Boost Converters

Boost converters (also called step-up converters) are a particular topology of switch-mode power supply (SMPS) that use a switching element, inductor, diode (or second switch) and capacitor to convert an input voltage into a higher (or equal) output voltage.

To learn more about EMI/EMC considerations regarding boost converters, see the EMC Design Guidelines page.

Schematics

Boost converters use a switching element, inductor, diode, and capacitor to convert an input voltage $V_{IN}$ into a higher or equal output voltage $V_{OUT}$.

The basic schematic of a boost converter. SW1 is typically a MOSFET switched by control logic (not shown).

How A Boost Converter Works

For an ideal boost converter (no component losses, operating in continuous conduction) the output voltage is determined purely by the input voltage and the duty cycle $D$:

$$\begin{align} V_{OUT} = \frac{V_{IN}}{1 - D} \end{align}$$

$D$ is the duty cycle (the fraction of each switching cycle the switch is on) and varies from $0$ to $1$. Since $1 - D \leq 1$, the output is always greater than or equal to the input — the converter can only step the voltage up.

Note

As with the buck converter, this simple relationship only holds for ideal components and continuous conduction (the inductor current never falls to 0). The duty cycle equation in the design procedure below folds the real-world losses into an efficiency term $\eta$.

The same “the average voltage across the inductor must be 0 over an entire switching cycle” argument used for the buck converter gives us this equation:

1. When the switch is closed ($t_{on}$), the inductor is connected directly across the input, so the voltage across it is $V_{IN}$. The inductor current ramps up, storing energy in its magnetic field.
2. When the switch opens ($t_{off}$), the inductor current can’t stop instantly, so it keeps flowing through the only path available: from the input, through the inductor and diode and into the output. To maintain this current, the inductor reverses its voltage polarity (its switch-node end swings above the input). The output voltage sits at approx. $V_{IN} + V_L$.
3. Setting the average (integral) of the inductor voltage over the whole cycle to 0: $$\begin{align} V_{IN} t_{on} + (V_{IN} - V_{OUT}) t_{off} &= 0 \nonumber \\ V_{IN}(t_{on} + t_{off}) &= V_{OUT} \, t_{off} \nonumber \\ V_{OUT} &= V_{IN} \frac{t_{on} + t_{off}}{t_{off}} \nonumber \\ &= \frac{V_{IN}}{1 - D} \nonumber \end{align}$$ using $D = \frac{t_{on}}{T}$ and therefore $\frac{t_{off}}{T} = 1 - D$.

Note

During the time the switch is off the input is still delivering energy to the output (its the input and inductor in series providing the energy). This is unlike a buck converter, in which the input only delivers energy when the switch is on.

Design Procedure

Duty Cycle

The duty cycle for a boost converter is given by:¹

$$\begin{align} D = 1 - \eta \cdot \frac{V_{IN}}{V_{OUT}} \end{align}$$

As you can see, the duty cycle is dependent only on the ratio between the input and output voltages. This has to be one of the simplest boost converter equations. This equation ignores the specific voltage drops across the switching element and rectifier, but rather lumps them together into the efficiency term.

Inductance

The inductance can be determined using:

$$\begin{align} L = (\frac{V_{IN}}{V_{OUT}})^2 \cdot (\frac{V_{OUT} - V_{IN}}{I_{OUT} \cdot f_s}) \cdot (\frac{\eta I_L}{\Delta I_L}) \end{align}$$

You don’t know $I_L$ or $\Delta I_L$ yet, but the trick is here to assume a maximum inductor ripple current $\Delta I_L$ as a percentage of the average inductor current, $I_L$. A rule-of-thumb is to assume a maximum ripple current of 35% (assuming it is operating in CCM). Thus,

$$\begin{align} \frac{\Delta I_L}{I_L} = 0.35 \end{align}$$

Now the equation for the inductance becomes:

$$\begin{align} L = (\frac{V_{IN}}{V_{OUT}})^2 \cdot (\frac{V_{OUT} - V_{IN}}{I_{OUT} \cdot f_s}) \cdot (\frac{\eta }{0.35}) \end{align}$$

which can be solved as we know all of the variables.

Output Capacitor

During $t_{on}$ the diode is reverse-biased and the load is supplied entirely by the output capacitor. This makes the capacitor’s job harder than in a buck converter — it sees a pulsed current rather than the inductor’s smooth triangular ripple, so both the RMS ripple current and the resulting output voltage ripple are larger.

The minimum output capacitance for a target output voltage ripple $\Delta V_{OUT}$ is:¹

$$\begin{align} C_{OUT(min)} = \frac{I_{OUT} \cdot D}{f_s \cdot \Delta V_{OUT}} \end{align}$$

where:
$I_{OUT}$ is the output current
$D$ is the duty cycle
$f_s$ is the switching frequency
$\Delta V_{OUT}$ is the maximum desired output voltage ripple

As with the buck converter, the capacitor’s ESR adds further ripple on top of this. Because the boost output capacitor also carries significant RMS ripple current, check the capacitor’s ripple-current rating, not just its capacitance and voltage rating.

Output Current

The maximum output current is given by:

$$\begin{align} I_{OUT(max)} = (I_{SW(max)} - \frac{\Delta I_L}{2})(1 - D) \end{align}$$

Diode Selection

The maximum reverse voltage of the diode must be at least equal to the output voltage of the boost converter. This is because the diode sees the full output voltage in reverse when the switch is closed.

Boost Converter Calculator

Use the generic boost-converter design helper below to compute the duty cycle, average and peak inductor current, and required inductance from the supply/output voltages, diode and switch drops, switching frequency, output current and acceptable inductor ripple.

Generic boost-converter design (CCM): enter the input/output voltages, diode and switch drops, switching frequency, output current and acceptable inductor ripple to get the duty cycle, average and peak inductor current, and required inductance. Diode drop is 0 for synchronous (active-rectifier) topologies.

V_in

V

The voltage provided to the input of the boost converter. Usually from a battery or a lower DC rail you want to step up.

V_out

V

The output voltage of the boost converter. Must be higher than V_in.

V_D

V

The forward voltage drop across the diode when fully conducting. Typically ~0.3 V for a Schottky. Use 0 for synchronous (active-rectifier) topologies.

V_SW

V

The voltage drop across the switching element when fully ON. Typically a MOSFET's V_DS(on).

f_SW

Hz

The switching frequency of the transistor (or other switching element).

I_out

A

The average (DC) output current of the boost converter. Note: this is lower than the input current — boosts trade voltage for current.

ΔI_L

%

The peak-to-peak inductor ripple current as a percentage of the average inductor current. A typical rule of thumb is 30–40%.

D

60.48 %

The on/off duty cycle, D = (V_out + V_D − V_in) / (V_out + V_D − V_SW).

I_L,avg

1.265 A

The average inductor (= input) current. For an ideal boost this is I_out / (1 − D).

I_L,peak

1.487 A

The peak inductor current. The inductor's saturation current rating must be above this.

L

6.692 µH

The required inductance to keep the inductor ripple current below the specified percentage.

PCB Routing

The same rules apply for routing boost converters as with any SMPS. See the PCB Routing section on the SMPS page for more information.

Light Load Instabilities

Bad things can happen when boost converters are operated with light/no load. If the controller isn’t smart enough to reduce the duty cycle down to near 0 when there is no or little load, the voltage across the capacitor can build up to a point where it causes damage to part of the circuitry.

Also, if the converter is in DCM and the load current suddenly increases, the output voltage can sag greatly.

Turning Off/Disabling

While most boost controllers have an enable/disable pin, this doesn’t actually disconnect the input from output, as the switching device is not in series with input and output, as it is in a buck converter. Thus if you need the load completely disconnected from the input, you will need to add something like a P-Channel MOSFET or load switch to the front-end of the boost converter.

Start-up vs. Runtime Minimum Input Voltage Requirements

Some boost converters have differing start-up and runtime minimum input voltage requirements. Typically, the boost converter requires a higher minimum input voltage to start (e.g. 18V), but once running, can run off a lower voltage (e.g. 500mV).

Bypass

Some boost converters designed for ultra-lower power applications have a bypass mode. When the output voltage is not needed to be higher than the input voltage, the converter enters a bypass mode in where most of the control circuitry is disabled, the converter stops switching, and the input voltage is “bypassed” straight to the output.

Max. Current Ratings

One gotcha: The “max. current” rating that a manufacturer will provide with a boost controller with an integrated switch will usually be the maximum current rating of the switch. This is not the maximum output current, but rather the maximum input current. The maximum output current, assuming you have a higher output voltage, will be less than this.

Compensation Loop

The compensation loop is part of the feedback mechanism.² The below diagram shows a current-mode controlled boost converter with a transconductance amplifier ($g_m$) providing the feedback.

A simplified diagram of a current-mode boost converter with a transconductance amplifier (gm). Image from Texas Instruments Application Report SLVA452 - Compensating the Current-Mode-Controlled Boost Converter.

The above model is only valid for when the boost converter is acting like a current-mode controlled regulator. This is the case when the ripple current is within the normal operating region (0.2-0.4 times the average input current). When the inductor is oversized to further reduce current ripple through the inductor (less than 0.2 times the average input current), the boost converter behaves more like a voltage-mode controlled regulator and this above model is no longer valid.

External compensation can be added if the manufacturer provides a compensation pin (typically called COMP). The pin is the output of the internal transconductance amplifier.

The external loop compensation components for the TPS61087 boost regulator. Image from the Texas Instruments TPS61087EVM User’s Guide with annotations.

A resistance between 5–100 kΩ and a capacitance between 1–10 nF is typical. A higher resistance corresponds to a faster response time. A lower capacitance corresponds to a higher phase margin.

Down Conversion

Some boost converters also have a built in regulator to provide regulation when the input voltage exceeds the desired output voltage. This is normally a linear regulator, so your efficiency will drop and you will have to take into account the thermal dissipation. This is normally called down conversion.

The internal schematic of a boost converter with in-built down conversion capability (the ability to drop the input voltage).

The price you pay for this added down conversion feature is a slightly higher cost, and slightly higher quiescent current (e.g. some of TI’s boost converters have 19uA quiescent current without down conversion, and 25uA with down conversion).

Input Voltage Range

Typically, boost ICs with an internal switch (a converter) can support lower input voltages than those that require an external switch (a controller). A typical minimum input voltage for a converter is in the range 0.3-0.9V, while a controller’s minimum is in the range 0.9-1.8V.

EMC

The output current loop “hot loop” of a boost converter is the most important part to worry about when it comes to EMC. See the EMC Design Guidelines page for more information.

Footnotes

1. Brigitte Hauke. Basic Calculation of a Boost Converter’s Power Stage [Application Report]. Texas Instruments. Retrieved 2026-05-27, from https://www.ti.com/lit/an/slva372c/slva372c.pdf. ↩ ↩²

2. Texas Instruments. Compensating the Current-Mode-Controlled Boost Converter (SLVA452) [Application Report]. Retrieved 2026-05-27, from https://www.ti.com/lit/an/slva452/slva452.pdf. ↩