What is a Full Bridge Inverter : Working & Its Application

The inverter is an electrical device that converts DC input supply to symmetric AC voltage of standard magnitude and frequency at the output side. It is also named as DC to AC converter. An ideal inverter input and output can be represented either in a sinusoidal and non-sinusoidal waveforms. If the input source to the inverter is a voltage source, then the inverter is said to be called a voltage source inverter (VSI) and if the input source to the inverter is a current source then it is called as current source inverter (CSI). Inverters are classified into 2 types according to the type of load being used i.e, single-phase inverters, and three-phase inverters. Single-phase inverters are further classified into 2 types of half-bridge inverter and full-bridge inverter. This article explains the detailed construction and working of a full-bridge inverter.

What is a Single Phase Full Bridge Inverter?

Definition: A full bridge single phase inverter is a switching device that generates a square wave AC output voltage on the application of DC input by adjusting the switch turning ON and OFF based on the appropriate switching sequence, where the output voltage generated is of the form +Vdc, -Vdc, Or 0.

Classification of Inverters

Inverters are classified into 5 types they are

According to the output characteristics

• Square wave inverter
• Sine wave inverter
• Modified sine wave inverter.

According to the source of the inverter

• Current source inverter
• Voltage source inverter

According to the type of load

Single-phase inverter

• Half-bridge inverter
• Full bridge inverter

Three-phase inverters

• 180-degree mode
• 120-degree mode

According to different PWM technique

• Simple pulse width modulation (SPWM)
• Multiple pulse width modulation (MPWM)
• Sinusoidal pulse width modulation (SPWM)
• Modified sinusoidal pulse width modulation (MSPWM)

According to the number of output levels.

• Regular 2 level inverters
• Multi-level inverter.

Construction

The construction of full-bridge inverter is, it consists of 4 choppers where each chopper consists of a pair of a transistor or a thyristor and a diode, pair connected together that is

• T1 and D1 are connected in parallel,
• T4 and D2 are connected in parallel,
• T3 and D3 are connected in parallel, and
• T2 and D4 are connected in parallel.

A load V0 is connected between the pair of choppers at “AB” and the end terminals of T1 and T4 are connected to voltage source VDC as shown below.

An equivalent circuit can be represented in the form of the switch as shown below

Working of Single Phase Full Bridge Inverter

The working of single-phase full-bridge using RLC load inverter can be explained using the following scenarios

Overdamping and Underdamping

From graph at 0 to T/2 if we apply DC excitation to RLC load. The output load current obtained is in the sinusoidal waveform. Since the RLC load is being used the reactance of the RLC load is represented in 2 conditions as XL and XC

Codition1: If XL> XC, it acts like lagging load and is said to be called as an overdamped system and

Condition2: If XL< XC, it acts like leading load and is said to be called an underdamped system.

Conduction Angle

Conduction angle of each switch and each diode can be determined using the waveform of V0 and I0.

At Lagging Load Condition

Case1: From φ to π, V0 > 0 and I0 > 0 then switches S1, S2 conducts
Case2: From 0 to φ, V0 > 0 and I0 < 0 then diodes D1, D2 conducts
Case3: From π + φ to 2 π, V0 < 0 and I0 < 0 then switches S3, S4 conducts
Case4: Form π to π + φ, V0 < 0 and I0 > 0 then diodes D3, D4 conducts.

At Leading Load Condition

Case1: From 0 to π – φ, V0 > 0 and I0 > 0 then switches S1, S2 conducts

Case2: From π – φ to π, V0 > 0 and I0 < 0 then diodes D1, D2 conducts

Case3: From π to 2 π – φ, V0 < 0 and I0 < 0 then switches S3, S4 conducts

Case4: Form 2 π – φ to 2 π, V0 < 0 and I0 > 0 then diodes D3, D4 conducts

Case 5: Prior to φ to 0, D3, and D4 conduct.

Therefore conduction angle of each diode is “φ” and the conduction angle of each Thyristor or Transistor is “π – φ”.

Forced Commutation and Self Commutation

Self Commutation Situation can be Observed in Leading Load Condition

From the graph, we can observe that “φ to π – φ”, S1and S2 is conducting and after “π – φ”, D1, D2 are conducting, at this point, the forward voltage drop across D1 and D2 is 1 Volt. Where S1 and S2 are facing negative voltage after “π – φ”  and so S1 and S2 turn off. Hence self commutation is possible in this case.

Forced Commutation Situation can be Observed in Lagging Load Condition

From the graph, we can observe that “o to φ”, D1 and D2 are conducting, and from π to φ, S1, and S2 are conducting and are short-circuited. After “φ” D3 and D4 conduct only if S1 and S2 are turned off, but this condition can be satisfied only by forcing S1 and S2 to turn off. Hence, we use the concept of forced commutation.

Formulas

1). The conduction angle of each diode is φ

2). The conduction angle of each Thyristor is π – φ.

3). Self-commutation is possible only in leading power factor load or underdamped system at of circuit turn off time t_c = φ / w_0.  Where w0 is the fundamental frequency.

4). Fourier series V₀ (t) = ∑_n=1,3,5^α [ 4 V_DC / nπ ] Sin n w₀ t

5). I₀ (t) = ∑_n=1,3,5^α  [ 4 V_DC / nπ l z_n l  ] Sin n w₀ t + φ_n

6). V_01max = 4 V_dc / π

7). I_01max = 4 V_dc / π Z₁

8). Mod Z_n = R² + ( n w₀ L – 1/ n w₀ C) ; where n = 1,2,3,4…..

9). φ_n = tan ^-1 [ ( / R ]

10). Fundamental Displacement factor F_DF = cos φ

11). Diode current equation I_D and waveform is given as follows

 I_{D01 (avg)} = 1/2π [ ∫₀^φ  I_{01 max}  Sin ( w₀ t – φ₁ ) ]dwt

I_{D01 (rms)} = [ 1/2π [ ∫₀^φ  I₀₁² _max  Sin² ( w₀ t – φ₁ ) dwt ] ]^1/2

12). Switch or thyristor current equation I_T and waveform is given as follows

I_{T01 (avg)} = 1/2π [ ∫_φ^π  I_{01 max}  Sin ( w₀ t – φ₁ ) ]dwt

I_{T01 (rms)} = [ 1/2π [ ∫_φ^π  I₀₁² _max  Sin² ( w₀ t – φ₁ ) dwt ] ]^1/2

Advantages of Single Phase Full Bridge Inverter

The following are the advantages

• Absence of voltage fluctuation in the circuit
• Suitable for high input voltage
• Energy efficient
• The current rating of the power devices is equal to the load current.

Disadvantages of Single Phase Full Bridge Inverter

The following are the disadvantages

• The efficiency of the full-bridge inverter ( 95% ) is less than half the bridge inverter (99%).
• Losses are high
• High noise.

Applications of Single Phase Full Bridge Inverter

The following are the applications

• Applicable in applications like low and medium power example square wave / quasi square wave voltage
• A sinusoidal wave which is distorted is used as input in high power applications
• Using high-speed power semiconductor devices, the harmonic contents at the output can be reduced by PWM techniques
• other applications like AC variable motor, heating induction device, standby power supply
• Solar Inverters
• compressors, etc

Thus, an inverter is an electrical device that converts DC input supply to asymmetric AC voltage of standard magnitude and frequency at the output side. According to the type of load a single-phase inverter is classified into 2 types, like half-bridge inverter and full-bridge inverter. This article explains about full bridge single phase inverter. It consists of 4 thyristors and 4 diodes which together act like switches. Depending upon the switch positions the full-bridge inverter operates. The main advantage of the full-bridge over half-bridge is that the output voltage is 2 times input voltage and output power is 4 times compared to a half-bridge inverter.