July 10, 2018 by Bruce Rose
Regulatory agencies have created many designators for power supplies which conform to different sets of specifications. The LPS (Limited Power Source) requirements are specified in the IEC 60950‑1 standard and are used to define power supplies with relatively low maximum voltage, current and power capabilities. The benefit to customers of LPS power supplies is system installers can follow relaxed requirements regarding the wiring and physical installations of loads powered by modules certified as LPS. Understanding the basic specifications for LPS power supplies will help to explain why certain power supplies qualify as LPS while others do not.
Power supplies which qualify as LPS are recognized as unlikely to cause electrocution or a fire due to the limitations on the output current and voltage they can deliver to a load. The following is a summary of the specifications for power supplies certified as LPS with inherent power delivery limits:
VA = Volts * Amps
Voc = Open circuit output voltage (no load)
The characteristics of LPS power supplies with inherent power delivery limits are also described in the following graph.
A) Imax and Isc
B) Imax limited to 100 VA
C) Isc limited to 8 A
D) Isc limited to 150/Voc
An inherently limited power supply can employ one of three methods to ensure the supply conforms to the limitations specified above.
This class of circuits does not require additional design considerations to ensure the limited power delivery capability as internal components are incapable of delivering power in excess of the limits. The classic example of a component limiting power delivery capacity is the winding resistance of an isolation transformer. In a well-designed power supply, the components limiting the power delivery capability will not be damaged when they are the limiting factor in the power delivery.
An impedance in the form of a conventional resistor or PTC resistor can be placed in series with the power conductors to limit the power delivery capability of the power supply. While being simple to implement, conventional resistors are seldom employed for this purpose due to the power dissipation of the resistors causing a reduction of the conversion efficiency of the power supply. The use of PTC resistors maintains the simplicity of the implementation while reducing the associated power losses during normal operation.
This method is common in modern power supplies due to the low cost and wide availability of the required integrated circuits. However, care must be taken in the power supply design and testing to ensure the required limits are adhered to during both normal and single fault operating conditions.
Power supplies with external current limiting devices can be classified as LPS even if they do not contain one of the three means of limiting power delivery listed above. A power supply can be certified as conforming to LPS if it employs an over current protection device (i.e. fuse or circuit breaker) to appropriately limit the current available to be supplied to the load. The current limiting device must be either a fuse or a non-adjustable, non-auto resetting, and electromechanical device (i.e. a circuit breaker). The fuses or circuit breakers must break the circuit within 120 seconds with a current equal to 210% of the current specified in the regulations. The following is a summary of the specifications for power supplies certified as LPS with non-inherent power delivery limits:
Characteristics of LPS power supplies with current limiting devices are described in the following graph.
A) Current limiting device limits currents to 5 A
B) Current limiting device limits power to 100 VA
C) Maximum current without current limiting device limited to 1000 VA/Voc
D) Maximum power without current limiting device limited to 250 VA
Compliance with LPS standards is typically included in the Certification Body (CB) Scheme report of the power supply with test results performed by safety agencies, such as UL, CSA, or TUV. Marking "LPS" on the power supply label itself is optional, though most LPS compliant models from CUI will include the LPS mark, as shown below.
As the presentation of LPS power supply characteristics can be dry and unexciting (perhaps not desired in most reading materials), an awareness of the specifications can also allow a dry and unexciting system design (often strongly desired in system designs).
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