Inductive Loads and Freewheeling Diode Related Applications

Inductive Loads and Freewheeling Diode Related Applications

  1. The role of the freewheeling diode

In order to remove the negative part of the output voltage, a diode VD can be connected in parallel across the load, as shown in Figure 1a, this diode is called a “freewheeling diode”. When the AC voltage u2 is positive, the thyristor is triggered and turned on. At this time, the voltage across the load is positive, and the freewheeling diode is not turned on by the reverse pressure. The voltage waveform on the load is the same as that without the freewheeling diode. When the AC voltage u2 changes from zero-crossing value to negative, the freewheeling diode is turned on due to the forward voltage, and the thyristor is turned off due to the negative voltage. At this time, the load current will be formed by the freewheeling diode under the action of the induced electromotive force. The loop continues to flow along the load and the freewheeling diode, and the voltage across the load is approximately zero.

When the inductance Ld is large (ωL4>10Rd), that is, the so-called large inductive load, the load current id basically tends to be stable due to the filtering effect of the inductance at this time, which can be regarded as a straight line parallel to the horizontal axis. The load current consists of the current iVT flowing through the thyristor and the current iVD of the freewheeling diode. The flow path of the load current is: when the thyristor is turned on, it flows through the thyristor, and the conduction angle of the thyristor in the waveform diagram is represented by θVT; when the thyristor is turned off, the load current flows through the freewheeling diode, and the freewheeling diode The conduction angle is denoted by θVD, as shown in Figure 1b. It can be seen from the waveform diagram in Figure 1b that the load current id of the large inductive load is approximately a horizontal line, while the thyristor current and the freewheeling diode current iVT are approximately rectangular waves.

Figure 1
  1. Calculation of large inductance load circuit with freewheeling diode

Since the negative part of the output voltage waveform of the circuit has been removed, the output voltage waveform is the same as that with a resistive load, and the formula for calculating the average value of the rectified output voltage is also the same as that with a resistive load. which is

Formula 1

The phase shift range is the same as with resistive load, both are 0°~180du3.
The average value of the load DC current is

Formula 2

It can be seen from formula 2 that this load DC current is provided by two paths, the thyristor and the freewheeling diode. The average value IdVT and the effective value IVT of the thyristor current are respectively:

Formula 3
Formula 4

The average value IdVD and the effective value IVD of the freewheeling diode current are respectively

Formula 5
Formula 6

The maximum voltage on both the thyristor and the freewheeling diode is the peak value of the AC voltage, which is √2U2.


Although the circuit structure of the single-phase half-wave controllable rectifier circuit is simple, it has disadvantages such as large output DC voltage ripple when the resistive load is applied, and the DC current component in the secondary winding of the rectifier transformer causes the DC magnetization of the iron core. The controlled rectifier circuit is only suitable for occasions with small capacity and low requirements. In the single-phase controllable rectifier circuit, the single-phase bridge type fully-controlled rectifier circuit and the single-phase bridge type half-controlled rectifier circuit are widely used.