# What is Full Bridge Inverter? – Circuit, Working & Advantages

Inverters are circuits that convert dc power into ac power at desired output voltage and frequency. The inverters achieve this by using thyristors as switches and hence circuits with fewer components have non-sinusoidal output waveform. By adding circuit complexity, nearly a sinusoidal output voltage is possible to obtain.

Depending upon commutation circuits i.e., some external means that are needed to forcefully turn OFF the thyristor. The inverter circuits are classified broadly as,

In the last article, we have learned about half-bridge inverters. The main problem of a half-bridge inverter is the requirement of a 3-wire dc supply. This drawback can be eliminated by using a full-bridge inverter. In this article, let us learn about the full-bridge inverter with circuit diagrams and waveforms.

## Full Bridge Inverter With R Load :

The below figure illustrates the single-phase full-bridge inverter circuit using thyristors as switching devices. Here the inverter circuit uses four thyristors divided into two pairs (T1, T2, and T3, T4). One pair of devices operates simultaneously.

### Working of Full-Bridge Inverter With R Load :

In the above circuit diagram, the commutating circuit of the thyristors is not shown for simplicity. Assume that on removing gate pulses to the thyristors they are turned OFF. The full-bridge inverter operates in two modes in one cycle of ac voltage at the output side.

#### Mode-I : During the interval between 0 and T/2 i.e., for 0 ≤ t ≤ T/2, thyristor pair T1 and T2 are triggered and they start conducting. When T1 and T2 are turned ON, the load voltage follows the positive source voltage Vdc. At the instant t = T/2, the triggering pulses are removed for both the thyristors T1 and T2 and are turned OFF.Mode-II : At the same time i.e., at t = T/2, when T1 and T2 get turned OFF, the triggering pulses are given to thyristor pair T3 and T4. This pair will conduct for the interval between T/2 and T (for T/2 ≤ t ≤ T) and the load voltage follows the negative source voltage -Vdc as shown below.The thyristor pair T3, T4 conducts until thyristor pair T1, T2 have triggered again at t = T, and the cycle repeats. The below shows the waveforms of gating signals, output voltage, and currents of thyristors and diodes. Since load is assumed to be purely resistive, the load current will be equal to load voltage.

From the above two modes, we can observe that the direction of current flowing through the load in mode-I is opposite to the current flowing through the load in mode-II. The thyristor pair T1, T2 conduct for time period 0 to T/2 for producing positive half cycle, and thyristor pair T3, T4 conduct for time period T/2 to T for producing negative half cycle. Thus an alternating output is obtained at the output side from a dc power.

## Full Bridge Inverter With RL Load :

In the above operation of the full-bridge inverter, we know that load connected is of resistive type. But generally, in practice, most of the loads are resistive-inductive (RL) type. Now let us see the operation of a full-bridge inverter with RL load. The below shows the circuit diagram of a full-bridge inverter with RL load.

Compare to the circuit diagram with resistive load, in this circuit, two extra diode pairs (D1, D2, and D3, D4) are used. The four diodes are connected in anti-parallel to the four thyristors. The purpose of diodes is discussed below in working.