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Buck vs Boost Converter: Key Differences Explained

Both are switching DC-DC converters built from the same parts (a switch, an inductor, a diode or second switch, and a capacitor), but the parts are arranged differently. A buck converter places the switch between the input and the inductor, chops and averages the input voltage according to the fraction of on-time (duty cycle D), and ideally outputs a lower voltage of D times the input. A boost converter places the inductor on the input side, uses the switch to connect it to ground to store energy in the inductor, then releases that energy to the output, ideally outputting a higher voltage of 1/(1−D) times the input.

AspectBuck (step-down) converterBoost (step-up) converter
Output voltage rangeLower than the input (0 to the input voltage)Higher than the input (the input voltage or above)
Ideal conversion ratio (continuous conduction mode)Vout/Vin = DVout/Vin = 1/(1−D)
Example (D = 0.5)12 V to 6 V12 V to 24 V
Component layoutSwitch between the input and the inductor; inductor on the output side. Diode between ground and the node between the switch and the inductorInductor on the input side; switch between ground and the node after the inductor. Diode between that node and the output
OperationWhen on, current flows through the inductor to the output; when off, the inductor current keeps flowing through the diodeWhen on, energy is stored in the inductor; when off, the inductor voltage adds to the input and the energy is released to the output
Input currentDiscontinuous (flows only while the switch is on). The input capacitor is importantContinuous (keeps flowing through the inductor)
Output currentContinuous (keeps flowing from the inductor). Output ripple is smallDiscontinuous (delivered only while the switch is off). The ripple current in the output capacitor is large
EfficiencyAbout 80 to 95% depending on conditionsAbout 80 to 95% depending on conditions. Drops at high boost ratios
ControlThe control loop is easy to designIn continuous conduction mode there is a right-half-plane zero, making it hard to raise the control loop bandwidth
When switching stopsThe input and output are disconnectedThe input voltage remains at the output through the inductor and diode. A separate switch is needed to disconnect completely
Typical usesGenerating CPU or SoC core voltages (around 1 V) from 12 V or 5 V, generating low voltages from batteriesGenerating 5 V from a lithium-ion battery (about 3.0 to 4.2 V), driving series LEDs, power factor correction (PFC) in AC power supplies

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Buck (step-down) converter
When you want to efficiently generate a voltage lower than the input at high current. CPU and SoC core supplies, and generating various voltages from 12 V or 5 V.
Boost (step-up) converter
When you need a voltage higher than the input. Generating a 5 V USB output from a battery, driving series-connected LEDs, or generating a high DC voltage in PFC.
Buck-boost
When the battery voltage crosses above and below the output voltage as it is used, use a buck-boost converter, which can do both.

FAQ

How are the conversion ratios D and 1/(1−D) derived?

In steady state, the voltage across the inductor averages to zero over one cycle (the volt-second balance). In a buck converter, (Vin − Vout) is applied during the on-time and −Vout during the off-time, so (Vin − Vout) × D = Vout × (1 − D), giving Vout = D × Vin. In a boost converter, Vin × D = (Vout − Vin) × (1 − D) gives Vout = Vin/(1 − D).

Why does a boost converter output a voltage higher than its input?

While the switch is on, current flows through the inductor and stores energy. When the switch turns off, the inductor tries to keep the current flowing and generates a voltage, which adds to the input voltage and flows through the diode to the output, so the output is higher than the input.

Can the boost ratio be made arbitrarily large by bringing D close to 1?

Ideally 1/(1−D) grows without limit, but in practice losses from the resistance of the inductor and switch increase, the output voltage levels off and efficiency drops. Practical boost ratios are up to a few times; for higher ratios, approaches such as transformer-based topologies are chosen.

What is synchronous rectification?

It replaces the diode with a MOSFET that is turned on when the diode would conduct. It reduces the loss from the diode's forward voltage (about 0.3 to 0.5 V even for a Schottky barrier diode), so efficiency improves more as the output voltage gets lower.

Check your understanding

In an ideal step-down chopper (buck converter) with an input voltage of 12 V and a switch duty cycle of 0.25, what is the output voltage?

  1. A3 V
  2. B9 V
  3. C48 V
Show answer and explanation

Answer: A. 3 V

An ideal buck converter outputs input voltage × duty cycle: 12 V × 0.25 = 3 V. Varying the switch's on-time ratio adjusts the output, and an inductor and capacitor average it into smooth DC. Synchronous rectification, using a MOSFET instead of a diode, pushes efficiency above 90%. The low supply voltages of CPUs and memory are produced by multiphase buck converters on the motherboard.

Which DC-DC converter outputs a DC voltage higher than its input?

  1. AStep-up chopper (boost converter)
  2. BStep-down chopper (buck converter)
  3. CBridge rectifier
Show answer and explanation

Answer: A. Step-up chopper (boost converter)

A boost converter stores energy in the inductor while the switch is on, then adds the inductor's voltage to the input when it turns off. Ideally the output is input ÷ (1 − duty cycle)—twice the input at a duty of 0.5. Applications include driving LEDs from a single battery cell, boosting solar-panel voltage, raising motor-drive voltage in hybrid cars, and PFC circuits.

What is the main advantage of raising the switching frequency of a power converter?

  1. ASwitching loss always decreases
  2. BPassive components such as inductors and capacitors can be made smaller
  3. CElectromagnetic noise always decreases
Show answer and explanation

Answer: B. Passive components such as inductors and capacitors can be made smaller

Raising switching frequency reduces the energy inductors and capacitors must store each cycle, so they can be smaller and supplies become compact and light; transformer size, for example, shrinks roughly in inverse proportion to frequency. Switching loss and noise rise with frequency, however, so low-loss, fast SiC and GaN devices are what make high-frequency operation practical.

  1. Compared with a linear regulator (such as an LDO), which is a characteristic of a switching regulator?
  2. What is the purpose of a diode (flyback diode) connected in parallel with an inductive load such as a relay coil?

Practice Power semiconductors questions

Key terms explained

DC-DC converter
A circuit that converts one DC voltage into another. Using switches, inductors and capacitors, buck types lower the voltage and boost types raise it.
Duty cycle
The fraction of each period during which a periodic pulse signal is high. In PWM it sets the average power.
Schottky barrier diode
A diode that uses a metal-semiconductor junction (Schottky junction). It has a low forward voltage and switches fast because there is no minority-carrier storage.
Power management IC (PMIC)
An IC that generates multiple voltages from a battery or similar and manages power-up sequencing and voltage changes.

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