Single phase half bridge inverter

Inverter is a device that converts dc power into ac power at desired output voltage and frequency.The output voltage waveform of an ideal inverter should be sinusoidal. However, the waveform of practical inverter are non sinusoidal and contain certain harmonics. For low and medium power applications, square wave or quasi square wave voltages may be acceptable. For high power applications, low distorted sinusoidal waveform is required.

Types of inverter

According to nature of input source, there are 2 types of inverter

Voltage source inverter
Current source inverter

Voltage source inverter has a stiff dc voltage while a current source inverter has a high impedance in series with the dc source.

The output voltage of a VSI is independent of the nature of the load.

Types of voltage source inverter

Single phase inverter
Three phase inverter

In single phase inverter, there are 2 types namely

Half bridge inverter

Full bridge inverter

In this article we will see about the single-phase half bridge inverter

Single-phase half bridge inverter

It consists of two switches and two diodes

Requires three wire DC supply.

The amplitude of output voltage and power is half of full bridge inverter

Single-phase half bridge inverter-R load

In case of R load, there is no use of feedback diodes.

When T1 is turned on, the load current flows from A to B. Applying KVL, the voltage across the load is + Vs/2.
When T1 is turned off and T2 is turned on, the load current flows from B to A. Applying KVL, the voltage across the load is – Vs/2.
For resistive load, voltage and current are in phase.

Fourier analysis of output voltage of half bridge inverter

The output voltage of the inverter is a periodic function. It is a square wave with a frequency (1/T). We are more concerned about the fundamental component which contributes to useful work. We want to remove the unwanted other harmonic components. Hence a fourier analysis on the output voltage waveform can help to identify the harmonics and eliminate it.

Fourier series is an infinite series used to represent a periodic function.

It is not difficult to find the Fourier coefficients if we know the symmetry of inverter output waveform . The inverter output voltage waveform exhibits odd symmetry and half wave symmetry. In such cases, only odd harmonics exists and coefficients ao and an will be zero.

Effect of waveform symmetry on Fourier coefficient

if we find the total harmonic distortion in the inverter output voltage waveform, it is a very high value 48.4%. and its value is independent of the magnitude of the output voltage.

Single-phase half bridge inverter-L load

The property of the inductance is that it will not allow to change its current direction instantaneously. So it is necessary to use feedback diodes in anti parallel with the switches to allow bidirectional current. In case of L load, the current through the inductor will be a triangular waveform as a dc voltage is applied across the inductor.

Single-phase half bridge inverter-RLC load (overdamped)

Depending upon the RLC values a sinusoidal waveform appear as the current waveform. In this case also current lags the voltage. Operation of the inverter remains same as the L load.

Single-phase half bridge inverter-RLC load (underdamped)

In this case, the current leads the voltage by an angle. The current through the thyristor T1 becomes zero beforeT2 is gated. This leads to load commutation where T1 is turned off without any forced commutation circuit.

In all the cases, we can observe that the output voltage remains the same while the output load current shape depends on the nature of the load.

Points to remember

VSI-Output voltage is independent of nature of load
Output current depends on the load type.
Inverter output -only odd harmonics exist
Magnitude of fundamental component > magnitude of inverter output voltage
THD is independent of output voltage magnitude