Topology of DC switching power supply
Switching power supply can be divided into DC switching power supply and AC switching power supply, according to the output to distinguish, AC switching power supply output is alternating current, and DC switching power supply output is direct current, here is the DC switching power supply. With the development of related components, DC switching power supply has been widely used in many occasions instead of linear power supply because of its high efficiency.
Dc switching power supplies are generally more expensive than linear power supplies, but in some special occasions they are simpler and cheaper, and even almost only switching power supplies can be used, such as voltage boost and polarity reversal. Dc switching power supply can also be divided into isolated and not isolated two, isolated is the use of transformers to achieve electrical isolation between the input and output, transformer is also easy to achieve multiple different voltages or multiple outputs of the same voltage.
Dc switching power supply structure is complex, design and analysis have a special set of theories and methods, here mainly introduces 6 basic non-isolated DC switching power supply structure form and its characteristics, easy to choose according to the application.
Ideal assumption: For ease of analysis, the following ideal state is often assumed
1. Electronic device ideal: electronic switching tube Q and D turn-on and turn-off time is zero, on-state voltage is zero, off-state leakage current is zero
2. Inductors and capacitors are ideal energy storage components without loss, and the switching frequency is higher than the resonant frequency of LC
3. In a switching cycle, the input voltage Vin remains unchanged
4. In a switching cycle, the output voltage has a small ripple, but it can be considered to remain basically unchanged, and its value is Vo
5. Do not consider line impedance
6. Converter efficiency is 100%
Buck converter: also known as buck converter, is a single tube unisolated DC converter whose output voltage is less than the input voltage.
In the figure, Q is a switching tube whose driving voltage is generally a Pulse width modulation (PWM) signal, and the signal period is Ts, then the signal frequency is f=1/Ts, the on-time is Ton, the turn-off time is Toff, then the period is Ts=Ton+Toff, and the duty cycle Dy= Ton/Ts.
Buck converters work in two basic ways:
CCM(Continuous current mode) : Inductor current continuous mode, the output filtered inductor Lf current is always greater than zero
DCM(Discontinuous current mode) : Inductive current discontinuous mode, there is a period of time when the current of Lf is zero during the switching tube disconnection