Almost every rail on a laptop board comes from a buck (step-down) converter: two MOSFETs chop the system voltage into pulses, an inductor and capacitors smooth them into a lower DC voltage. A few numbers describe how hard each part works. This calculator gives the duty cycle, the inductor's ripple and peak current and the average input current — useful when checking a replacement inductor, reading a scope trace or understanding why a phase runs hot.
How to use it
- Enter the input voltage (the system rail), output voltage and load current.
- Enter the switching frequency from the controller's datasheet or a scope measurement.
- Enter the inductor value from the schematic or its marking.
- Optionally enter the efficiency; 85–92 % is typical for laptop rails.
The formulas
- Duty cycle: D ≈ Vout ÷ (Vin × η). Ideal converters use D = Vout ÷ Vin.
- High-side on-time: t_on = D ÷ f.
- Inductor ripple: ΔI = (Vin − Vout) × D ÷ (f × L), peak to peak.
- Peak inductor current: I_pk = I_out + ΔI ÷ 2.
- Input current: I_in = Vout × I_out ÷ (Vin × η).
Designers usually aim for ripple of 20–40 % of the load current. Much lower needs a large inductor; much higher stresses capacitors and MOSFETs.
Worked example: a 1.05 V rail
From 19 V to 1.05 V at 8 A, 500 kHz, 0.47 µH and 90 % efficiency: the duty cycle is about 6 %, on-time about 0.12 µs, and the ripple close to 4.7 A — more than half the load, so the peak inductor current is about 10.3 A. That tells you the replacement inductor's saturation current must comfortably exceed 10 A, not just 8 A. The very short on-time also explains why these converters are sensitive to layout and why their switch node looks like narrow spikes on a scope. Converter basics are in Buck Converters on Laptop Boards: How a Rail Is Made and buck converter.
Where it matters on a laptop board
- Choosing a replacement inductor with enough saturation current.
- Interpreting a switch-node waveform: the measured duty cycle should roughly match Vout ÷ Vin.
- Estimating how much current a regulator draws from the system rail.
Multi-phase processor regulators split the load between phases; enter the current per phase. Phases are described in VCore Phases, Inductors and Output Capacitors.
FAQ
Why is my measured duty cycle higher than calculated?
Losses: the converter must stay on longer to make up for resistance in MOSFETs and inductor. Light-load modes also change the pattern.
Can I replace an inductor with a different value?
Only with care. A smaller value raises ripple and peak current; a larger one slows the response. Keep the original value and at least its current rating.
What is the switching frequency of a laptop regulator?
Typically a few hundred kilohertz to around a megahertz; check the controller datasheet or measure the switch node.