Isolated PWM suits low frequencies

摘要

Many industrial- and medical-system circuits require isolation from the mains-ac power. You can often send a signal across the isolation barrier using a small transformer; transformers do not pass low-frequency signals well. The circuit in this Design Idea converts a low-frequency PWM (pulse-width-modulated) signal to a higher frequency, which you pass across the transformer, and retains the duty cycle. Once the frequency is on the other side, you can then convert the PWM signal back to an analog voltage. The circuit converts a 1-kHz PWM signal to a 100-kHz signal with the same duty cycle (Figure 1). This 100-kHz signal easily couples across an isolation transformer. You then filter it to provide a dc control voltage on the isolated side. IC_2, an LTC6992-2, is a voltage-controlled PWM IC. A voltage ranging from 0 to 1V on the MOD input pin varies the duty cycle of the output from 5 to 95%. The duty cycle does not reach 0 or 100%, which would not pass through the transformer. Resistor R_(SET) fixes an internal master-oscillator frequency of 100 kHz. The voltage on the DIV pin sets a divider ratio. An internal 4-bit ADC translates the analog voltage to a digital-divider value. With the input shored to ground, the divider ratio is one, and the circuit outputs the oscillator frequency.