An Overview of Limited Power Source (LPS) Requirements
July 10, 2018 by Bruce Rose - 6 Minute Read
What is an LPS Rated Power Supply?
An LPS rated power supply is designed for safety reasons to conform to a maximum permitted output voltage, output current and output power rating. 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 the maximum performance capabilities mentioned above. 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)
- Dc voltage less than or equal to 30 Vdc or substantially sinusoidal ac voltage less than or equal to 30 VACrms
- Maximum short circuit current of 8 A
- Maximum VA of 100
- Maximum marked output power rating of 5 A * Voc
- Maximum marked output current rating of 5 A
- Dc voltage with ripple greater than 10% of the peak or non-sinusoidal ac voltage
- Maximum peak voltage of 42.4 V
- Maximum short circuit current of 8 A
- Maximum VA of 100
- Maximum marked output power rating of 5 A * Voc
- Maximum marked output current rating of 5 A
- Dc voltage greater than 30 Vdc and less than or equal to 60 Vdc
- Maximum short circuit current of 150 VA/Voc
- Maximum VA of 100
- Maximum marked output power rating of 100 VA
- Maximum marked output current rating of 100 VA/Voc
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.
1. Inherent Power Limiting
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.
2. Linear or Non-Linear Impedance Providing Power Limiting
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.
3. Regulating Network Providing Power Limiting
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.
Devices Limiting Current Delivered to the Load
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:
- Dc voltages ≤ 20 Vdc and substantially sinusoidal ac voltages ≤ 20 VACrms
- Short circuit current must be less than 1,000 VA/Voc
- Current rating of the over current protection device must be ≤ 5 A
- Maximum VA must be ≤ 250
- Maximum marked output power rating of 5 A * Vmax
- Maximum marked output current rating of 5 A
- Dc voltages > 20 Vdc and ≤ 60 Vdc and substantially sinusoidal ac voltages > 20 VACrms and ≤ 30 VACrms
- Dc voltages with ripple greater than 10% of the peak and non-sinusoidal ac voltages must have Vp ≤ 42.4 V
- Short circuit current must be less than 1,000 VA/Voc
- Current rating of the overcurrent protection device must be %le 100 VA/Voc
- Maximum VA must be ≤ 250
- Maximum marked output power rating of 100 VA
- Maximum marked output current rating of 100 VA/Vmax
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
LPS Certification and Labeling
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.
Conclusion
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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Bruce Rose
During his many years in the electronics industry working in design, sales, and marketing, Bruce Rose has focused on analog circuits and power delivery. His range of work experience includes organizing and chairing international workshops, publishing and presenting in more than 40 technical conferences and journals, and having been awarded seven patents. While he enjoys his time at work, Bruce further enjoys the time he is able to spend with his family hiking, biking, and canoeing as well as pursuing his passion of full scale and model aviation.