There are several other characteristics that impact power-supply operation. Among these are those employed to protect the supply, which are listed below.
Overcurrent: A failure mode caused by output load current that is greater than specified. It is limited by the maximum current capability of the power supply and controlled by internal protection circuits. It can also damage the power supply in some cases. Short circuits between the power-supply output and ground can create currents within the system that are limited only by the maximum current capability and internal impedance of the power supply. Without limiting, this high current can cause overheating and damage the power supply as well as the load and its interconnects (printed circuit board traces, cables). Therefore, most power supplies should have current limiting (overcurrent protection) that activates if the output current exceeds a specified maximum.
Overtemperature: A temperature that is above the power supply’s specified value must be prevented or it can cause power-supply failure. Excessive operating temperature can damage a power supply and the circuits connected to it. Therefore, many supplies employ a temperature sensor and associated circuits to disable the supply if its operating temperature exceeds a specific value. In particular, semiconductors used in the supply are vulnerable to temperatures beyond their specified limits. Many supplies include overtemperature protection that turns off the supply if the temperature exceeds the specified limit.
Overvoltage: This failure mode occurs if the output voltage goes above the specified dc value, which can impose excessive dc voltage that damages the load circuits. Typically, electronic system loads can withstand up to 20% overvoltage without incurring any permanent damage. If this is a consideration, select a supply that minimizes this risk. Many supplies include overvoltage protection that turns the supply off if the output voltage exceeds a specified amount. Another approach is a crowbar zener diode that conducts enough current at the overvoltage threshold so that it activates the power-supply current limiting and it shuts down.
Soft Start: Inrush current limitation may be needed when power is first applied or when new boards are hot plugged. Typically, this is achieved by a soft-start circuit that slows the initial rise of current and then allows normal operation. If left untreated, the inrush current can generate a high peak charging current that impacts the output voltage. If this is an important consideration, select a supply with this feature.
Undervoltage Lockout: Known as UVLO, it turns the supply on when it reaches a high enough input voltage and turns off the supply if the input voltage falls below a certain value. This feature is used for supplies operating from utility power as well as battery power. When operated from battery-based power UVLO disables the power supply (as well as the system) if the battery discharges so much that it drops supply’s input voltage too low to permit reliable operation.
Electromagnetic Compatibility (EMC): Involves design techniques that minimize electromagnetic interference (EMI). In switch-mode power supplies, a dc voltage is converted to a chopped or a pulsed waveform. This causes the power supply to generate narrow-band noise (EMI) at the fundamental of the switching frequency and its associated harmonics. To contain the noise, manufacturers must minimize radiated or conducted emissions.
Power-supply manufacturers minimize EMI radiation by enclosing the supply in a metal box or spray coating the case with a metallic material. Manufacturers also need to pay attention to the internal layout of the supply and the wiring that goes in and out of the supply, which can generate noise.
Most of the conducted interference on the power line is the result of the main switching transistor or output rectifiers. With power-factor correction and proper transformer design, connection of the heat sink, and filter design, the power-supply manufacturer can reduce conducted interference so that the supply can achieve EMI regulatory agency approvals without incurring excessive filter cost. Always check to see that the power-supply manufacturer meets the requirement of the regulatory EMI standards.
Post time: Apr-23-2018