Principle and Application of Diodes

1. Basic Features of Power Diodes Figure 1 (a) and Figure 1 (B) show the circuit symbols and static voltammetry characteristics of the power diode. As we know, when the diode is under positive voltage and the positive bias at both ends of the tube is very small (about 1 V), it starts to turn on. If the two ends of the diode are at reverse voltage, only a very small negligible leakage current flows through the device before breakdown. When working normally, the reverse voltage added to the diode should be less than the limited voltage value for breakdown. The voltage and current changes during the working process are compared according to the characteristics of only a very small leakage current when the diode is in the blocking state (under the reverse voltage) and the tube pressure drop is very low during the on-going state, we can obtain its ideal voltammetry characteristics, as shown in 1 (c. This ideal feature can be used to analyze the basic structure and working principle of the converter. However, in circuit analysis that requires a precise diode model or design of an actual converter (for example, when estimating the heat dissipation condition of a device, you must know the forward pressure drop of the tube, although the value of this parameter may be small), it is commonly used to represent another ideal feature, as shown in 2 (a), corresponding equivalent circuit 2 (B ). In Figure 2 (B), E represents the forward conduction bias of the diode, and RD represents the equivalent resistance when the positive wizard is connected. As a basic principle, we will use the ideal characteristic diagram 1 (c) to simplify the replacement when diode is involved in circuit analysis in the future. As the conduction speed of the diode is much faster than the transient change process of the power circuit, the diode can be regarded as an ideal switch. However, during the shutdown process, due to the need for reverse charging of the semiconductor PN junction, in order to balance the block voltage, the current in the diode will flow reversely and take a period of time before it degrades to 0, this time is called the reverse recovery time TRR, as shown in 3. This phenomenon may lead to over-voltage of the circuit containing inductance, but in most power frequency Rectification circuits, it has little impact on the switching characteristics of the converter. Therefore, when the diode is turned off, it can still be regarded as an ideal switch. According to the actual application requirements, you can select the following different types of power diodes. (1) The diode. Unlike the commonly used PN junction rectification diode, it is a single polarity diode, so it is not affected by the charging process and reverse recovery time TRR (generally dozens of nanoseconds. In terms of the device structure, the compromise results are taken between reducing the forward tube voltage drop and increasing the Reverse Leakage stream. It is suitable for low output voltage and requires a lower forward tube pressure drop (typically 0.3 V) converter circuit. The reverse blocking voltage of this diode ranges from 50 to 50 ~ Between v. (2) fast recovery diode. It is used in a high-frequency circuit that works with a controllable switch. These circuits require a very small reverse recovery time. For example, the TRR should be smaller than several microseconds under hundreds of volts and hundreds of amps. (3) Power Frequency diode. The forward voltage drop of the pilot mode is very low, but the reverse recovery time is long, which is acceptable in the power frequency circuit. It is suitable for large-capacity Converters with reverse blocking voltage of thousands of volts and working current of thousands of amps. Theoretically, the series and parallel connections of this diode can meet the requirements of any voltage and current values. 2. Basic Applications of diodes. (1) The most basic application of the diode is to realize the rectification and transformation by using the asymmetric nonlinear characteristics of the diode at positive and reverse bias ends (see Figure 1b), as shown in Figure 4 (. (2) continuous-flow diodes, as shown in Figure 4 (B. When switching s to cut off the inductor circuit, in order to prevent the inductance from generating a high back potential E = L · di/dt, and damage the device, access a Diode D, the inductance current has a circuit that continues to flow, so that the voltage at both ends of the switch does not exceed the power supply voltage when the switch S is turned off. This avoids high voltage at both ends of the switch device due to inductance cut-off. (3) The diode limiting circuit shown in 4 (c) can be used to limit the amplitude of the signal voltage to a certain range when the input signal voltage us has a large variation range. Set the diode threshold voltage to uth. When us <uth, the diode does not turn on and all signals are output. uo = us. When us> uth, the diode is turned on, the diode voltage is limited to forward conduction voltage. A silicon tube is about 0.7 V, and a GE tube is about 0.3 V. Several diodes can be connected in series to obtain different limiting values. (4) clamping figure 4 (d) when the load resistance RL changes, as long as the Diode D is in a positive turn-on State; the output voltage uo is equal to the power supply voltage UG and diode Forward Voltage Drop UF, UO = ug + UF and is not related to the load. That is, the uo is clamped to UG + UF. Of course, to keep the diode always in a positive turn-on state, rl cannot be too small, rl too small flow on the R current is too large, r voltage drops too large (E-R I) <ug, the diode offset ends. The uo changes with the changes of RL, And the clamp circuit is ineffective. (5) regulator (Dz in Figure 4 (e) is also a semiconductor diode. The normal work zone of this diode is in the reverse breakdown zone, the third quadrant zone in 1 (B. When the reverse current of the diode changes after reverse breakdown, the reverse end voltage remains unchanged. Therefore, when the power supply voltage is changed in Figure 4 (e), the pressure drop on the series resistance R is changed by changing the reverse current of the voltage regulator, while the load voltage uo is basically unchanged. This simple voltage regulator circuit is also widely used.