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Equivalent circuit and switching characteristics of semiconductor diodes

first, the capacitance effect of diodes

diodes have capacitance effect. The capacitance of real-time digital signal processing includes barrier capacitance CB and diffusion capacitance CD

1. Barrier capacitance CB (CR)

as mentioned earlier, the PN junction lacks conductive carriers, and its conductivity is very low, equivalent to a medium; The conductivity of P and N regions on both sides of PN junction is high, which is equivalent to metal conductor. From this structure, PN junction is equivalent to a nearly blank capacitor in most places

in fact, when a forward voltage is applied at both ends of the PN junction, the PN junction becomes narrower and the amount of space charge in the junction decreases, which is equivalent to the "discharge" of the capacitor. When a reverse voltage is applied at both ends of the PN junction, the PN junction becomes wider and the amount of space charge in the junction increases, which is equivalent to the "charge" of the capacitor. This phenomenon can be simulated by a capacitor, which is called barrier capacitance. The difference between barrier capacitance and ordinary capacitance is that its capacitance is not constant, but related to the applied voltage. When the applied reverse voltage increases, the barrier capacitance decreases; When the reverse voltage decreases, the barrier capacitance increases. At present, the widely used varactor diode is made of the characteristics that the PN junction capacitance changes with the applied voltage

2. Diffusion capacitance CD

when the PN junction is forward biased, the electrons in the N region diffuse to the p region, forming a certain non-equilibrium carrier concentration distribution in the p region, that is, the concentration near the PN junction is high, and the concentration away from the PN junction is low. Obviously, electrons are accumulated in the p region, that is, a certain amount of negative charges are stored; Similarly, holes are also accumulated in the N region, that is, a certain number of positive charges are stored. When the forward voltage increases, the diffusion increases. At this time, the number of electrons diffused from n to P and the number of holes diffused from P to n will increase, resulting in the formation of charge accumulation in the two regions, which is equivalent to the charging of capacitors. On the contrary, when the forward voltage decreases, the diffusion weakens, that is, the number of electrons diffused from n region to p region and the number of holes diffused from P region to n region decrease, resulting in the reduction of charge in the two regions, which is equivalent to the discharge of capacitors. Therefore, it can be simulated by a capacitor, which is called diffusion capacitance

in a word, the diode presents two kinds of capacitance, and its total capacitance CJ is equivalent to that of the two and therefore harmless to human health, that is, CJ = CB + CD. When the diode is forward biased, the diffusion capacitance is much larger than the barrier capacitance CJ ≈ CD; In reverse bias, the diffusion capacitance can be ignored, and the barrier capacitance plays a major role, CJ ≈ CB

2. Equivalent circuit of diode

diode is a nonlinear device, and the analysis and calculation of nonlinear circuit is relatively complex. In order to simplify the analysis of the circuit, the circuit composed of linear elements can be used to simulate the diode. If the voltage and circuit relationship of the linear circuit are approximately consistent with the external characteristics of the diode, then this linear circuit is called the equivalent circuit of the diode. Obviously, the equivalent circuit is an approximation under certain conditions

When a diode is applied to a DC circuit, it is usually equivalent to an ideal diode model, which can be regarded as an ideal switch. In case of positive bias, it is equivalent to that the "switch" is closed (on), the resistance is zero, and the voltage drop is zero; During reverse bias, it is equivalent to that the "switch" is off, the resistance is infinite, and the current is zero. Because the ideal diode model highlights the most basic characteristic of the diode - unidirectional conductivity, it is widely used in DC circuits and switching circuitsif the influence of diode resistance and threshold voltage is considered in DC circuit. 286 the actual diode of glass ball for automatic fire extinguishing system can be equivalent to the circuit shown in figure z0112

under the condition of adding DC bias voltage at both ends of the diode and working in AC small signal, it can be equivalent with a simplified circuit. RS in the figure is the body resistance of diode P and N regions

III. switching characteristics of diode

the diode is on when it is positively biased, which is equivalent to the switch on; The cut-off during reverse bias is equivalent to the disconnection of the switch, indicating that the diode has switching characteristics. However, an ideal switch has zero resistance and zero voltage drop when it is turned on, and infinite resistance and zero current when it is turned off. Moreover, it is required to have the above characteristics when it is turned on and off at high speed, without switching time. However, the actual application of diode as a switch is not ideal. Because when the diode is conducting in the forward direction, its forward resistance and forward voltage reduction are not zero; When reverse stabbing stops, the reverse resistance is not infinite, and the reverse current is not zero. And it takes time for the diode to switch on and off, which limits its switching speed. Therefore, when switching, the switching diode with small forward resistance RF, large reverse resistance RR and small switching time should be selected. (end)

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