Push Pull Inverter (With RL Load)
Circuit Diagram of Push Pull Inverter (With RL Load) :
For the operation of push pull transformer the output transformer is a must. The circuit diagram for the push pull inverter is as shown in figure A.
Note that this circuit is same as that of the parallel inverter. The only change being that the SCRs in the parallel inverter circuit have been replace by power transistors.
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Figure A |
Operation of Push Pull Inverter with Resistive Load :
Mode I :
- When Q1 is turned on the de source voltage appears across the left half of the primary OA.
- The primary current flows from O to A. Due to the transformer action the voltage between AB is 2 V Volts.
- The load voltage is positive, so is the load current (Figure B). After half time at t = T / 2 Q1 is turned off by reducing its base drive to zero.
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Figure B |
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Figure C |
Mode II :
- At t = T / 2 transistor Q2 is turned ON .
- The input dc voltage now gets connected across winding OB. The primary current flows from O to B, through Q2 as shown in Figure C.
- The load voltage changes its polarity, and the direction of load current is reversed. Q2 conducts for half time and is turned off at t = T.
- The square output waveform is thus obtained across the load. The load voltage waveform is as shown in Figure D.
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Figure D |
Operation of Push Pull Inverter with Inductive (RL) Load :
- The operation of the circuit can be divided into four modes.
Mode II (t1 – t2) :
- At t = t1 diode D1 is turned off and transistor Q1 comes into conduction.
- The input DC supply V is connected across winding OA of centre tapped transformer as shown in Figure A.
- The load voltage is positive and equal to + V if the transformer ratio is assumed to be unity.
- The load current increases exponentially in the positive direction .
- As both the load voltage and current are positive in this mode, the load stores the energy. At instant t = t2 transistor Q1 is turned off.
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Figure E |
Mode III (t2 – t3) :
- As soon as Q1 is turned off, the load voltage polarity reverses due to the self induced voltage which appears in order to maintain the load current in the same direction as mode II.
- The load voltage is negative and load current positive therefore energy feedback takes place through diode D2 as shown in Figure B.
- The load current reduces exponentially and eventually reduces to zero at t = t3. D2 is therefore turn off at t = t2 and transistor Q2 can be retriggered.
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Figure F |
Mode IV (t3 –T) :
- If t = t3 transistor Q2 is retriggered, this will reverse the direction of load current.
- The primary current now flows through winding OB of primary as shown in Figure C.
- As both the load voltage and load current are negative the load will store the energy during this mode of operation .
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Figure G |
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Figure H |
Mode l (t0 –
t1) :
- At t = T or t0 transistor Q2 turned off.The load voltage polarity reverses due to the self induced voltage which appears in order to maintain the load current in the same direction as mode IV.
- The load voltage is positive but load current continues to be negative therefore energy feedback takes place through diode D1 as shown in Figure D.
- The load current reduces exponentially to zero at t = t1. D1 is therefore turned off at t = t1 and transistor Q1 can be triggered.
- The cycle then repeats. The voltage and current waveforms for the push pull inverter with RL load are as shown in Figure E.
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Figure I |
Some of the important points about the push pull inverters are as follows :
- The control circuit for push pull inverter must be designed in order to avoid the cross conduction . The simultaneous conduction of both the transistors will effectively short circuit the primary winding which will lead to a large device current.
- The transformer must be designed in such a way that it will not enter into saturation. Because if it happens , then there is no induction on the secondary side and the transistors will be damaged due to high primary current.
- The two transistors must have very close matching characteristics.
- The two lives of the primary winding of the transformer should have identical number of turns.
- In order to reduce the amplitude of voltage spikes across nonconducting transistor the leakage inductance between the transformer windings shoe kept as low as possible.
- Due to the centre tap transformer action the minimum voltage that appears across a non contacting transistor is 2 volts.
The duration of conduction of the two transistor Q1 and Q2and should be exactly identical. This ensure equal and opposite flow of primary current for same duration and will avoid saturation.