The essence of a commonly used PWM circuit is a square wave cycle with a certain duty ratio adjustable circuit. Its basic working principle is to compare a sawtooth wave signal with a certain frequency and a DC control voltage in the comparator. When the DC control voltage changes , the output duty cycle changes accordingly. In cases where a dedicated PWM circuit is not available, the circuit described below can be used. 1. Duty cycle adjustable circuit composed of dual comparators
Figure 1 is an adjustable duty cycle circuit composed of dual comparators. The circuit uses a dual comparator. The comparator (1) is a square wave oscillator. The sawtooth wave is drawn from its oscillating capacitor and sent to the comparator (2). The inverting input terminal and the non-inverting input terminal of the comparator (2) are connected to the control voltage. Adjusting RP can adjust the output duty ratio (that is, the output pulse width). The working waveform is shown in Figure 2. It can be seen from the figure that the higher the DC control voltage, the wider the output pulse width and the larger the duty cycle; on the contrary, the narrower the pulse width, the smaller the duty cycle, and the output frequency is determined by the frequency of the sawtooth wave.
figure 1
