Diode as Switch and Junction Simple Explanation

Power diode is the simplest static devices among power electronics (PE). This device can act as an electronic switch. So it seems convenient to use a diode as switch.

The circuit symbol can be seen in Figure.(1), it is a two-terminal device where the terminal A stands for the anode and the terminal K stands for the cathode.

power diode as switch
Figure 1. Power diode

Power Diode as Switch

If terminal A gets higher potential than terminal B, the device will be operated in forward biased mode and the current flowing in the direction as shown is called forward current (IF).

This produces a relatively small voltage drop across the device (< 1 V), and can be ignored in ideal condition.

Before we move on to the explanation, make sure you read what is electric current in order to understand the operation principle.

In opposite, when reverse-biased, it does not let current flowing through and a practical power diode will have a small current flowing in the reverse direction called the leakage current.

When assuming an ideal device, both voltage drop and leakage current will be ignored. Usually, in PE we will use a calculation based on ideal condition.

Its practical characteristics from the ideals of zero forward and infinite reverse impedance, as illustrated in Figure.(2a).

When operated in forward-biased, a potential barrier associated with the distribution of charges in the vicinity of the junction, together with the other effects, makes a voltage drop.

In a practical diode, its characteristic is drawn in Fig.2b.

power diode
Figure 2a. Typical static characteristic
power diode
Figure 2b. Characteristic in practical condition

Notice the characteristic graph, the forward characteristic is drawn as a threshold voltage Vo and a linear incremental or slope resistance, r. The reverse characteristic remains the same over the range of possible leakage current irrespective of voltage within the normal working range.

PN Junction Diode

From the characteristic, it is easy to notice that when it is forward biased, the current flowing through it rises as the voltage is increased.

The current in reverse biased mode is relatively small until the breakdown voltage is reached.

Once the applied voltage surpasses the limit, the current will flow rapidly to a very high value limited only by external resistance.

Its work principle is based on passing or blocking the voltage and current.

Power Diode Characteristics

DC diode characteristics. The most important parameters are the followings :

  • Forward voltage, VF is the voltage drop of a diode across A and K when it is forward biased.
  • Breakdown voltage, VB is the voltage drop across the diode when it is beyond reverse biased, also known as an avalanche.
  • Reverse current, IR is the current at a specific voltage, which is below the breakdown voltage.

AC diode characteristics. The most commonly used parameters are :

  • Forward recovery time, tFR is the required time for its voltage to drop to a particular value after the forward current starts to pass.
  • Reverse recovery time, trr is the time interval between the application of reverse current and reverse voltage dropped to a particular value as drawn in Figure.(3). Parameter ta is the interval between the zero crossings of its current to when it becomes IRR. In opposite, tb is the time interval from the maximum reverse current to approximately 0.25 of IRR. The value ratio of ta and tb is called by the softness factor (SF). Power diodes with abrupt recovery characteristics are applied for high-frequency switching.

It is important to calculate the reverse recovery time. Lower trr indicates it can be switched faster.

power diode
(1)

If tb can be ignored (common case), then the following equation is valid :

power diode
(2)

And the reverse recovery current

power diode
(3)

where QRR is the storage charge.

For example,

A selected diode has the rate of fall of its current di/dt = 20 A/µs, and the reverse recovery time trr = 5 µs. Calculate the peak reverse current.

Solution,

The peak reverse current is,

power diode
(4)

The storage charge QRR is,

power diode
(5)

Hence,

power diode
(6)
  • Diode capacitance, CD is the capacitance of net diode along with the junction (CJ) plus package capacitance (CP).