Half Bridge Inverter with Inductive Load

Half Bridge Inverter with Inductive Load

Operation with inductive load :

Let us divide the operation into four intervals . We start explanation from the second time interval t₁ to t₂ because at the beginning of this interval transistor Q₁, will start conducting.

Interval II ( t₁ – t₂ ) :

Q₁ is turned on at instant t₁ , the load voltage is equal to + V / 2 and the positive load current increases gradually.
At instant t₂ the load current reaches the peak value. The transistor Q₁ is turned off at this instant.
Due to the same polarity of load voltage and load current, the energy is stored by the load. Refer Figure A.

Figure A

Figure B

Interval III ( t₂– t₃ ) :

Due to inductive load, the load current direction will be maintained same even after Q₁ is turned off.
The self induced voltage across the load will be negative. The load current flows through lower half of the supply and D₂, as shown in Figure B.
In this interval the stored energy in load is fed back to the lower half of the source and the load voltage is clamped to – V / 2.

Interval IV ( t₃ – t₄ ) :

At the instant t₃, the load current goes to zero, indicating that all the stored energy has been returned back to the lower half of supply.
At instant t₃, Q₂ is turned on. This will produce a negative load voltage V_o = -1/2 and a negative load current. Load current reaches a negative peak at the end of this interval. (See Figure C).

Figure C

Figure D

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

Transistor Q₂is turned off at instant t₄. The self induced voltage in the inductive load will maintain the load current.
The load voltage changes its polarity to become positive V / 2, load current remains negative and the energy stored in the load is returned back to the upper half of the dc source. (see figure B).
At t₅, the load current goes to 0 and Q₁. can be turned on again. The cycle of operation repeats.

Note : The output voltage waveform remains square wave with purely resistive or RL type of load.

Conduction period of transistors :

Conduction period of the transistors depends upon the load power factor. For purely inductive load, a transistor conducts only for T_o/2 or 90°. Depending on the load power factor, the conduction period of the transistor will vary between 90° to 180° (180° for purely resistive load).
The voltage and current waveforms for RL load are as shown in Figure E.

Figure E