The unidirectional conductivity of the diode is due to the unidirectional conductivity of the space charge region. When the PN junction is added with a forward voltage, the external electric field is opposite to the internal electric field and is much stronger than the internal electric field, so that the dispersion motion of the many sons is much larger than that of the few sons (the many sons of the P or N semiconductor to the other region to form a small number of carriers called the few sons), and there can be a large forward dispersion current, that is, low resistance, called the PN junction conduction;

When the applied voltage frequency of the PN junction diode is high to a certain extent, the single conduction property is lost. Because the PN junction constitutes a depletion layer when the reverse cutoff is equivalent to a small capacitor of an intermediate insulation layer, when the voltage frequency increases, the high-frequency signal will pass through the small capacitor, that is, the reverse signal can also pass through the diode, and the single conductivity will be lost.

When the forward current flowing through the diode is small, the internal resistance of the diode is mainly the resistance of the low-doping N-region as the substrate, and its resistance value is high and constant, so the voltage drop in the tube is added with the rise of the forward current. When the forward current flowing on the PN junction is large, because the dispersive motion P region is injected and accumulated in the low-doping N region, the minority hole concentration will be large, in order to maintain the electrical neutral condition of the semiconductor, the multi-particle concentration is also added accordingly, so that its resistivity fluctuates greatly, that is, the conductivity is greatly increased, which is the conductivity modulation effect.

When the forward voltage is applied to a silicon PN junction diode, there will be a non-equilibrium accumulation of a few carriers in the PN junction, which constitutes the so-called charge storage effect. When the diode is converted from positive conduction to external negative voltage cutoff, the disappearance of these stored small number of carriers requires a certain time, which is the reverse recovery time of the diode, and the current formed in the diode during this period is the reverse recovery current of the diode. In the reverse recovery moment, the reverse recovery current change rate is very large, will constitute the reverse voltage overshoot, there is a reverse voltage drop at both ends of the diode, that is, the diode turn-off instant voltage undershoot. Therefore, when the PWM is turned on in the buck circuit, the continuous current diode passes from the positive guide to the reverse cutoff period, and there will be an upsurge at the switch node SW, whose fluctuation is the reverse voltage drop of the input power plus the continuous current diode.