POWER ELECTRONICS HANDBOOK
Power operating limits: How to choose the right power supply Not all power supplies react to over and under-voltages and currents in the same way. It pays to know the tradeoffs of different approaches to protection.
POWER
supplies can experience operating conditions outside normal specified limits, such as input under- or over-voltage, or variations in load and ambient temperature. These conditions can cause responses such as shutdowns, performance degradation, or component failures. To minimize such difficulties, product designers must know how their supply will perform outside its specified limits. At 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). These all have known failure modes when exposed to voltages above their rated maximum. X-capacitors, for example, are designed to fail short and will typically open the fuse, shutting down the power supply. Y-capacitors, on the other hand, are designed to fail open. This fault may go unnoticed for some time, although the capacitor will cease to filter common-mode noise effectively. The effects of over-voltage on the fuse can depend on the fuse voltage rating or its withstand voltage. If the voltage across the fuse exceeds this rating, arcing may prevent the fuse from protecting the circuit as intended.
Typical ac input network
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DESIGN WORLD — EE NETWORK
CUI v2 — Power Electronics HB 02-19.indd 30
2 • 2019
RON STULL | CUI INC.
This condition increases the risk of fire and can cause problems at the input or in downstream circuitry. Over-voltages may also interact with parasitic elements in the power supply circuitry, possibly boosting voltage-related stress on power semiconductors. In a flyback converter, the peak voltage across Input components the power switch vulnerable to voltage is determined by a stress typically include combination of the input those for protection voltage and output and filtering such as voltage as well as the X-capacitors (CX1 and transformer turns ratio CX2), Y-capacitors (CY1 and leakage inductance. and in the figure CY2), This peak voltage can and metal oxide varistors be difficult to calculate (MOV). All have known and typically must be failure modes when measured directly. exposed to voltages Conversely, above their rated under-voltage causes maximum. X-capacitors, higher currents in also known as “acrosscomponents such as the-line capacitors,” are the fuse, rectifier and used between the wires carrying the incoming ac current. A capacitor failure in this position will usually cause a fuse or circuit breaker to open. 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.
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2/19/19 3:34 PM
