The change of forward and reverse biasing of the diode makes the diode a switch. The diode is a two-terminal semiconductor device that has a PN junction. When the PN junction is in forward biased state it acts as a close circuit. When the junction is in a reverse bias state, the diode acts as open-circuited. Thus, when the state of the PN junction changes from reversed biases to forward bias or vice versa the diode acts as a switch. The PN junction diode acts as an electrical switch.
Advantages of Electrical Switches over Mechanical Switches
The followings are the advantages of electrical switches over mechanical switches.
- The mechanical switches have movable contacts. As a result, these need more maintenance. Whereas in the case of electrical switches there are no moving parts and these do not require maintenance.
- The contacts of mechanical switches are made of metals and exposed to the environment. Therefore, metals are apt to oxidize. On the other hand, the electrical switches do not have the problem of contact oxidization
- The mechanical switches undergo more stress and strain in comparison to electrical switches.
- The wear of the contacts of mechanical switches affects the working and thus they are not reliable.
- The power loss across the contact of the mechanical switches is more than the electrical switches.
- The switching time of the mechanical switches is more as compared to electrical switches.
Hence an electrical switch has better features than a Mechanical switch.
Working of Diode as a Switch
The principle of working of a diode as a switch is nothing but the forward and reverse biasing of the diode. When a forward voltage is more than the cut-in voltage of the PN junction diode, the current flows through the junction. Thus, the diode junction becomes a short circuit. The diode comes in the reverse bias when the voltage at the diode’s anode is more negative than the voltage at the cathode. In this condition, the diode junction is an open circuit.
For understanding the above phenomenon, let us understand it with the help of the below diagram.
A diode has a PN junction. In a diode, P-region has lightly doped holes as the majority carriers. and N- region has highly doped electrons as the majority carriers. When the switch is at the ON position, the anode of diode D gets a positive supply and the cathode of diode D gets a negative supply. In this condition, the diode gets forward biasing and it starts conducting.
Now, when the switch position changes from ON to OFF state, the anode of the diode gets the negative voltage at the anode. Under this condition, the current that was flowing in the forward bias state drops to zero, and the diode becomes an open circuit. Here, note that the diode forward current does not drop to zero instantly, but it takes some time. This time is called the reverse recovery time. Now, the diode draws negligible leakage current in the reverse bias.
A phenomenon of ringing happens when the diode undergoes from forward biased to a reverse-biased state.
What is Ringing in Diode?
On application of reverse voltage to the diode after forward current flow, reverse current flows for an instantaneous moment rather than instantly switching off. This reverse current is called recovery current.
During the flow of the recovery current, the recovery current may create a few oscillations. These oscillations are called ringing.
The ringing condition causes power loss and it is desirable to have minimum ringing.
The switching time is a very vital part of an ideal switch. The switch must have the shortest turn-on and turn-off time. Now, let us understand the switching times of a diode.
Diode Switching Times
While changing the bias conditions, The diode undergoes a transient response during forward bias to reverse bias and vice versa.
The time taken to respond under transient conditions is the important criterion for an electrical switch.
The time taken before the diode recovers its steady state is called as Recovery Time.
Forward Recovery Time
The time taken by a diode to switch from a reverse-biased state to a forward-biased state is called Forward Recovery Time.
On application of forward biasing in an OFF state diode, the voltage across the diode instantly shoots up to about 1–2 volts before settling down to its final value of 0.7 volts because injection of minority carriers takes some time. The time taken to cross 10% of the final value to peak and fall to the final value is the forward recovery time. It is an important consideration in the selection of diodes for SMPS. For fast switching diodes, it is about 50 ns.
Reverse Recovery Time
The reverse current flows for a small period of time when the diode is conducting and suddenly it turns off. The time interval taken by the diode to switch from a forward-biased state to a reverse-biased state is called as Reverse Recovery Time. And, the reverse current is called reverse recovery current.
The recovery time is in order of a few nanoseconds to a few microseconds or even more. It all depends on the device and the current levels. The main reason for this reverse recovery current and reverse recovery time is that minority charges require time to recombine with the majority charges in the depletion region. This cause a significant power loss in switching regulators.
The time period for which the diode remains in the conduction state even in the reverse-biased state is called as Storage time.
Applications of diode Switching circuits
The diode is used as a switch for the following applications.
- High speed rectifying circuits
- High-speed switching circuits
- RF receivers
- General-purpose applications
- Consumer applications
- Automotive applications
- Telecom applications etc.