Power-supply requirements for three-phase electric-energy meters used in industrial applications have grown quickly in recent years. For example, distributed factory automation requirements for monitoring power consumption have driven the adoption of wireless communications in high-end electric-energy meters. In such meters, the power supply may need to deliver up to 20 W of regulated low-voltage dc output, while operating from mains voltages as high as 690 Vac.
The electric-energy meter manufacturer typically would like to implement such power supplies as inexpensively and flexibly as possible. Furthermore, the manufacturer may wish to reuse the same approach, topology and semiconductors it employs in single-phase electric-energy meters, which operate at much lower ac line voltages. A quasi-resonant flyback converter design that incorporates high-voltage bipolar transistors can meet these seemingly conflicting requirements.
Electric-Energy Metering Background
Along with the rest of the power electronics industry, electric-energy metering equipment has quickly evolved into more feature-rich and complex systems. In the past, most electric-energy meters were simple magnetic machines that measured the electricity consumption of the utilities' residential and commercial customers. The meters were entirely mechanical and magnetic, or electromechanical, with no real electronics, so internal power-supply issues were never raised.
In our networked world, however, electronic metering, automated meter reading and communication have become tools in the quest to optimize power consumption. Consequently, electromechanical meters no longer suit the needs of either the consumer or the professional meter markets. Electronic meters are required to convert metering data into standard digital formats, and to store and communicate this data with the outside world.
The first step in digitizing the electromechanical meters was to implement electronic metering using analog ICs, sometimes in combination with more complex microcontroller-based systems. This approach required the availability of stabilized voltages to keep those silicon ICs well-biased, and as reliable and precise as the law requires the metering system to be. Regardless of the country and the end-user domain, this first step toward modernization required a stable power supply providing roughly 0.5 W to 2 W.
The next obvious step in the metering modernization process is the implementation of wireless communication (either RF, Bluetooth or WiFi) protocols, together with LCD (possibly touchscreen) displays. These features may increase the meter's power requirements by up to 10 times, pushing consumption of the meter's electronics as high as 20 W.
As mentioned previously, electric-energy metering systems are supposed to be plugged into the same voltage mains, so the house/utility power consumption should be measured as plugged in.
Implementations of electric-energy metering must take into account standard domestic and commercial use worldwide for voltage mains going from 100 Vac up to 575 Vac, either single phase or three phase. For example, the latest environmental requirements for the automotive and factory automation industries have led manufacturers to monitor the power consumption of all of a factory's equipment, with the aim of optimizing overall consumption, avoiding network-disturbing power peaks and saving money. With each piece of equipment being monitored, the upper limit of mains voltage moves further up. New motors and robots consuming 1 MW and higher do not operate off the existing 575-Vac standard, but rather off the new standard of 690 Vac. This increase in operating voltage reduces the currents both in the power distribution circuit and in the motor itself.
Powering Industrial Meters
The two main electrical parameters to take into account when developing a suitable power supply for high-end metering systems are well defined:
POUT: up to 20 W
VMAINS: up to 690 Vac.
The ever-present requirement of getting high efficiency from a low-cost topology leads designers to think about switch-mode power supplies (SMPS; either isolated or nonisolated), and likely using the inexpensive and easy-to-design flyback topology.
Although this implementation is feasible and even simple when the mains voltage remains below 264 Vac, it begins to get complicated when the ac line voltage rises above this value, boosting the dc link to more than 600 V. In the case of 690-Vac line voltage (plus the usual margin designers must take into account), the dc link reaches values up to 1250 Vdc.
When designing a standard flyback converter, the voltage capability of the switch must follow the basic formula below, which takes into account the dc link, the reverted voltage design, the leakage-inductance effect and an appropriate margin:
It is easy to calculate that when 1250 Vdc applies, the formula becomes:
Based on an emitter-switching concept, emitter-switched bipolar transistor (ESBT) technology was specifically developed to cope with stringent industrial requirements, with the advantage of maintaining a simple and inexpensive flyback single-switch topology at higher voltages. This technology has been used to create a 3-A, 2200-V switch, the STC03DE220HP from STMicroelectronics (Fig. 1).
This transistor is housed in a special TO-247-4LHP, which is a fully isolated package that complies with IEC norms for creepage to achieve its high-voltage rating. Furthermore, the device allows designers to operate the power converter at switching frequencies up to 150 kHz, enabling the SMPS to achieve small size through the use of low-value passives and magnetic components.
Another point to consider when designing an SMPS for electric-energy metering is efficiency. The meter itself is not meant to consume any power, just to monitor it. Any losses introduced by the SMPS result in added consumption at the system level.
In designing the SMPS, the two main losses that must be taken into account are switching losses and conduction losses. Since ESBT technology is bipolar based and employs minority carriers, it offers lower conduction losses than would be possible with MOSFETs. As for switching losses, the main advantage of an ESBT is its turn-on losses, which are roughly based on the following formula:
PON = ½ C V2.
When the dc link goes as high as 1250 Vdc, this loss is predominant when compared with conduction losses and turn-off losses.
ESBT technology offers the possibility of implementing a quasi-resonant topology, also known as a valley-switching topology — where the device is turned on after leakage-inductance demagnetization, when the first valley appears as a result of the intrinsic circuit resonance.
Fig. 2 illustrates how this smart control method can help to improve efficiency in any SMPS, but becomes especially valuable when applied to a very-high-voltage bus.
As shown in Fig. 3, ESBTs are four-pin rather than the standard three-pin devices because of the cascode construction of the monolithic switch. Although this is not the standard switch construction, there is much literature and many reference designs available to support applications using ESBT technology.
A power-supply reference design for a single-switch quasi-resonant flyback converter meets the requirements listed in the table. Although this reference design was conceived to deliver 5 W, with very small modifications its output power can be increased up to 20 W. Fig. 4 portrays the full circuit implementation of the reference design, where special attention must be paid to the ESBT base biasing design and the proper design of the quasi-resonant topology in order to achieve the best performance in a high-voltage environment.
Obviously, 20 W represents a modest amount of power for such a smart topology/technology mix. The reference design can deliver more than 360 W while still using a single-switch topology if transistors with higher current ratings are used.
Beyond the 1000-V Limit
ESBT technology offers industrial system designers, of either SMPS or any other switching application, the possibility to think beyond the usual 1000-V limit for MOSFETs, widening the breakdown voltage capability to 2200 V and keeping both the switching and conduction losses very low.
These benefits are thanks to the fast switching characteristic of the ESBT technology and its intrinsically low saturation voltage, which is typical of any minority-carrier-based technology. Electric-energy metering applications can benefit from the combination of ESBT technology with a quasi-resonant topology, achieving more than 80% efficiency in ultrasmall, cool-running and reliable power supplies for demanding environments.