Single Phase Full Bridge Inverter (Square Wave Output)

• At t = 1 / 2, Q₃ and Q₄ are turned on and Q₁ , and D₂ are turned off. The load voltage is – V and load current flows from B to A. The equivalent circuit for mode II is as shown in Figure B. At t = T_o, Q₃ and Q₄ are turned off and Q₁, and Q₂ are turned on again.
• As the load is resistive it does not store any energy. Therefore the feedback diodes are not effective here.
• The voltage and current waveforms with resistive load are as shown in Figure D.

1. The important observations from the waveforms of Figure D are as follows :
2. The load current is in phase with the load voltage.
3. The conduction period for each transistor is π radians or 180°.
4. Peak current through each transistor = V / R.
5. Average current through each transistor = V / 2R.
6. Peak forward voltage across each transistor = V Volts

Single Phase Bridge Inverter with RL Load :

The operation of the circuit can be divided into four intervals or modes.The waveforms are as shown in Figure I.

Interval (t₁ – t₂) :

• At instant t₁, the pair of transistors Q₁ and Q₂ is turned on. The transistors are assumed to be ideal switches. Therefore point A gets connected to positive point of dc source V through Q₁, and point B gets connected to negative point of input supply.
• The output voltage V_o = + V as shown in Figure A. The load current starts increasing exponentially due to the inductive nature of the load.
• The instantaneous current through Q₁ and Q₂ is equal to the instantaneous load current. The energy is stored into the inductive load during this interval of operation.

Figure E Figure F

Interval II (t_{2 –} t₃ ) :

• At instant t₂ both the transistors Q₁ and Q₂ are turned off. But the load current does not reduce to 0 instantaneously, due to its inductive nature.
• So in order to maintain the flow of current in the same direction there is a self induced voltage across the load. The polarity of this voltage is exactly opposite to that in the previous mode.
• Thus output voltage becomes negative equal to – V. But the load current continues to flow in the same direction through D₃ and D₄. as shown in Figure A.
• Thus the stored energy in the load inductance is returned back to the source in this mode. The diodes D₁ to D₄ are therefore known as the feedback diodes.
• The load current decreases exponentially and goes to 0 at instant t₃ when all the energy stored in the load is returned back to supply. D₃ and D₄ are turned off at t_4.

Interval III (t_{3 –} t₄  ) :

• At instant t₃ , Q₃ and Q₄ are turned on simultaneously. The load voltage remains negative equal to – V but the direction of load current will reverse and become negative.
• The current increases exponentially in the negative direction. And the load again stores energy in this mode of operation. This is as shown in Figure A.

Figure G

Figure H

Interval IV (t_{4 –} t₅) or (t_{0 –}
t₁) :

• At instant t_{4 or} t₀ the transistors Q₃ and Q₄ are turned off. The load inductance tries to maintain the load current in the same direction, by inducing a positive load voltage.
• This will forward bias the diodes D₁ and D₂. The load stored energy is returned back to the input dc supply. The load voltage V_o = + V but the load current remains negative and decreases exponentially towards 0. This is as shown in Figure B.
• At t₅ or t₁ the load current goes to zero and transistors Q₁ and Q₂ can be turned on again.

Conduction period of devices :

• The conduction period with a very highly inductive load, will be T_o/4 or 90° for all the transistors as well as the diodes. (see Figure F).
• The conduction period of transistors will increase towards T / 2 or 180 ° with increase in the load power factor. (i.e. as the load becomes more and more resistive).

Output voltage waveform  of Single Phase Full Bridge Inverter :

Circuit The output voltage waveform remains a square wave independent of whether the load is resistive or reactive.

Figure I