DC/DC conversion circuit

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DC/DC conversion

Converting one DC voltage into another (fixed or adjustable) DC voltage is called DC/DC conversion (also known as DC converter). This technology is widely used in the continuously variable transmission of trolleybuses, subway trains, and battery-powered motor vehicles, thereby achieving smooth acceleration, deceleration, and fast response performance. The electric vehicles that emerged in the 1980s are an example.

The following introduces a typical DC/DC conversion circuit using self-shutdown devices.

The most basic chopper circuit is shown in Figure 3.1. The chopper load is R. When the switch S is closed, uo=uR=Ud, and it lasts for t1 time. When the switch is turned off, uo=uR=0, and it lasts for t2 time. T=t1+t2 is the working cycle of the chopper. The output waveform of the chopper is shown in Figure 3.1(b). If the duty cycle of the chopper is defined as D=t1/T, then the average output voltage can be obtained from the waveform diagram as

　

t1 is the chopper on-time, T is the on-off cycle, and there are usually two working modes of the chopper:

(1) Pulse width modulation working mode: keep T unchanged and change t1.

(2) Pulse frequency modulation working mode: keep t1 unchanged and change T.

Pulse width modulation is commonly used because frequency modulation is prone to harmonic interference and filter design is also difficult.

Figure 3.1 Buck chopper circuit principle
(a) Circuit (b) Waveform