Switches – Types & Characteristics

A switch is an electrical component that can “make” or “break” an electrical circuit. A switch mainly works with ON and OFF mechanism.

Types of Switches

1. Ideal Switch

2. Practical Switch

Ideal Switch

1. It should have zero resistance or zero voltage drop in ON state and infinite resistance or zero current during OFF state. In other words, power loss across the switch should be zero both in ON and OFF state.

2. It should be closed and open instantaneously independent of current through it.

3. In the closed state, it should conduct current in one direction only. The resistance to reverse direction must be infinite.

During ON state

here I_sw = switch current, V_sw = voltage across switch, and P_sw = Switch power loss

V_sw = 0 Volt (always)

I_sw = value depending upon the circuit condition

P_sw = V_sw × I_sw = 0 Watt

turn on time, t_on = 0 seconds

During OFF state

V_sw = value depending upon the circuit condition

I_sw = 0 Amp (always)

P_sw = V_sw × I_sw= 0 Watt

turn off time, t_off = 0 seconds

Consider OFF switch condition as shown in figure 1.

V_sw = V volt

I_sw = 0 Amp

P_sw = V_sw × I_sw = 0 Watt

turn off time, t_off = 0 seconds

Consider ON switch condition as shown in figure 2.

V_sw = 0 Volt

I_sw = (V/R) Amp

P_sw = V_sw × I_sw = 0 Watt

turn on time, t_on = 0 seconds

The switching characteristics of an ideal switch are shown below. Here I_c is the controlling signal that controls the ON and OFF of the switch. From time 0 to t₁, the switch is closed (i.e. ON condition). From time t₁ to T, the switch is open (i.e. OFF condition) and so on.

Practical Switch

1. A practical switch has non-zero resistance in ON state and finite resistance in the OFF state.

2. The time taken to the switch to change from OFF state to ON state i.e. turn ON time and turn OFF time is not same.

During ON state

V_sw ≠ 0 Volt

I_sw = value depending upon the circuit condition

P_sw = V_sw × I_sw ≠ 0 Watt

turn on time, t_on ≠ 0 seconds

During OFF state

V_sw = value depending upon the circuit condition

I_sw ≠ 0 Amp

P_sw = V_sw × I_sw ≠ 0 Watt

turn off time, t_off ≠ 0 seconds

The switching characteristics of a practical switch are shown below.

t_on = The time taken to reach steady-state ON condition from the steady-state OFF condition.

T_ON = The time during which the switch remains in steady-state ON condition.

t_off = The time taken to reach steady-state OFF condition from the steady-state ON condition.

T_OFF = The time during which the switch remains in steady-state OFF condition.

T = Time period

T = t_on + T_{ON +} t_off + T_OFF

Note: Normally the ON-state voltage drop across the switch and OFF-state leakage current through the switch are negligible as compared to circuit voltage and current. Hence may be neglected.

Losses in practical switch

1. ON-state loss

The power loss during the ON state of the switch is known as ON state loss. It is given by

P_s-ON = V_s-ON × I_s-ON

2. OFF-state loss

The power loss during the OFF state of the switch is known as OFF state loss. It is given by

P_s-OFF = V_s-OFF × I_s-OFF

3. Switching loss

Some power loss occurs in the switch when it changes state from ON to OFF state or OFF to ON state. These losses are combined together are called switching loss. It depends upon the turn ON and turn OFF time of the switch and also depends on the frequency of switching. As the turn On or turn OFF time increases, power loss increases. As the switching frequency increases, power loss increases.