PWM watchdog circuit ensures upper bound

-- EDN, November 7, 1996

Design Ideas

PWM watchdog circuit ensures upper bounds

Shyam Tiwari, IGCAR, Kalpakkam, India

 With PWM control of power factor, you need a continuous check on excess pulse width and dead periods to protect a variety of systems that may not incorporate such protection. You also need regular pulsed power to ensure good averaged, reconstructed power-signal waveforms for which the dead period for the PWM signal must not exceed limits. The circuit in Figure 1 monitors both excess high-level pulse width and excess dead period and visually warns the user of problems in the PWM function.

  Excess high-level pulse width turns on the normally off, red LED, and excess dead period turns on the normally off, green LED. If the PWM function suffers from both excess high-level pulse width and excess dead period, both the red and green LEDs glow. The circuit compares the upper bounds on pulse width and dead period with the charging time of capacitors C¹ and C². Variable resistors R¹ and R² control the capacitors' charging rates. The resistors set the upper-boundary limits at 10 µsec to 1 msec, according to user requirements. R¹ sets the high pulse-width limit, and R² sets the dead-period limit. C¹ and C² are fixed, 0.01-µF, low-leakage Teflon or polypropylene capacitors. (DI #1941)

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