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What is Inverter?

A device that converts DC power into AC power of any desired frequency and magnitude. Power switches (IGBTs or MOSFETs) are connected in a bridge and turned on and off by PWM to create an AC voltage. It is used for motor speed control, electric-vehicle drives, power conditioners for solar power and uninterruptible power supplies (UPS). It is different from the inverter (NOT gate) of logic circuits, which flips 0 and 1.

Read the beginner explainer: What Is a Power Semiconductor? SiC, GaN and IGBTs Explained

Japanese name: インバータ

Inverter appears in 25 practice questions across 5 topics, including Power semiconductors, Power Electronics Circuits, Automotive Semiconductors and Functional Safety.

Field: Power Electronics Circuits

Circuit configuration

A single-phase half bridge uses two switches, a full bridge four and a three-phase inverter six, each with a diode in antiparallel to carry reverse current. If the upper and lower switches are on at the same time, the DC supply is short-circuited, so a dead time with both off is inserted at every changeover. The gates of the high-side switches are driven by bootstrap circuits or isolated gate drivers.

Controlling the output

Sinusoidal PWM switches by comparing the desired sine wave with a triangular carrier; the modulation index sets the output voltage magnitude and the frequency of the reference sets the output frequency. With three-phase sinusoidal PWM at a modulation index of 1, the RMS fundamental line voltage is about 0.61 times the DC voltage (about 367 V from 600 V). Induction-motor drives use V/f control, which keeps the voltage-to-frequency ratio constant, or vector control, which controls the current as separate flux and torque components.

Voltage-source and current-source types

An inverter that holds the DC voltage constant with a smoothing capacitor and makes voltage pulses with its switches is a voltage-source inverter; one that holds the DC current constant with a large inductor is a current-source inverter. Today almost all motor drives and power supplies use the voltage-source type. Because of the capacitor on the DC side, care is needed when the motor acts as a generator, as the returning energy raises the DC voltage.

Try it: questions about Inverter

What is the main role of an inverter in power electronics?

  1. ATo convert DC to AC
  2. BTo convert AC to DC
  3. CTo convert light to electricity
Show answer and explanation

Answer: A. To convert DC to AC

An inverter switches power semiconductors at several kHz to tens of kHz and varies the pulse widths (PWM) to synthesize AC of any frequency and voltage from DC. It drives variable-speed motors in air conditioners and washing machines, EVs, solar power conditioners and trains, saving considerable energy by running motors only as fast as needed. Converting AC to DC is done by a converter (rectifier).

What is the main advantage of using SiC Schottky barrier diodes as freewheeling diodes in inverters and similar circuits?

  1. AThey are much cheaper than silicon diodes
  2. BThey emit light so their state can be seen
  3. CThey have almost no reverse recovery current, reducing switching loss
Show answer and explanation

Answer: C. They have almost no reverse recovery current, reducing switching loss

A Schottky barrier diode operates with majority carriers only, so it has almost none of the reverse-recovery current caused by stored minority carriers in pn diodes. Silicon Schottky diodes struggle to reach high voltages, but SiC versions are made at 600–1700 V. As freewheeling diodes in inverters and PFC circuits, they cut not only their own losses but also the turn-on losses of the complementary switch.

FAQ

What does "inverter control" in home appliances do?

It rectifies the mains AC into DC and turns it back into AC of a different frequency and voltage with an inverter, so that the motor of an air-conditioner compressor or a washing machine turns at just the speed needed. Compared with switching on and off repeatedly, it keeps temperatures and the like steady while using less power.

How does it differ from the inverter in logic circuits?

The inverter of logic circuits is the NOT gate, a small-signal circuit that flips an input 0 or 1. The inverter of power conversion is a device that turns DC into AC to deliver power; they share only the name. Both are called "inverter" in English, so the context tells them apart.

Inverter in the explanations

Questions that mention Inverter (25)

  1. What is the main role of an inverter in power electronics?
  2. What is the main advantage of using SiC Schottky barrier diodes as freewheeling diodes in inverters and similar circuits?
  3. What is the main advantage of using SiC power devices in electric vehicle inverters?
  4. Which metric indicates how long a power device can withstand a short circuit, such as both upper and lower switches of an inverter turning on at once, without being destroyed?
  5. What is the purpose of providing a short period in which both switches are off (dead time) when switching the upper and lower switches of an inverter?
  6. How many switching devices does a typical three-phase inverter for driving a three-phase motor use?
  7. When one switch of an inverter turns on, which loss increases due to the reverse recovery of the diode in the opposite arm?
  8. In sinusoidal PWM for an inverter, which of the following correctly describes how the switches are turned on and off?
  9. For inverters that make single-phase AC from a DC voltage E, which of the following correctly compares the output voltage of a half bridge (two switches) and a full bridge (four switches)?
  10. An inverter leg on a 600 V DC supply is operated with sinusoidal PWM at a modulation index of 0.8. Which of the following is the amplitude (peak) of the fundamental of the leg output voltage with respect to the midpoint of the DC supply?
  11. A three-phase inverter on a 400 V DC supply is operated with sinusoidal PWM at a modulation index of 1. Which of the following is closest to the RMS value of the fundamental of the output line-to-line voltage?
  12. In a sinusoidal PWM inverter, which of the following correctly describes what happens when the modulation index is made larger than 1 (overmodulation)?
  13. In a PWM inverter driving a motor, which of the following correctly describes the effect of raising the carrier frequency?
  14. In a voltage-source PWM inverter, which of the following correctly describes the effect of the dead time on the output voltage?
  15. When the speed of an induction motor is varied with an inverter, which of the following is the main reason the voltage-to-frequency ratio (V/f) is kept roughly constant?

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Related terms

SiC (silicon carbide) Power semiconductor device Pulse-width modulation (PWM) Sinusoidal PWM Tail current Schottky barrier diode On-resistance IGBT Dead time GaN (gallium nitride)

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