What If The Load Is Not Resistive?
What if the load is not resistive?
Fig: A push-pull active amplifier circuit & A push-pull switched mode circuit
- Transistors used in the circuit of A push-pull active amplifier circuit are meant to carry only unidirectional current (from collector to emitter) and thus if the upper (n-p-n) transistor is on, the current must enter the star (*) marked terminal of the load and this same terminal will get connected to the positive dc supply ( E), other load terminal being at ground potential.
- When n-p-n transistor turns off and p-n-p type turns on, the load voltage and current polarities reverse simultaneously (p-n-p transistor can only carry current coming out of star marked end of load). Such one to one matching between the instantaneous polarities of load voltage and load current can be achieved only in purely resistive loads.
- For a general load the instantaneous current polarity may be different from instantaneous load-voltage polarity. The inverter switching-pattern fixes the output waveform irrespective of the load. Thus the magnitude, phase and frequency of the fundamental voltage output by a VSI is independent of the nature of load. Thus it turns out that for a non-resistive load the switches in the circuit of A push-pull switched mode circuit should be able to carry bi-directional current and at the same time be controllable.
Fig: Bi-directional controlled switch & Modified push-pull circuit
- If an anti-parallel diode is connected across each transistor switch the combination can conduct a bi-directional current. Now the transistor in anti-parallel with the diode may be considered as a single switch. In spite of unidirectional voltage blocking capability, the new electronic switch suffices for the inverter application as pointed out in the following paragraphs.
- The push-pull circuit operation is now revisited using bi-directional current carrying switches. It may be noted that both IGBT and BJT type transistors, when bypassed by anti-parallel diode, qualify as bi-directional current carrying switches. However, IGBT switch is controlled by gate voltage whereas the BJT switch is controlled using base current.
- In the circuit of Modified push-pull circuit, n-p-n transistor (Q1) together with diode (D1) constitutes the upper switch (SW1). Similarly lower switch (SW2) consists of p-n-p transistor (Q2) in anti-parallel with diode (D2). By applying positive base-to-emitter voltage of suitable magnitude to transistor ‘Q1’, the upper switch is turned on. Once the upper switch (diode ‘D1’or transistor ‘Q1’) is conducting star end of load is at ‘ E’ potential and diode ‘D2’ of lower switch gets reverse biased. Transistor ‘Q2’ is also reverse biased due to application of positive base voltage to the transistors. Thus while switch ‘SW1’is conducting current, switch ‘SW2’ is off and is blocking voltage of magnitude ‘2E’. Similarly when applied base voltage to the transistors is made negative, ‘Q1’ is reverse biased and ‘Q2’ is forward biased. This results in ‘SW1’ turning off and ‘SW2’ turning on. Now ‘SW1’ blocks a voltage of magnitude ‘2E’. It may be interesting to see how diodes follow the switching command given to the transistor part of the switches.