Power & Energy
How does a boost converter work?
A boost converter makes a voltage higher than its input. A coil sits on the input, a switch runs from the coil to ground, and a diode leads from there to the output capacitor. Each cycle the switch fills the coil with current, then lets go, and the coil's kick lands on top of the input voltage. The output is about Vin ÷ (1 − D), where D is the fraction of time the switch is on.
Fill, then kick
- Switch on: the coil sits straight across the input, and its current builds up. Energy is stored in its magnetic field.
- Switch off: the coil's current can't stop at once, so the voltage at the switch end leaps up until the diode conducts. The current flows on into the output capacitor and the load, at a voltage above the input.
The diode stops the capacitor emptying back through the switch when it turns on again. It's like a hydraulic ram pump, where a slammed valve uses moving water's surge to lift some of it higher than it started.
Live experiment
Pump it up, one cycle at a time
A 6 V battery, a 0.1 H coil, M1 switching to ground at 500 Hz, a diode and 220 µF into a 240 Ω load. Step one cycle at a time and watch the current's path, then let it run: the output overshoots, then settles at about 11.5 V.
“Step it up” is lesson 5 of 6 in Buck Bay, Power & Energy World 2. Free with an account. Lessons unlock in order, so you’ll do the 11 before it first.
The step-up formula
On half the time, the output is twice the input; on three quarters, four times. Example: 6 V at D = ½ gives 12 V in theory; with the diode's drop and the coil's resistance, about 11.5 V. On a quarter of the time, 8 V in theory and about 7.7 V in practice.
Power in equals power out
A boost makes more volts, not more energy. Power in is roughly power out, so twice the voltage means the input supplies about twice the current: in the example, the battery gives about 94 mA while the load takes 48 mA. A power bank boosting a 3.7 V cell to 5 V at 1 A draws about 1.5 A from the cell, plus losses.
That's why a boost converter's coil and switch must handle the input current, which is larger than the output's.
Only up
A boost can't step down. With the switch off, current flows from the input through the coil and diode anyway, so the output already sits at the input minus a diode drop, and switching can only raise it. Buck and boost use the same four parts, a switch, a coil, a diode and a capacitor, just in a different order: switch first steps down, coil first steps up.
Where you'll meet it
A one-cell torch boosts 1.5 V to light a 3 V white LED. Power banks boost their 3.7 V cells to USB's 5 V. LED backlights, solar chargers and electric cars use boosts too, some at hundreds of volts.
Free lesson
Now build it yourself
Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.
“Step it up” is lesson 5 of 6 in Buck Bay, Power & Energy World 2. Free with an account. Lessons unlock in order, so you’ll do the 11 before it first.
Or start from the very first lesson, no account neededPractice questions
Three quick questions from the lessons. Pick an answer to check it.
Show the answer
Answer: No: with the switch off, the output is already the battery less a diode drop, and switching only raises it. Even never switching, current flows from the battery through the coil and diode, so the output sits near 5.7 V. A boost only goes up; a buck only goes down.
- Why not “Yes, with less on-time”? Less on-time only lowers the boost towards the input minus a diode drop. It never goes below that.
- Why not “Only with a bigger coil”? The coil's size changes ripple and current, not which way the converter steps.
- Why not “Yes, at 50%”? 50% on-time doubles the input, the opposite of stepping down.
Show the answer
Answer: A bigger output capacitor. Ripple ≈ ΔI ÷ (8 f C): bigger C, smaller ripple.
- Why not “A smaller coil”? A smaller coil makes the current wobble more, which adds ripple.
- Why not “A slower switch”? Slower switching means bigger gulps of charge each cycle, so more ripple, not less.
- Why not “A higher input voltage”? A higher input voltage doesn't remove the wobble. The output capacitor is what smooths it.
Show the answer
Answer: A switch, a coil, a diode and a capacitor, arranged differently. Same four parts. Switch first steps down; coil first steps up.
- Why not “Different parts”? They share the same four parts. Only the order changes: switch first steps down, coil first steps up.
- Why not “Transformers”? Neither uses a transformer. A single coil stores and releases the energy each cycle.
- Why not “Zeners”? Zeners hold a voltage by wasting power. Switchers avoid that by switching fully on and off.
“Step it up” is lesson 5 of 6 in Buck Bay, Power & Energy World 2. Free with an account. Lessons unlock in order, so you’ll do the 11 before it first.
Questions people ask
What is the output voltage of a boost converter?
V_out ≈ V_in ÷ (1 − D), where D is the fraction of the time the switch is on. On half the time, the output is about twice the input; on three quarters, four times. Real converters lose a diode drop and a little in the coil.
Can a boost converter step voltage down?
No. Even with the switch never turning on, current flows from the input through the coil and diode, so the output sits at the input minus a diode drop. Switching only raises it. To step down, use a buck; for both, a buck-boost.
Why does a boost converter draw more current than it delivers?
Power in is roughly power out. Doubling the voltage means the input supplies about twice the load's current, plus a little for losses. A 5 V, 1 A USB output from a 3.7 V cell draws about 1.5 A from the cell.
What happens if a boost converter's output is shorted?
A simple boost has a direct path from the input, through the coil and diode, to the output, so the switch can't stop the current. A short draws whatever the source can give. Converters that must survive a short add a separate switch or a fuse.
Related topics
- How does a buck converter work?Switch, coil, diode and capacitor: the step-down converter that halves a voltage with almost no heat.
- Linear regulator vs buck converterBrakes or gears: a linear regulator burns the spare volts, a buck trades them for amps. When each wins.
- How does an inductor work?A coil that fights changes in its current, the high-voltage spike it makes, and how to tame it.
- Batteries in series vs parallelChain cells for more volts, put them side by side for more hours, and never mix a tired one with a fresh one.