Using Duty Cycle To Detect Power Supply Overload (Part 1): Why It's Needed

by Gregory Mirsky, Continental Automotive Systems, Deer Park, Ill., How2Power Today, Apr 17 2017

Focus:
A majority of switching power supplies use sensing resistors to detect overloads and output shorts. At high output current, a sense resistor value becomes comparable to the resistance of the board traces and wires as well as the diode bonding wire resistances, which reduces the overload detection circuit accuracy. However, if the power supply is made correctly, its feedback sensing node would be placed on or near the output terminals to efficiently compensate for the voltage drop across the printed board foil, inductor winding resistance, control MOSFET RDS(ON), etc. In this case, when the load current changes, the feedback loop commands the control PWM duty cycle to compensate for the voltage drop. At very heavy loads or shorts the feedback may increase the control PWM signal duty cycle to very high values. This effect is used in the proposed protection circuit. This part 1 article explains how the duty cycle depends on the load current in a buck converter and how to estimate the thresholds for the protection circuit tripping at overload.

What you’ll learn:

• How to use duty cycle to detect overloads in PWM-based power converters

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