Compliance with energy efficiency standards plays an increasingly important role for electronics. Statutory requirements for "pure" power supplies have long been established worldwide. Now chargers have increasingly begun to capture the legislature's attention. The article describes the standards for the energy efficiency requirements for charging technology and circuitry solutions to conform with them.
Around the globe power supply engineers encounter a multitude of standards for energy efficiency. In addition to voluntary programs for energy conservation such as Energy Star, or the Code of Conduct (CoC), there are also specific legal requirements for the efficiency of devices in local markets. For example, the ErP (Energy-related Products) specifications must be met for the European market to distribute the products. The U.S. market, however, requires EISA standards (Energy Independence and Security Act), MEPS applies to Australia (Minimum Energy Performance Standards), and Korea asks for KEMCO (Korea Energy Management Corporation). An example of the constant regulatory tightenings can be found in China, where currently a legal obligation instead of the previously voluntary China Energy Conservation Program (CECP) is being discussed.
Basically, all these programs differentiate their regulations in the product categories EPS (External Power Supplies) and BCS (Battery Charging Systems). EPS refers to voltage and current power supplies for external devices, BCS to a battery charging system, i.e. a charger with attached battery plus charging control. The BCS category also includes devices with a fixed battery, which can be charged via a charging device and an EPS device. Almost all mandatory energy efficiency standards ascertain that charging systems are currently specifically excluded from the regulations. Thus, for example, article 1, "Purpose and scope" of the EuP Directive 2009/125/EC in paragraph 2 states:
This Regulation shall not apply to:
a) voltage converters
b) uninterruptible power supplies
c) battery chargers
d) converters for halogen lamps
e) external power supplies for medical devices
The requirements of efficiency programs for EPS, can however be often found in the customer's product specifications. Most of them can be neglected due to the lack of legal necessity; nevertheless existing energy-efficient concepts of pure power supplies help to comply with the standards for battery chargers. Here is a brief summary of vital issues which need to be observed:
To check the efficiency standards for compliance, efficiency and no-load losses of the device must be tested. A decisive factor for determinating the minimum energy efficiency for all standards are the specifications on the type label of the unit.
During testing, the specified test method is to be observed:
- Selection of three test devices at random.
- Adjustment of the mains voltage to the rated voltage of the device.
- Recording of all readings after thirty minutes of operation.
- Testing of the equipment under four load conditions: 25 % / 50 % / 75 % / 100 %.
- The mean average of the efficiencies, measured for all four loads must be in accordance with the standard.
- The no-load losses shall comply with the standard.
It should be noted that the specifications of the efficiency programs Energy Star and ErP also differ in regard to standard and low voltage devices. The current limit values can be found in the following table:
|Current limit values acc. to ErP2 (2009/125/EC) and EnergyStar|
|Standard power supplies||Low voltage power supplies|
|Po ≤ 1 W||0,48 * Po + 0,14||0,48 * Po + 0,14||0,497 * Po + 0,067||0,497 * Po + 0,067|
|1 W < Po ≤ 51 W||0,063 + In(Po) + 0,622||0,075 * In(Po) + 0,0561|
|1 W < Po ≤ 49 W||0,626 + In(Po) + 0,622||0,075 * In(Po) + 0,561|
|> 51 W||0,87||0,86|
|> 49 W||0,87||0,86|
|No load power consumption|
|Po ≤ 51 W||≤ 0,3||≤ 0,3|
|Po > 51 W||≤ 0,5||≤ 0,5|
|Po < 50 W||≤ 0,3||≤ 0,3|
|Po ≥ 50 W||≤ 0,5||≤ 0,5|
CEC – World's only binding standard for chargers
Currently the only global mandatory standard for energy efficiency for chargers is Title 20 of the California Code of Regulations (paragraphs 1601-1608). This title covers almost all consumer electrical devices that contain circuits for battery charging. The spectrum ranges from notebooks through power tools to eBike chargers. From 2017 onwards, the standard also regulates applications which are not attributable to the consumer sector.
The standard was defined by the Ministry of Energy of the State of California, the California Energy Commission (CEC). Since this authority is solely responsible for the energy policy and planning within the California state boundaries, a high regional consideration could be assumed. A closer look reveals, however, that this standard is relevant for the entire U.S. market: companies which export their products to the USA, cannot exclude the distribution and use of the equipment in the state of California - and therefore should comply with the standard from the outset.
But which energy efficiency limits are ruled by the CEC title? Attention should be made to the fact, that the standard distinguishes between the categories of large chargers (large BCS), with an input power of more than 2 kW and small chargers (small BCS), with a lower input power. For the following, the regulation for small chargers should be given special attention. For a better understanding of the regulations and the following optimization of a charger, it is necessary to clarify some important CEC terms:
|Active charge mode:||Main charge until the battery is fully charged|
|Battery maintenance mode (Pm):||Trickle charge. The battery is charged but stays connected to the charger|
|24h charge and maintenance energy (E24h):||Total energy in watt hours which is consumed within 24 hours by the charging system (during main charge and trickle charge)|
|No battery mode (Pstby):||Standby usage without battery|
Figure: Energy consumption of a charge system acc. to CEC
With their standard, the CEC sets two mandatory key figures: firstly a fixed maximum watt hours (Wh) for the "24h charge and maintenance energy", on the other hand a maximum value for the total "battery maintenance mode" and "no battery mode". With the help of the energy content of the battery (Eb) used and the number of charging bays of the charger used (N), the limit values can be determined according to CEC. The corresponding formulas are shown in the following table:
|Current limit values acc. to CEC|
|Maximum 24h charge and maintenance energy (E24h)|
|For charging systems with Eb ≤ 2,5 Wh||16 * N|
|For charging systems with Eb > 2,5 Wh and ≤ 100 Wh||12 * N + 1,6 Eb|
|For charging systems with Eb > 100 Wh and ≤ 1.000 Wh||22 * N + 1,5 Eb|
|For charging systems with Eb > 1.000 Wh||36,4 * N + 1,486 Eb|
|Maximum power for maintenance mode and no battery mode (Pstby + Pm)|
|Limit value for total of Pm and Pstby in W||N + 0,0021 * Eb|
A typical e-bike charging system, for example, with a single charging bay (N = 1) and a battery with 36 V and 11 A (Eb = 36 V · 11 A = 396 W) generates the following threshold values which need to be met:
(1) E24h = 22 · 1 + 1,5 · 396 = 616 Wh
(2) Pstby + Pm = 1 + 0,0021 · 396 = 1,83 W
In the area of classical power tools, a typical charging system might look like this: A charging slot (N = 1), associated battery 18 V / 2,6 Ah (Eb = 18 V • 2.6 A = 46.8 W). According to CEC the following threshold values are to be maintained:
(3) E24h = 12 · 1 + 1,6 · 46,8 Wh = 86,88 Wh
(4) Pstby + Pm = 1 + 0,0021 · 46,8 W = 1,09 W
CEC requires special tests for multiple chargers, which can load batteries with different voltages and capacities. The charging processes of three predefined types of batteries have to be checked, each must comply with the threshold limit values, so that the total system meets the standard. The following battery types must be checked:
- Lowest voltage / lowest capacity
- Highest voltage / lowest capacity
- Largest energy content
The different cell chemistries like SLA, NiCd, NiMH or Li are not relevant when selecting the batteries.
Circuits to optimize energy efficiency
If the threshold values defined by the CEC are not reached by the existing charging system, the battery charger must be optimized. There are several suitable starting points for the fulfillment of the values. For compliance with the limit E24h the efficiency of the power unit is critical. A very good efficiency can be primarily achieved by the choice of an efficient topology, eg. LLC, flanked by other measures such as a synchronous rectification. If the observance of the limit value (Pstby + Pm) turns out to be problematic, the no-load losses must be reduced. In this case an auxiliary power supply is recommended.
Figure: Set-up of a charging system
Efficiency by LLC topology
In comparison with the flyback converter used in classic chargers, the LLC topology generates higher efficiencies. The advantage lies in a voltage-free switching of the MOSFETs, the so-called Zero Voltage Switching (ZVS). Compared to conventional switching, switching losses can be significantly reduced by ZVS, which leads to a higher efficiency of the entire system.
Another advantage is the soft switching. In this case, the switching interferences can be minimized, which in turn allows a smaller EMC filter and leads to lower efficiency loss in the filter circuit. LLC also stands for lower voltage stress on the primary sided MOSFETs and the secondary-sided rectifiers. This allows the use of more powerful semiconductors, resulting in further minimization of efficiency loss.
Efficiency by synchronuous rectifiers
A large part of the charger's power losses, is caused by rectifiers in the output stage. The following rule applies: the higher the output current, the greater the losses. In times like these, where high output currents and thus short charging times play an increasingly important role: Cyclists do not want to take unnecessarily long breaks with their e-bikes, craftsmen wish to use their tools after a short charging time.
In order to meet the CEC threshold values despite high output currents, these power losses must be limited as far as possible. This could be done by a synchronous rectification. Here the rectifier of the classic charger concept - typically a diode - is replaced by a switched FET. The advantage of the MOSFET is a considerably lower voltage drop at high output currents.
Figure: Equivalent circuit of a synchronous rectification
Efficiency by auxiliary power supply
Even if the threshold values comply with CEC for E24h, the compliance (Pstby + Pm) could turn out to be problematic. Due to strong dependencies on factors such as battery chemistry or permanent battery displays, the power consumption during the "battery maintenance mode" Pm can hardly or not at all be influenced by the charger. To comply with the threshold value (Pstby + Pm) the optimization of standby losses is crucial. Generally speaking: The lower the battery capacity, the more important the standby issue for CEC standard compliance (see table “Current limit values acc. to CEC”).
An auxiliary power supply might solve the problem. With a low capacity power supply, the power unit of a charger can be actively switched off, which significantly reduces the no-load losses of a charging system. Hence, charger engineers can profit from existing circuit concepts of very efficient small power supplies according to ErP / Energy Star specifications.
Submission and marking in accordance with CEC
If the charging system needs to be approved for the State of California, proof must be furnished by a CEC certified testing agent that the threshold values are reached. The agent will forward the test results directly to CEC for examination. If the results are positive, the manufacturers have to safeguard, in the context of a self-certification, the labeling of their charging systems. To this end, each approved charging system must be marked with the certification mark "BC", which can be done either on the type label of the unit or on the individual packaging and instructions for use.
The CEC maintains a database of all approved devices, which is publicly available online: http://www.appliances.energy.ca.gov/QuickSearch.aspx
Standards for charging systems in the future
Due to the problems in defining where the product shall be distributed in the US (the local Californian or the total U.S. market), it is probably only a matter of time before the CEC standard will become mandatory for the entire USA. It is therefore strongly recommended to already comply with the standards today.
In addition to the CEC mandatory standard for battery chargers there is the voluntary efficiency standard EnergyStar of the Environmental Protection Agency (EPA). Originally, new threshold values were planned for EnergyStar 2.0. Since the stricter CEC policy has already become effective and no further savings can be identified, the plan was abandoned. Accordingly, EPA has decided to phase out the EnergyStar program for charging systems by December 31st, 2014.
Energy efficiency guidelines for battery chargers are currently being discussed also for the European market. The ErP directive, which currently still excludes chargers, is supposed to be expanded accordingly. So far no concrete proposals for threshold values are known. In the course of globalization, however, it can be assumed that the ErP's parameters will follow those of the CEC.