Limited Power Sources (LPS): IEC62368-1 vs. UL1310 Class 2 Power Supply for AC DC Power supplies and Adapters

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2/20/2020

Introduction

IEC62368-1 and UL1310 (NEC Class 2 Power Units) are product safety standards which aim to keep humans safe by lowering the possibility of a hazardous situation. Both standards agree that limiting the output power of power supplies connected to mains results in a reduced risk of starting a fire, especially during an overload condition. While the two share similar ideas, the requirements and definitions differ slightly.

Overview

IEC62368-1 gives the name “Limited Power Source” (LPS) to a class of AC to DC and DC to DC power supplies which meet a set of criteria defined in Annex Q of the standard. It is important to note that a power supply certified to IEC62368-1 does not imply that it also LPS compliant; LPS compliance is optional. GlobTek aims to include LPS ratings and LPS certification in as many product families as possible.

(Note: The definition of LPS in IEC62368-1 is identical to the definition in IEC60950-1, as of February 2020.)

NEC Class 2 Power Units or UL1310 rated power supplies require that all AC/DC power supplies and AC/DC adapters carrying UL1310 certification have limited output power capability. Thus, there is no need for a term like LPS, since all Class 2 Power Units rated with UL1310  must be limited. The limits are defined in Section 28 of the standard. UL 1310 AC adapters and power supplies are also known as Class 2 Transformers.

The following discussion applies to “inherently limited” power sources, that is, power sources which cannot exceed certain power limits by virtue of their design. Switch-mode power supplies and AC Adapters which are IEC62368-1/LPS or UL1310/Class 2 compliant, are typically “inherently limited” by an electronic regulation network or due to the intrinsic power limitation of certain components.

Definitions

IEC62368-1:
An inherently limited power source (LPS) is an isolated circuit which…
  1. …cannot provide more than 100VA of continuous apparent power under any loading condition
  2. …cannot supply substantial current under a short circuit condition
  3. …has a SELV compliant open circuit voltage of 60V DC or 42.4V peak AC max
UL1310 or Class 2 Power Unit:
An inherently limited unit is an isolated circuit which…
  1. …cannot provide more than 100VA of continuous apparent power under any loading condition
  2. …cannot supply substantial current under a short circuit condition
  3. …has a SELV compliant open circuit voltage of 60V DC or 42.4V peak AC max
  4. …has a limited nameplate current rating

As evident in the above list, the major difference between the two definitions is UL1310’s explicit inclusion of a maximum nameplate current rating. While IEC62368-1 does not discuss nameplate ratings directly, it still implies maximum nameplate ratings, as long as the power supply is constant voltage (CV) in nature. That is, its open-circuit output voltage is essentially the same as its full-load output voltage.

Discussion

The graph below shows the restrictions on current according to the two standards. Notice that the Class 2 Power Unit's UL1310 nameplate rating (yellow) is limited to 5A when the open-circuit output voltage is ≤20V. On the other hand, an IEC62368-1 power supply (red) may be rated up to 8A for open-circuit output voltages ≤12.5V. To reiterate, there are no explicit nameplate restrictions per IEC62368-1, but it is essential to keep the output current below the red curve in order to comply with IEC62368-1/LPS, for a CV power supply.

graphThe blue curve may be a source of confusion to some. To understand its place, it is important to remember that the x-axis is the open-circuit output voltage and that both standards require that apparent power never exceeds 100VA under any circumstance. Basically, the blue curve only applies to power supplies which reduce their output voltages under heavy loading.

For example, a 20V nameplate power supply is only allowed to provide 8A of output current if its loaded output voltage drops sufficiently to ensure that 8A * Vout-loaded ≤ 100VA. In this case, Vout-loaded must collapse to ≤12.5V to comply.

However, since most applications prefer fully constant voltage operation, this type of power limiting is generally undesirable. For applications which require power sources near the 100VA limit, GlobTek’s GTM96900P 90W LPS power supplies are designed to operate entirely in constant voltage mode, and employ an electronic over-current limiter to prevent the unit from supplying greater than 100VA, if overloaded. GlobTek offers LPS power supplies in wall plug-in, desktop, and open-frame configurations from 3-100W.

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