Other protection mechanisms allow the power supply to continue operating, although the performance may suffer. High-wattage power supplies often feature brown-out protection to initiate shutdown if the input voltage falls below a specified threshold. Of course, there are several ways to protect the power supply against excessive input-voltage variations. A boost-PFC converter will cease to regulate if the input voltage rises above the output.
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Overvoltage can also affect PFC circuitry. However, there is a minimum frequency below which further reduction lowers the gain and so can cause malfunctioning or power-supply failure. If the input voltage drops, the frequency slows to boost the input-to-output gain and keep the output voltage stable. In LLC resonant converters, a varying of the operating frequency regulates the output voltage. In other topologies, low input voltage can affect the operating frequency or duty cycle and cause a supply malfunction. In some topologies, such conditions can cause potentially damaging peak currents in power switches, a rise in their operating frequency, a loss of energy efficiency, or the power supply may shut down. High current can also introduce a loss of inductance or a saturation of magnetic components such as the PFC (power factor correction) choke. The result can be extra internal heating that can lead to rapid failure or poor reliability. This peak voltage can be difficult to calculate and typically must be measured directly.Ĭonversely, under-voltage causes higher currents in components such as the fuse, rectifier and power switches. In a flyback converter, the peak voltage across the power switch is determined by a combination of the input voltage and output voltage as well as the transformer turns ratio and leakage inductance. Over-voltages may also interact with parasitic elements in the power supply circuitry, possibly boosting voltage-related stress on power semiconductors. This condition increases the risk of fire and can cause problems at the input or in downstream circuitry. If the voltage across the fuse exceeds this rating, arcing may prevent the fuse from protecting the circuit as intended. The effects of over-voltage on the fuse can depend on the fuse voltage rating or its withstand voltage. This fault may go unnoticed for some time, although the capacitor will cease to filter common-mode noise effectively. Y-capacitors, on the other hand, are designed to fail open.
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X-capacitors, for example, are designed to fail short and will typically open the fuse, shutting down the power supply. These all have known failure modes when exposed to voltages above their rated maximum. Y-capacitors, also known as “line-to-ground capacitors,” are used where capacitor failure could lead to the danger of electrical shock if the ground connection is lost.Īt the power supply input, voltage fluctuations on the ac supply line can over-stress mandatory protection and filtering components such as X-capacitors, Y-capacitors and metal oxide varistors (MOV). A capacitor failure in this position will usually cause a fuse or circuit breaker to open. X-capacitors, also known as “across-the-line capacitors,” are used between the wires carrying the incoming ac current. All have known failure modes when exposed to voltages above their rated maximum.
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Input components vulnerable to voltage stress typically include those for protection and filtering such as X-capacitors (CX1 and CX2), Y-capacitors (CY1 and in the figure CY2), and metal oxide varistors (MOV). To minimize such difficulties, product designers must know how their supply will perform outside its specified limits. These conditions can cause responses such as shutdowns, performance degradation, or component failures. Power supplies can experience operating conditions outside normal specified limits, such as input under- or over-voltage, or variations in load and ambient temperature. It pays to know the tradeoffs of different approaches to protection. Not all power supplies react to over and under-voltages and currents in the same way.