Working principle of series switching power supply (b)

This is the schematic diagram of a series switching power supply . Transistor
VT is a switching tube, L is an energy storage inductor, C is an energy storage and
filter capacitor, and VI) is a freewheeling diode. Under the control of the oscillation pulse
, the switching tube VT is in a switching working state, that is, it is on
(open) for a while and cut off (off) for a while. Its working waveform is as shown in the figure.
    When a forward control pulse is applied to the base of the switch tube VT,
the switch tube is turned on and enters a saturated state. At this time, the diode VI)
is cut off due to reverse bias, and the input voltage Vi passes through the switch tube to the inductor L and
the load RL. Since the current in the inductor L cannot change suddenly, it
can only gradually increase with the degree of conduction of the switch tube. At this time, the input
voltage vi is transmitted to the inductor L and the energy is stored in the inductor (converted from
electrical energy to magnetic energy). The longer the switching tube is on, that is,
the wider the positive pulse, the greater the current increase, and the more magnetic energy is stored. Since the capacitor
C is connected in parallel with the load, the capacitor C is also charged when the power supply supplies power to the load
. As shown in Figure (b),
    when a negative pulse or a positive pulse is applied to the base of the switch vr and
disappears, the switch VT is cut off. At this time,
although the current in the inductor L stops growing, it cannot stop completely suddenly, but occurs.
A self-induced electromotive force is generated, and its polarity is negative on the left and positive on the right, as shown in Figure
(c). Under the action of this electromotive force, the circuit will generate an induced
current. This induced current continues to charge the capacitor C, and the freewheeling diode forms a charging loop.
The loop and current direction are shown by the dotted line in Figure (c). This current also supplies power to the load. Afterwards, when the current in the inductor L drops to a
certain level, the capacitor C starts to discharge to maintain the current required by the load. When the electric energy on the capacitor C is released to a certain extent
and the voltage at both ends of the load is about to decrease, the switch tube VT becomes conductive again, and the next working cycle begins. This cycle continues
over and over again, so that the output voltage is maintained at a corresponding value.
    Since the capacitor C is connected in parallel with the output terminal, the output voltage Vo is the voltage across the capacitor. The level of this voltage
is determined by the amount of charge stored in the capacitor. These charges are supplied by
the conversion Therefore, as long as enough charge is provided, the voltage across the capacitor can be guaranteed. That is, the output voltage, and the value of o
remain basically unchanged.
    It can be seen that although the current in the switch tube is intermittent, due to the effect of the energy storage circuit, the output voltage
is continuous and the value does not fluctuate greatly. In the energy storage circuit, the inductor L plays the role of storing and supplying energy:
energy is stored when the switch tube is turned on; energy is released when the switch tube is turned off, which ensures the continuity of the current .
In addition to energy storage, the capacitor C in the energy storage circuit mainly plays a regulating and filtering role: it sometimes charges and sometimes discharges to maintain the output voltage
at a certain value. The function of the diode VD is to provide a path for the inductor L to release energy, so it is called a freewheeling diode
. These three components are the key to the energy storage circuit. They complement each other and are indispensable.