested differently?
Their power pins are tested like a MOSFET pair; their control pins are compared with a sibling stage on the same board.
Is a hot MOSFET always faulty?
Not always. Power-stage MOSFETs under load run warm. One that is much hotter than its siblings is suspicious.
Bottom line
MOSFETs on laptop boards fall into power stages, charger and input switches, load switches and signal switches. Test each group the way its circuit works: switch-node shorts for power stages, gate and drain voltages for load switches, gate drive and body diodes for input switches, and sibling comparison for everything. Find the cause before replacing a failed MOSFET.