Guidelines for Class 2 / LPS Investigations
GENERAL: The following is intended to provide a quick reference guide when conducting Class 2 or Limited Power Source (LPS) investigations.
USE: Circuits defined as Class 2 have been investigated to the Standard for Class 2 Power Units, UL1310; Class 2 and Class 3 Transformer, UL1585; or Information Technology Equipment UL60950. UL60950 also offers a Limited Power Source (LPS) circuit, which is similar to Class 2 but is not marked Class 2. UL1310 and UL60950 devices are intended for use in a Controlled or Pollution Degree 2 environment; and UL1585 is intended for use indoor, dry locations.
Based upon the evaluation of Class 2 limits these products are allowed
to relax the following requirements:
? Output Wiring
? Enclosure flammability rating ? Testing
The limits for Class 2 / Limited Power Source (LPS) are also noted in
the National Electric Code 2002 Article 725.41 FPN No. 2 Table 11(A) and 11(B) for reference purposes.
DEFINITIONS:
Inherently Limited – In general, an Inherently Limited Power Source
limits the output power by circuit and component design (i.e. high impedance, etc.), not by supplemental protection such as overcurrent protective devices. The typical limiting factors are:
? Transformer design ? Regulating Network
?
PTC or Resistor
Non-Inherently Limited – A Non-Inherently Limited Source is not
limited by a circuit or component design and must rely on reliable
protective devices to limit its output power. This design exceeds the
requirements of an Inherently Limited circuit and in addition to conforming to the above similar factors, must meet the following:
? Suitably rated and calibrated protective device (i.e. fuse, circuit breaker
NOTE: The above description incorporates NEC Class 2, UL1310, and UL60950 LPS requirements in general terms. Specific notes/requirements shall be taken into account for the particular standard/investigation.
Page 1 of 4
Guidelines for Class 2 / LPS Investigations
InputOvercurrentProtectionPulse WidthModulationController(PWM)MOSFETTransformerOutputFeedbackNetwork
The illustration above notes the major components of a switch mode power supply. When applying the requirements for Class 2 or LPS we need to source the maximum current/power available from the secondary/output utilizing a variable resistor under normal operating conditions and abnormal operating conditions. The most common abnormal operations are in an effort to disable the power supply foldback limiting circuitry. The following are
typical examples of circuitry to apply abnormal on, although not inclusive, it provides a starting point for abnormal selection:
TRANSFORMERVoutInputOutputFBComponents ToConsider FaultTestingINSULATIONPRIMARYSECONDARY
Power Mosfet – Typically, this devise has a impedance component connected to the side commonly connected to the –DC rail (-side of the bulk capacitor). Performing component
faults on this impedance (in an effort to decrease the impedance) will typically allow our variable resistor connected to the secondary to source more current and power.
Page 2 of 4
Guidelines for Class 2 / LPS Investigations
TRANSFORMERVoutInputComponents ToConsider FaultTestingOutputFBPWMPRIMARYRTCTINSULATIONSECONDARYComponents ToConsider FaultTesting
The current sense feedback connects to the Pulse width modulating (PWM) chip. Typically, it receives its signal from a winding in the power transformer. The signal could also be derived from a independent current sense transformer. Component faults (opening and
shorting) of the capacitors and resistors connecting the current sense winding to the PWM in this circuitry will typically allow our variable resistor connected to the secondary to source more current and power or the power supply will foldback. Please note component faults in the PWM support circuitry may also allow our output voltage to double or even triple in value. These types of faults may also be beneficial for SELV, LVLE, LVISC
reliability testing. Additionally, the secondary capacitors connected across the output are typically not rated for three times the output voltage which creates a potential of capacitor venting (potential risk of fire or shock hazard).
Page 3 of 4
Guidelines for Class 2 / LPS Investigations
TRANSFORMERInputOutputPWMPRIMARYFBINSULATIONSECONDARYComponents ToConsider FaultTestingComponents ToConsider FaultTestingOPTOCOUPLERComponents ToConsider FaultTesting
The secondary feedback senses the output voltage and signals to the PWM when the correct output voltage is achieved. With this signal disabled, the PWM attempts to deliver more power to the output. Typically, the primary to secondary feedback loop consists of a
optoisolator. The signal could also be derived from a independent feedback transformer. Component faults (opening and shorting) of the components connecting to the optoisolator or to the PWM in an effort to disable this signal will typically allow our variable
resistor connected to the secondary to source more current and power and may also allow our output voltage to double or even triple in value or the power supply will foldback. These types of faults may also be beneficial for SELV, LVLE, LVISC reliability testing. Additionally, the secondary capacitors connected across the output are typically not rated for three times the output voltage which creates a potential of capacitor venting (potential risk of fire or shock hazard).
Page 4 of 4
百度搜索“77cn”或“免费范文网”即可找到本站免费阅读全部范文。收藏本站方便下次阅读，免费范文网，提供经典小说综合文库Class2-LPSevaluation在线全文阅读。
相关推荐：