Transistor as a Switch

What does a switch do?
It offers open circuit when it is put in ‘OFF’ position and offers short circuit when it is put in ‘ON’ position. In other words, it can be said that a switch provides infinite resistance during its OFF condition and provides zero resistance during its ON condition. Hence a switch can be imagined as an ON-OFF controlled resistor, which provides either zero or infinite resistance to the circuit without any intermediate value.A transistor on the other hand can be considered as a controlled resistor, as the resistance between emitter and collector is controlled by current in the base-emitter junction. By controlling base-emitter current, the emitter-collector resistance can be made either nearly infinite or nearly zero.

A transistor gives quite a large resistance to the circuit but it is not ideally infinite also gives very small resistance but it is also not ideally zero. In a transistor characteristic, there are three regions. They are –

  • Cutoff Region
  • Linear Region
  • Saturation Region
transistor as switch In linear region, for wide range of collector – emitter voltage (VCE) collector current (IC) remains constant. As the voltage has a wide range and collector current is almost constant, there will be signifiant power loss if the transistor is operated in this region.

But in a practical switch, when it is OFF, the voltage across it will be equal to open circuit voltage but the current is zero hence there is no power loss. Similarly when the switch is ON, current through the switch is as high as short circuit current but the voltage across the switch is zero hence, there is no power loss again. When we want a transistor to be operated as a switch, it must be operated in such a way that the power loss during ON and OFF condition should be nearly zero, or very low. It is only possible when the transistor is only operated in the marginal region of the characteristics. There are two marginal regions in the characteristics of a transistor and there are cut-off region and a saturation region. In the figure when base-emitter current or simply base current is zero, the collector current (IC) will have very small constant value for a wide range of collector-emitter voltage (VCE). So when the transistor is operated with base current ≤ 0, the current through the collector to emitter (IC) is very tiny, hence the transistor is said to be in OFF condition but at the same time, power loss across the transistor switch i.e. IC × VCE is negligible because of very tiny IC. Hence the transistor is said to be operated as open circuited switch or OFF switch. Now, suppose the transistor is connected in series with a load of resistance RL. In normal condition, the voltage across the load is VL. Hence current through the load is If the transistor is operated with a base current Ib1 for which collector current is IC1. IL is less than IC1, then the transistor is operated in the saturation region. Here, for any current less than IC1 through the transistor’s collector to emitter, there will be very tiny collector-emitter voltage VCE. Hence at this situation the current through the transistor is as high as load current, but the voltage across the transistor (VCE) quite low, hence power loss in the transistor again is negligible. The transistor behaves as approximate short circuited switch or ON switch. So for using the transistor as a switch we should make sure that the applied base-emitter current must be sufficiently high to keep the transistor in the saturation region, for given load current.

So, from the above explanation we can conclude that the transistor behaves as a switch only when it is operated in cut - off and saturation region of its characteristic. In switching application, the linear region of characteristics is avoided. As we already told, the power loss in transistor switch is very low but not zero. So, it is not an ideal switch but accepted for specific applications. Now, for regulating output DC power in load, we must operate a transistor switch in such a way that it periodically switch ON and OFF the circuit to provide desired output power. For that, we need a specific waveform of base current due to which transistor goes to its cutoff and saturation region periodically for a given load current. The typical periodic base current waveform is generally obtained from a microprocessor based pulse generator.

When we choose a transistor as a switch, we have to careful about the rating of the transistor. Because during ON condition, the entire load current will flow through the transistor, if this current is more than a safe value of collector-emitter current carrying capacity of the transistor, then the transistor may be permanently destroyed due to overheating. Again at OFF condition, the entire open circuit voltage of the load will appear across the transistor. The transistor must be able to withstand this voltage otherwise collector-emitter junction will breakdown and transistor becomes ‘ON’ instead of being ‘OFF’. Another thing must be considered during a transistor as a switch. A heat sink of suitable size and design is always required with the transistor. Each transistor takes a finite time to go from OFF to ON condition and vice verse.

Although this finite time is very small, may be less than few microsecond but still it is not zero. During switch ON period, the current (IC) will be building up while the collector-emitter voltage VCE will be falling towards zero. As the current increases from zero (ideal) to its maximum value and voltage falls from the maximum value to zero (ideal) there will be an instant where both of them will be maximum. At that point, peak power loss happens. In the same way, maximum power loss happens in a transistor when it comes at OFF state from ON state. Hence maximum power loss happens in a transistor during the transition period of changing state but still the amount of energy dissipated is quite moderate as the period of transition is quite small. For low-frequency operation, the generated heat may be moderated. But if the frequency of operation is quite high, there will be significant power loss and corresponding generation of heat.

This is to be noted that, heat generation does not occur only during transient condition also during steady ON or OFF condition of the transistor but the amount of heat during steady condition is quite small and negligible. transistor as switch

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