A boost converter steps up a DC voltage to a higher DC voltage using an inductor, a switch, a diode, and a capacitor. Building one yourself is straightforward once you understand how these four components work together and how to size each one for your specific input and output requirements. The core principle is simple: store energy in an inductor’s magnetic field, then release it at a higher voltage.
How a Boost Converter Works
A boost converter operates in two phases that repeat thousands of times per second. In the first phase, a transistor switch closes and connects the inductor directly across the input voltage source. Current flows through the inductor, and energy builds up in its magnetic field. The output capacitor sustains the load during this phase.
In the second phase, the switch opens. The inductor resists the sudden change in current by generating a voltage spike that adds to the input voltage. This combined, higher voltage forward-biases the diode and charges the output capacitor while also powering the load. The ratio of time the switch spends closed versus the total cycle time is called the duty cycle, and it directly controls how much the voltage gets boosted.
Calculate Your Duty Cycle First
The fundamental equation for an ideal boost converter in continuous conduction mode is:
Vout = Vin / (1 – D)
Where Vin is your input voltage, Vout is your desired output voltage, and D is the duty cycle (a value between 0 and 1). Rearranging to solve for D gives you:
D = 1 – (Vin / Vout)
So if you want to boost 5 V to 12 V, your duty cycle is 1 – (5/12) = 0.58, meaning the switch is on 58% of each cycle. This number feeds into nearly every other component calculation, so nail it down before selecting parts. Keep in mind that real-world losses from the switch, diode, and inductor resistance mean you’ll need a slightly higher duty cycle than the ideal formula predicts.
Choosing the Inductor
The inductor is the energy storage heart of your converter. Two specifications matter most: inductance value and saturation current rating.
A standard design rule is to target an inductor ripple current of about 30% of the maximum load current, peak to peak. This 30% figure, recommended across most switching regulator datasheets and application notes from companies like Analog Devices, represents a good tradeoff between inductor size, efficiency, and output ripple. Too little ripple means an oversized, expensive inductor. Too much ripple increases stress on every other component and worsens output noise.
The inductance formula for continuous conduction mode is:
L = (Vin × D) / (ΔI × f)
Where ΔI is the desired ripple current and f is the switching frequency. For a 5 V to 12 V converter switching at 100 kHz with a 1 A load, your ripple current target would be 0.3 A. Plugging in: L = (5 × 0.58) / (0.3 × 100,000) = about 97 µH. A standard 100 µH inductor would work.
Critically, make sure the inductor’s saturation current rating exceeds your peak current. Peak inductor current equals the average current plus half the ripple current. If the peak current pushes the core into saturation, inductance drops sharply and current spikes, potentially destroying your switch. Choose an inductor rated for at least 20% above your calculated peak current.
Selecting the MOSFET Switch
The switching transistor in a boost converter is almost always an N-channel MOSFET. Three specifications drive your selection: voltage rating, on-resistance, and gate charge.
The MOSFET’s voltage rating needs to exceed your output voltage with margin for voltage spikes caused by parasitic inductance in your layout. A common practice is choosing a MOSFET rated for at least 1.5 times your output voltage.
On-resistance directly determines conduction losses. Every amp of current flowing through the switch dissipates power as heat proportional to I² × RDS(on). For low-voltage applications (under 30 V), on-resistance is dominated by the channel resistance of the transistor, and modern trench MOSFETs minimize this effectively. For higher-voltage MOSFETs (say 600 V rated), the drift region resistance dominates and on-resistance climbs significantly, so efficiency naturally drops.
Gate charge matters because it determines how much energy your driver circuit spends turning the MOSFET on and off each cycle. Lower gate charge means faster switching transitions and less power wasted in the gate driver. At higher switching frequencies, gate charge losses become increasingly significant.
Picking the Right Diode
The diode in a boost converter conducts current from the inductor to the output only during the switch-off phase, and it needs to block the output voltage during the switch-on phase. For most boost converters operating at tens to hundreds of kilohertz, a Schottky diode is the best choice.
Schottky diodes have reverse recovery times measured in nanoseconds, compared to hundreds of nanoseconds for standard fast-recovery diodes. This matters because during each switching cycle, a slow diode continues conducting briefly in reverse when the switch turns on, wasting energy and generating noise. Schottky diodes also have a lower forward voltage drop (typically 0.3 to 0.5 V versus 0.7 to 1.2 V for fast-recovery types), which directly reduces power loss.
The tradeoff is that Schottky diodes generally top out at reverse breakdown voltages below 100 V. If your output voltage exceeds that, you may need a fast-recovery diode instead, accepting the higher losses. For the common case of boosting battery voltages to 12 V, 24 V, or similar, a Schottky diode is the clear winner.
Sizing the Output Capacitor
The output capacitor smooths the pulsed energy delivery into a steady DC voltage. Its size determines how much the output voltage ripples during each switching cycle. The formula is:
C = D / (R × (ΔVout/Vout) × f)
Where R is the load resistance (Vout/Iout), ΔVout/Vout is your acceptable ripple as a fraction (for example, 1% ripple = 0.01), and f is the switching frequency. Tighter ripple requirements and lower switching frequencies demand larger capacitors.
Beyond raw capacitance, pay attention to the capacitor’s equivalent series resistance (ESR). A high-ESR capacitor adds its own ripple component on top of the capacitive ripple. Low-ESR ceramic capacitors or polymer electrolytic capacitors work well here. For output voltages above what ceramic capacitors comfortably handle, aluminum electrolytic capacitors with a small ceramic capacitor in parallel give you both bulk energy storage and low high-frequency impedance.
Where Efficiency Gets Lost
A well-designed boost converter can reach 90% or higher efficiency, but losses creep in from several sources. Conduction losses in the MOSFET (I² × RDS(on)) and the diode’s forward voltage drop are the two largest contributors. The inductor’s DC resistance adds a third source of conduction loss, which is why choosing an inductor with low DCR matters for high-current designs.
Switching losses occur during the brief moments when the MOSFET transitions between on and off states. During these transitions, both voltage across and current through the device are nonzero simultaneously, dissipating power. Higher switching frequencies multiply these losses. The diode’s reverse recovery behavior also contributes to switching loss, which is another reason Schottky diodes improve efficiency.
Finally, inductor core losses from the alternating magnetic field add up, particularly at higher frequencies or with cheaper core materials. Ferrite cores generally perform well at typical boost converter frequencies.
PCB Layout Makes or Breaks Performance
Even with perfectly chosen components, a poor physical layout can cause high output ripple, voltage regulation problems, electromagnetic interference, and even component failure from voltage spikes. The switch node, where the MOSFET drain, inductor, and diode connect, is the noisiest point in the circuit and deserves the most layout attention.
Keep the switching loop as small and tight as possible. This loop runs from the input capacitor, through the inductor, through the MOSFET, and back to the capacitor’s ground. A large loop area acts like an antenna, radiating noise and adding parasitic inductance that causes voltage spikes when current changes rapidly. Place the inductor close to the switching IC or MOSFET, and use short, wide traces to connect the input capacitor.
Minimize the copper area of the switch node. While you might think more copper is always better for carrying current, a large switch node pad acts as a parasitic capacitance that couples noise into nearby traces. Size it just large enough to handle the current.
Keep sensitive signal traces, especially the feedback voltage divider trace, physically far from the switch node. Never route the feedback trace parallel to or underneath the switch node copper. Switching noise that couples into the feedback signal causes regulation problems and can make the output voltage unstable.
If you’re using a snubber circuit (a small resistor-capacitor network across the switch to dampen voltage ringing), keep its loop area as small as possible to minimize the very parasitic inductance you’re trying to suppress.
Putting It All Together
For a concrete example, here’s a 5 V to 12 V boost converter designed for 1 A output at 100 kHz switching frequency:
- Duty cycle: 0.58 (58%)
- Inductor: 100 µH, rated for at least 1.8 A saturation current
- MOSFET: N-channel, 20 V or higher rating, low RDS(on) in the tens of milliohms range
- Diode: Schottky, 20 V reverse voltage minimum, 2 A forward current rating
- Output capacitor: 47 µF or higher, low ESR, rated for at least 16 V
- Input capacitor: 10 to 22 µF ceramic, placed close to the circuit
You’ll also need a PWM controller IC or a signal generator to drive the MOSFET gate at the correct duty cycle. Dedicated boost controller ICs from companies like Texas Instruments or Analog Devices handle this automatically, adjusting the duty cycle through a feedback loop to maintain your target output voltage. For learning purposes, a 555 timer configured for the right frequency and duty cycle can work, though it won’t regulate the output voltage against load changes.
Start by building the circuit on a breadboard for low-power testing (under 100 mA), keeping wire leads short. For anything above that, move to a proper PCB where you can control loop areas and trace routing. Measure your output voltage ripple with an oscilloscope using a short ground lead to get an accurate picture of performance.