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What is Buck converter?

A DC-DC converter that produces a DC voltage lower than its input, using a switch, a diode (or a second switch), an inductor and a capacitor. The output is adjusted by the fraction of time the switch is on (the duty cycle D); the ideal output voltage in continuous conduction mode is D × the input voltage. It is the most widely used non-isolated converter, powering CPU and SoC cores and generating various rails from 12 V or 5 V.

Japanese name: 降圧チョッパ(バックコンバータ)

Buck converter appears in 11 practice questions across 2 topics, including Power Electronics Circuits, Power semiconductors.

Field: Power Electronics Circuits

Operation and conversion ratio

While the switch is on, (Vin − Vout) appears across the inductor and its current rises; while it is off, −Vout appears across it through the diode (the freewheeling diode) and the current falls. In steady state these two volt-second products balance, so (Vin − Vout) × D = Vout × (1 − D) gives Vout = D × Vin. For example, D = 0.25 turns 12 V into 3 V.

Choosing the inductor and capacitor

The inductor ripple current is ΔI = (Vin − Vout) × D/(L × f), and one rule of thumb is to choose L so that it is about 20 to 40% of the output current. When the output current falls below ΔI/2, the current drops to zero and the converter enters discontinuous conduction mode. With an ideal capacitor the output voltage ripple is ΔI/(8fC); in practice ΔI × ESR from the ESR is added.

Losses and higher efficiency

The main losses are the conduction losses of the switch and diode, switching losses, and the winding and core losses of the inductor. At low output voltages the diode forward voltage is a large fraction of the output, so synchronous rectification, which replaces the diode with a MOSFET, is standard. With a large step-down ratio such as 12 V to 1 V the duty cycle is only about 8%, and at 1 MHz the on-time is only about 83 ns, which challenges the control and the speed of the devices.

Try it: questions about Buck converter

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.

In a step-down chopper (buck converter), which of the following correctly describes the path of the inductor current while the switch is off?

  1. AIt keeps flowing from the input supply through the switch into the inductor
  2. BIt freewheels through the diode and keeps flowing from the inductor to the output
  3. CThe inductor current drops to zero the instant the switch turns off and flows nowhere
Show answer and explanation

Answer: B. It freewheels through the diode and keeps flowing from the inductor to the output

In a buck converter, while the switch is on, current flows from the input through the inductor to the output and energy is stored in the inductor. When the switch turns off, the inductor tries to keep its current flowing, so the diode (freewheeling diode) conducts and current flows from ground through the diode and the inductor to the output. The current on the output side is therefore uninterrupted, which keeps the output ripple small. Replacing this diode with a MOSFET is synchronous rectification.

FAQ

Why can the output of a buck converter not exceed its input?

The duty cycle D lies between 0 and 1, so the ideal output D × Vin never exceeds the input voltage. The output can be thought of as the average of the chopped input voltage. When a voltage higher than the input is needed, a boost converter or buck-boost converter is used.

Why does the output voltage rise at light load with the same duty cycle?

A buck converter that freewheels through a diode enters discontinuous conduction mode at light load. While the inductor current is zero, the switch node sits at the output voltage, which raises its average, so the output rises above D × Vin. Real power supplies keep the output constant by lowering the duty cycle with feedback control.

Buck converter in the explanations

Questions that mention Buck converter (11)

  1. 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?
  2. In a step-down chopper (buck converter), which of the following correctly describes the path of the inductor current while the switch is off?
  3. Which of the following is the correct basis for deriving the ideal output voltage Vout = D × Vin of a buck converter?
  4. For an ideal buck converter with an input voltage of 12 V, an output voltage of 3 V, an inductance of 15 µH and a switching frequency of 300 kHz, which of the following is the peak-to-peak inductor ripple current in continuous conduction mode?
  5. In an ideal buck converter (freewheeling through a diode) with an input voltage of 12 V, an output voltage of 3 V, an inductance of 15 µH and a switching frequency of 300 kHz, the load current is gradually reduced. Which of the following is the minimum output current that keeps continuous conduction mode (the boundary with discontinuous conduction mode)?
  6. Assume that the entire AC component of the inductor ripple current (0.6 A peak to peak) flows into the output capacitor of a buck converter. With a switching frequency of 300 kHz and an output capacitance of 25 µF (ignore the ESR), which of the following is the peak-to-peak output voltage ripple?
  7. In a synchronous buck converter, which of the following correctly describes the path of the inductor current during the dead time when both the upper and lower MOSFETs are off?
  8. For an ideal step-up chopper (boost converter) with an input voltage of 5 V and a switch duty cycle of 0.6, which of the following is the output voltage in continuous conduction mode?
  9. For an ideal buck-boost chopper (inverting buck-boost converter) with an input voltage of 12 V and a duty cycle of 0.6, which of the following is the magnitude of the output voltage in continuous conduction mode?
  10. Which of the following correctly describes continuous conduction mode (CCM) of a DC-DC converter?
  11. In a forward converter, which of the following correctly explains why a transformer reset winding (or a clamp circuit) is needed?

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Comparisons that include this term

Semiconductor, Electronics and Computing Calculators

DC-DC converter calculatorDuty cycle, inductor ripple current and output voltage ripple for buck, boost and buck-boost

Related terms

Duty cycle Continuous conduction mode Boost converter Synchronous rectification Buck-boost converter Dead time Body diode Reverse recovery time Schottky barrier diode DC-DC converter

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