What is the output voltage of a single-phase full-bridge inverter?

A full-bridge (H-bridge) inverter applies ±Vdc across the load, so the square-wave output has a peak of Vdc and an RMS of Vdc. Its fundamental has a peak of 4Vdc/π and an RMS of 2√2·Vdc/π ≈ 0.9·Vdc — double a half-bridge for the same DC bus. With sinusoidal PWM the fundamental peak is ma·Vdc in the linear region.

What is the difference between bipolar and unipolar PWM?

In bipolar PWM the two legs switch in opposite phase, so the output is a two-level ±Vdc waveform and the harmonics appear around the carrier frequency mf. In unipolar PWM each leg is modulated by its own reference, so the output is a three-level ±Vdc/0 waveform and the lowest harmonics move up to 2·mf — effectively doubling the switching frequency seen by the load and giving lower distortion for the same device switching.

What is selective harmonic elimination in a quasi-square-wave inverter?

A quasi-square wave has a notch of width 2β at each zero crossing. The nth harmonic amplitude is proportional to cos(nβ), so choosing β = 90°/n makes that harmonic zero. For example β = 30° eliminates the 3rd harmonic, β = 18° eliminates the 5th, and the fundamental is scaled to (4Vdc/π)cosβ. This is the basis of selective harmonic elimination (SHE) PWM.

How does a full-bridge inverter differ from a half-bridge?

A half-bridge uses two switches and a split-capacitor mid-point to apply ±Vdc/2 (fundamental 2Vdc/π). A full-bridge uses four switches to apply the full ±Vdc (fundamental 4Vdc/π), doubling the output for the same DC bus, and it also allows unipolar PWM for lower distortion.

Single-Phase Full-Bridge (H-Bridge) Inverter Simulator

An advanced, physics-accurate simulator of the single-phase full-bridge (H-bridge) DC-AC inverter — four switches that apply the full ±V_dc across the load. Compare square-wave, bipolar SPWM, unipolar SPWM and quasi-square modulation, use the notch angle for selective harmonic elimination, and drive an R or R-L load — watching true H-bridge, output-voltage and load-current waveforms on a real-time oscilloscope, validated live against Vo1 = 4·Vdc/π (square), ma·Vdc (SPWM) and (4Vdc/π)cosβ (quasi-square), with total & fundamental RMS, %THD and a real harmonic-spectrum FFT.

Single-phase full-bridge (H-bridge) inverter circuit diagram: a DC source and four switches arranged as an H-bridge with the load in the diagonal; diagonal switch pairs apply +Vdc and -Vdc across the load to produce a square or PWM AC output — Full-bridge (H-bridge) inverter — four switches apply the full ±V_dc across the load. Fundamental V_o1 = 4·V_dc/π (square), double a half-bridge.

Parameters

Modulation

DC bus voltage V_dc

Fundamental frequency 50 Hz

Modulation index m_a 0.80

Vo1 = ma·Vdc (linear); >1 = overmodulation.

Carrier ratio m_f = f_c/f₁ 21

Load

Load resistance R 10 Ω

Load inductance L 10 mH

Adds a lagging current and filters the PWM ripple. L=0 → pure R.

Sampling & display

Sampling rate 1200 pts/cycle

Points plotted per fundamental period

Fundamental periods shown

Run speed (DSO) 1.0×

Show sample points

Presets

Waveforms to display

Leg states Ref & carrier Output voltage Load current

Waveforms — one fundamental period —

sine ref carrier vo output vo fundamental io load current

LIVE

Modulation & harmonic elimination (topic-specific)

—

Harmonic spectrum

Harmonics shown 50

Output-voltage THD (relative to fundamental)—

FFT of the output. Bipolar SPWM → sidebands around m_f; unipolar → around 2·m_f (cleaner); quasi-square → the chosen harmonic vanishes.

Measurements

Live accuracy check — simulation vs closed-form theory

Fundamental output

RMS & distortion

What is a single-phase full-bridge inverter?

A full-bridge (H-bridge) inverter uses four switches in two legs, with the load connected across the bridge diagonal. Turning on one diagonal pair applies +V_dc to the load; the other diagonal pair applies −V_dc. Because it swings the full ±V_dc (versus ±V_dc/2 for a half-bridge), it delivers double the output from the same DC bus, and its two legs enable extra modulation schemes.

Square-wave operation

Vo1(peak) = 4Vdc/π · Vo1(rms) = 2√2·Vdc/π ≈ 0.9·Vdc · Vrms = Vdc · THD ≈ 48.3%

Bipolar vs unipolar PWM (topic-specific)

With sinusoidal PWM the fundamental peak is ma·Vdc in the linear region, but the two legs can be driven two ways:

Bipolar PWM: the legs switch in anti-phase, so the output is a two-level ±V_dc waveform. Harmonics cluster around the carrier m_f.
Unipolar PWM: each leg follows its own reference, so the output is a three-level ±V_dc/0 waveform. The lowest harmonics move up to 2·m_f — as if the switching frequency doubled — giving markedly lower distortion for the same device switching frequency.

Quasi-square wave & selective harmonic elimination

A quasi-square wave places a notch of width 2β at each zero crossing. The n-th harmonic amplitude scales as cos(nβ), so setting β = 90°/n drives that harmonic to zero while the fundamental becomes (4Vdc/π)·cosβ:

Vn ∝ cos(nβ) → β = 30° eliminates the 3rd, β = 18° the 5th, β = 12.9° the 7th

This is the essence of selective harmonic elimination (SHE). Try the "Kill 3rd / Kill 5th" presets and watch that bar vanish in the spectrum.

Half-bridge vs full-bridge

Inverter	Switches	Output swing	Fundamental (square)
Half-Bridge	2	±Vdc/2	Vo1 = 2Vdc/π
Full-Bridge	4	±Vdc	Vo1 = 4Vdc/π

See the half-bridge inverter and the three-phase 180° / 120° VSI simulators.

Applications

Single-phase UPS, solar string inverters, induction heating, variable-frequency drives and grid-tie stages. Unipolar PWM and SHE are widely used to meet harmonic standards with a small output filter.

Frequently asked questions

What is the output of a full-bridge inverter?

Square-wave: ±Vdc, RMS Vdc, fundamental peak 4Vdc/π. SPWM: fundamental peak ma·Vdc in the linear region.

Bipolar vs unipolar PWM?

Bipolar gives a 2-level ±Vdc output with harmonics near mf; unipolar gives a 3-level output with harmonics near 2·mf — lower distortion for the same switching frequency.

What is selective harmonic elimination?

A quasi-square notch of width 2β makes the n-th harmonic (∝ cos nβ) vanish at β = 90°/n, e.g. β = 30° kills the 3rd harmonic.

Half-bridge vs full-bridge output?

The full-bridge applies ±Vdc (fundamental 4Vdc/π) — double the half-bridge's ±Vdc/2 (2Vdc/π) from the same DC bus.

Power4All · Full-Bridge Inverter interactive simulator. All waveforms are produced by numerical integration of the actual switching circuit and validated against closed-form theory.