DC/DC conversion circuit diagram without isolation

For a converter circuit that does not need to be isolated from the input and output voltages but only uses a working switch and L, D, and C, the most basic forms are as follows: Buck Converter (Buck Converter ), Boost Converter Converter (BoostConverter) and buck/boost converter (Buck-Boost Converter). The circuit principle is shown in the figure.

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1. Buck converter

As shown in Figure (a) 9, in order to reduce the output ripple, an inductor L and a capacitor C are connected to the output end. VD in the figure is the freewheeling diode. The average value of the buck converter output voltage Vo is always less than the input voltage Vin. Whether the current iL in the inductor is continuous depends on the switching frequency, the parameter values ​​of the filter inductor L and the capacitor C.

When the operating frequency of the circuit is high, if the inductance value of the inductor and the capacitance of the capacitor are large enough and they are ideal components, the output voltage can be considered to be constant after the circuit enters a steady state. When the transistor VT is turned on, the current in the inductor rises linearly; when the transistor VT is turned off, the current in the inductor cannot change suddenly, and the induced electromotive force on it causes the diode to turn on. In the steady state, the inductor filter maintains the DC component and eliminates the harmonic components, so the average output current is Io=Vo/RL;, where RL is the resistance of the load resistor.

2. Boost converter

Figure (b) shows a boost converter, which consists of a power transistor VT, an energy storage inductor L, a diode VD and a filter capacitor C. When the transistor VT is turned on, the power supply stores energy in the inductor, the inductor current increases, the polarity of the induced electromotive force is left positive and right negative, and the load capacitor C supplies power. When VT stops, the inductor current decreases, the polarity of the induced electromotive force is negative on the left and positive on the right. The inductor releases energy, supplies power to the load through the diode VD together with the input voltage, and charges the capacitor C at the same time. This converts low-voltage direct current into high-voltage direct current.

3. Buck/boost converter

Figure (c) shows a buck/boost converter. The average value of the output voltage Vo is greater than or less than the input voltage Vin, and their polarities are opposite.