Boost (step-up) · topology: how it works

With Q on, the inductor is tied to ground and stores energy; when Q turns off, the inductor raises its voltage above the input and passes the energy through the diode to the output. Output ≈ input / (1 − duty cycle).

⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)

The longer the gate stays HIGH, the more energy the inductor stores; more duty cycle means a higher output: Vout ≈ Vin / (1 − duty cycle).

Measuring points: tap the numbers on the diagram

Phase 1/2: Q on

Q connects the inductor straight to ground: its current rises, storing energy. The diode is reverse-biased; the capacitor feeds the load.

1. Input

Flat DC input (e.g. 5 V or a Li-ion cell).

On the scope

Multimeter5 V

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Real circuit: circuit tour and fault scenarios

The same topology with every part you'd find on a real board. Follow how it works step by step with the "Circuit tour", or pick a fault scenario below to see what to measure, in order, on the schematic.

⏱ Slow motion: what the driver IC outputs and what the circuit does (the highlighted slice is the moment shown on the schematic)

The longer the GATE pulse, the higher the LED voltage and current. The IC adjusts the pulse width to hold ISEN steady. Brightness is set separately, by a much slower (hundreds of Hz) PWM on Qd.

Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.

Phase 1/2: Q on

The IC turns Q on from GATE; the inductor stores energy from 24 V. Meanwhile Cout feeds the LED bars.

Driver GATE output: HIGH (≈12 V, switch on)

1. Q's drain

Switches between 0 and the output voltage (≈130 V).

On the scope

Multimeteruse the scope

🛠 Fault scenarios: pick a symptom and see what to measure, in order, on the schematic

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🔍 How to recognise it on a board

✓The inductor at the input; the MOSFET after it to ground; the diode towards the output
✓No transformer; the output is higher than the input
✓On TVs, the LED driver circuit that raises 24 V to 100–200 V
✓In power banks, the circuit that raises a 3.7 V cell to 5 V

Where it's used

TV LED backlight drivers, power banks, PFC circuits

Common ICs

MT3608TV LED sürücü entegreleri

Go further

Simulator: buck and boost PFC: the boost on the mains

Other topologies

Flyback Half-bridge (LLC)PFC (power factor correction)Forward Buck (step-down)Full bridge (H-bridge) and SPWM Push-pull MPPT solar charge controller