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Converter vs Inverter: Rectifier, DC-DC Converter and Inverter

In power electronics, circuits that change the form of electric power are named by the direction of conversion. A rectifier (AC-DC converter) converts AC to DC, a DC-DC converter (chopper) converts DC to DC at another voltage, and an inverter converts DC to AC. In a broad sense, all of these are sometimes called power converters. Air conditioners and industrial motor drives commonly combine the two ends: a rectifier first turns AC into DC, and an inverter turns it back into AC with an adjustable frequency and voltage.

AspectRectifier circuit (AC to DC)DC-DC converter (DC to DC)Inverter (DC to AC)
Direction of conversionAC → DCDC → DC at another voltageDC → AC with adjustable frequency and voltage
Main switchesDiodes; thyristors if the output is to be controlledA switch such as a MOSFET plus a diode (synchronous rectification replaces the diode with a MOSFET)IGBTs or MOSFETs, each with an antiparallel diode
Basic circuitsSingle-phase bridge of 4 diodes, three-phase bridge of 6; followed by a smoothing capacitorBuck, boost and buck-boost choppers built from a switch, inductor, diode and capacitor; transformer-based topologies (such as flyback) when isolation is neededHalf bridge (2 switches), full bridge (4), three-phase bridge (6)
What sets the outputThe input AC voltage (with thyristors, adjusted by the firing angle α)The switch duty cycle DThe PWM modulation index m (voltage) and the frequency of the modulating wave (output frequency)
Ideal output equationsSingle-phase full-wave average is about 0.9 × RMS (about 90 V for 100 V); with a smoothing capacitor it approaches the peak of about 141 VIn continuous conduction mode, buck Vout = D·Vin and boost Vout = Vin/(1 − D)With sinusoidal PWM, the fundamental amplitude of a leg output is m·E/2 (E is the DC voltage); the three-phase line-to-line RMS is about 0.61·m·E
What remains on the outputRipple at twice the line frequency (full-wave), reduced by the smoothing capacitorRipple at the switching frequency, reduced by the inductor and capacitorHarmonics around the carrier frequency, reduced by the motor winding inductance or an LC filter
Input-side issuesCapacitor charging current is concentrated near the voltage peaks, and the harmonic-rich current lowers the power factor; power factor correction (PFC) improves itIn a buck converter the input current is pulsed, so the input capacitor is importantLarge ripple current flows on the DC side; braking a motor quickly returns power that raises the DC voltage
Typical usesThe input stage of AC adapters and appliance power supplies, the front end of motor drivesCPU and SoC core supplies, rails derived from batteries or 12 V, LED driversMotor speed control (inverter air conditioners, trains, electric vehicles), solar power conditioners, uninterruptible power supplies (UPS)

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

When to use which

Rectifier circuit (AC to DC)
When making the DC that electronics and batteries need from AC mains. In equipment above a certain power, a power factor correction stage often follows to make the input current closer to a sine wave.
DC-DC converter (DC to DC)
When a DC voltage must be changed efficiently to another voltage. The typical example is making the low voltages ICs need from a battery or from a 12 V or 48 V supply.
Inverter (DC to AC)
When making AC from DC, especially to control motor speed by changing the frequency, or to connect power from solar cells or batteries to the AC grid.

FAQ

How are the names converter and inverter used?

In the narrow sense, a device that converts AC to DC is called a converter and a device that converts DC to AC is called an inverter; the converter and inverter sections of a motor drive use the names this way. On the other hand, circuits that change the form of power are also often called converters (power converters) as a group, as in DC-DC converter, so the meaning has to be judged from context.

Why can an inverter change the speed of a motor?

Because AC motors such as induction motors turn at a speed set by the frequency of the applied AC. The synchronous speed of a 4-pole induction motor is 1500 min⁻¹ at 50 Hz and 1800 min⁻¹ at 60 Hz (120f/number of poles), so lowering the frequency with an inverter slows the motor and raising it speeds the motor up. When changing the frequency, the voltage-to-frequency ratio (V/f) is kept roughly constant so that the magnetic flux in the motor stays constant.

Why is the output of a rectifier hard to use as DC on its own?

The output of a diode bridge that only aligns the direction pulsates between 0 and the peak at twice the line frequency. A smoothing capacitor fills in the dips and brings it close to DC. The ripple is roughly load current ÷ (ripple frequency × capacitance): for full-wave rectification at 50 Hz, 2 A and 10,000 µF, it is 2 ÷ (100 × 0.01) = 2 V.

Is the output of an inverter a sine wave?

The output voltage itself is a train of pulses made by switching the DC voltage on and off. Sinusoidal PWM compares a sinusoidal reference with a triangular carrier to vary the pulse widths, so the fundamental obtained by averaging the pulses is a sine wave. In a motor, the winding inductance smooths the current; where a sinusoidal voltage is required, an LC filter is added.

Check your understanding

A sinusoidal AC voltage of 100 V RMS is full-wave rectified with an ideal diode bridge. Without a smoothing capacitor, which of the following is closest to the average output voltage?

  1. AAbout 90 V
  2. B100 V
  3. CAbout 141 V
Show answer and explanation

Answer: A. About 90 V

The peak of the sine wave is Vm = √2 × 100 ≈ 141 V. The average of the full-wave rectified waveform is (2/π) × Vm = (2√2/π) × V ≈ 0.90 × 100 ≈ 90 V. 100 V is the RMS value of the full-wave rectified waveform, the same as that of the input. With half-wave rectification the negative half cycles are missing, so the average is half of this, about 45 V.

Estimate on the assumption that the capacitor supplies all of the load current for one full ripple period. When 50 Hz AC is full-wave rectified, smoothed with a 10,000 µF capacitor, and a constant 2 A flows to the load, which of the following is the approximate peak-to-peak output voltage ripple?

  1. AAbout 1 V
  2. BAbout 2 V
  3. CAbout 4 V
Show answer and explanation

Answer: B. About 2 V

With full-wave rectification the ripple period is half a supply cycle, 1/(2 × 50 Hz) = 10 ms. If the capacitor supplies 2 A during that time, it gives up a charge of 2 A × 10 ms = 20 mC, and the voltage drops by ΔV ≈ I/(2fC) = 20 mC ÷ 10,000 µF = 2 V. In reality the next charging starts slightly before the peak, so the ripple is a little smaller than this estimate. With half-wave rectification the ripple period is 20 ms, giving about 4 V under the same conditions.

Which of the following is the correct basis for deriving the ideal output voltage Vout = D × Vin of a buck converter?

  1. AIn steady state the inductor current returns to its starting value every period, so the voltage across the inductor averages to zero over a period
  2. BIn steady state the output capacitor current is always zero, so the capacitor voltage stays constant regardless of whether the switch is on or off
  3. CWhile the switch is on no voltage appears across the inductor, so the output voltage equals the input voltage
Show answer and explanation

Answer: A. In steady state the inductor current returns to its starting value every period, so the voltage across the inductor averages to zero over a period

The change in inductor current is determined by the time integral of the voltage across it (V = L di/dt). In steady state the current returns to the same value every period, so the voltage across the inductor averages to zero over a period (volt-second balance). In a buck converter, (Vin − Vout) appears while the switch is on and −Vout while it is off, so (Vin − Vout) × D = Vout × (1 − D) gives Vout = D × Vin. The conversion ratios of boost and buck-boost converters are obtained in the same way.

  1. In sinusoidal PWM for an inverter, which of the following correctly describes how the switches are turned on and off?
  2. 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?
  3. Which of the following is the synchronous speed of a four-pole induction motor operated from three-phase AC at 50 Hz?

Practice Power Electronics Circuits questions

Key terms explained

Rectifier circuit
A circuit that converts AC into current flowing in one direction (DC). Types include the half-wave rectifier, which removes the negative half cycles with a diode, the full-wave rectifier, which turns both half cycles the same way with a bridge of four diodes, and the three-phase full-wave rectifier, which rectifies three-phase AC with six diodes. The output contains ripple, so a smoothing capacitor or an inductor brings it closer to DC. Using thyristors instead of diodes lets the firing angle adjust the output voltage.
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.
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.
Smoothing capacitor
A capacitor connected in parallel with the output of a rectifier or similar circuit to reduce the voltage ripple and bring the output closer to DC. It charges while the voltage is high and supplies current to the load while it is low. Large aluminum electrolytic capacitors are used in mains-frequency rectifiers, and ceramic capacitors among others at the outputs of switching power supplies. For full-wave rectification the peak-to-peak ripple can be estimated as load current ÷ (2 × supply frequency × capacitance).
Sinusoidal PWM
A PWM method for making a near-sinusoidal AC output with an inverter. A sine wave at the desired output frequency (the reference) is compared with a triangular wave of much higher frequency (the carrier); the upper switch is on while the reference is larger and the lower switch while it is smaller. The ratio of the reference amplitude to the carrier amplitude is the modulation index, and as long as it is 1 or less, the fundamental of the output voltage is proportional to it. Overmodulation beyond 1 increases low-order harmonics.
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.
Power factor correction
A circuit in equipment powered from an AC supply that shapes the input current into a sine wave of the same form and phase as the voltage, bringing the power factor close to 1 (a PFC circuit). The common approach places a boost converter after the rectifier and controls its switch so that the input current is proportional to the rectified voltage; the output is a DC voltage of about 380 to 400 V, higher than the peak input voltage. It reduces the harmonic currents of a rectifier with plain capacitor smoothing, and a downstream isolated DC-DC converter makes the required voltages from this DC.
Induction motor
An AC motor in which three-phase stator windings create a rotating magnetic field and the rotor turns by the force between that field and the currents induced in the rotor conductors. The rotor turns slightly slower than the field (the synchronous speed), and the fractional lag is called the slip. Simple and rugged, it is widely used in pumps, fans, machine tools and railway vehicles. Changing the frequency with an inverter adjusts its speed over a wide range.

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