Static and dynamic characteristics of power diodes

Static and dynamic characteristics of power diodes

The classification of power electronic devices can be divided into uncontrolled devices, semi-controlled devices and fully controlled devices according to their switching control performance. Power diodes are uncontrolled devices.

The basic structure and working principle of power diodes are the same as diodes in information electronic circuits, and they are all based on semiconductor PN junctions. Due to the working characteristics of the power diode, it is composed of a large area PN junction, two leads and a package. Figure 1 shows the shape, structure and graphic symbols of the power diode. From the appearance point of view, the power diode can have a variety of packages such as bolt type, flat type and module type. The electrical graphic symbols are shown in Figure 1d, where A is the anode and K is the cathode.

Static and dynamic characteristics of power diodes
Figure 1 – Shape, structure and graphic symbols of power diode

1. Static characteristics of power diodes

The static characteristics of power diodes mainly refer to the volt-ampere characteristics, that is, the relationship between the anode voltage of the power diode and the anode current flowing through the diode. As shown in Figure 2, the first quadrant is the positive characteristic area. From the curve of the first quadrant, when the applied forward voltage (potential of anode A is higher than cathode K) is less than UTO (threshold voltage), only a small forward leakage current flows through the device; when the applied forward voltage is greater than UTO (threshold voltage), the device starts to conduct. After the forward conduction, the current is determined by the load, and the device voltage drop UF=0.4~1.2v, at this time, UF basically does not change with current changes. The third limit is the reverse blocking state. When a reverse voltage is applied to the power diode (the potential of the anode A is lower than the cathode K), there is only a very small reverse leakage current at the beginning. When the reverse voltage increases to UB (breakdown voltage), the reverse current will increase sharply, destroying the PN junction’s reverse biased working state, which is called reverse breakdown. Reverse breakdown has two forms of avalanche breakdown and Zener breakdown according to different mechanisms. When reverse breakdown occurs, as long as measures are taken in the external circuit to limit the reverse current within a certain range, the PN junction can still return to its original state when the reverse voltage is reduced. But if the reverse current is not limited, so that the product of the reverse current and the reverse voltage exceeds the allowable power consumption of the PN junction, the temperature of the PN junction will rise due to heat not being dissipated, until it is overheated and burned. This is thermal breakdown.

In summary, the following conclusions can be drawn:

1) The power diode has unidirectional conductivity.

2) After the power diode is turned on, the current flowing through the device is determined by the load.

3) The value of the on-state voltage drop and reverse leakage current of the power diode is very small, which is usually ignored in circuit analysis.

2. Dynamic characteristics of power diodes

The amount of charge in the PN junction changes with the external voltage, presenting a capacitive effect, which is called junction capacitance (CJ), also called differential capacitance. The junction capacitance is divided into barrier capacitance GB and diffusion capacitance CD according to the difference of its generation mechanism and function. The barrier capacitance only works when the applied voltage changes. The higher the frequency of the applied voltage, the more obvious the effect of the barrier capacitance. The size of the barrier capacitor is directly proportional to the cross-sectional area of the PN junction and inversely proportional to the thickness of the barrier layer; while the diffusion capacitor only works when it is forward biased. In the forward bias, when the forward voltage is low, the barrier capacitance is the main component; when the forward voltage is high, the diffusion capacitance is the main component of the junction capacitance.

Because of the junction capacitance, the power diode must undergo a transition process when it switches between the three states of zero bias (zero applied voltage), forward bias, and reverse bias. Therefore, the characteristics of voltage and current changing with time are used to describe this transition process. This is the dynamic characteristics of power diodes, namely turn-on and turn-off characteristics, referred to as switching characteristics.