Supercap (EDLC) Cell Balancing IC

1. Evolution of the Power Electronics Market

In recent years power electronics have been garnering increased attention, especially in the automotive, industrial equipment and renewable energy fields. In the automotive sector in particular, improved fuel economy in conjunction with emissions regulations has become an important issue, and auto makers have been moving forward with research into new technologies. To develop vehicles with better fuel economy, in addition to attempts to increase power conversion efficiency by introducing next-generation power devices, efforts to achieve higher system power consumption efficiency by devising combinations of power storage devices are being considered.

Stricter fuel consumption targets have also been adopted for automobiles, including in the Japanese market, spurring development of more environmentally friendly vehicles. (Figure 1)

Figure 1. Fuel Economy Regulations Roadmap

* Source: Created by ROHM based on the ‘2013 General Survey of Automotive Electronic Devices and Components’ published December 4, 2012 by the Fuji Chimera Research Institute, Inc.

2. New Power Storage Device Application Technologies

The use of large capacity power storage devices is becoming increasingly common in the automotive field, as typified by electric and hybrid vehicles. And in addition to conventional lead-acid batteries, lithium ion batteries and large-capacitance capacitors are being integrated as electrical components, with research being actively performed on application technologies.

Power storage devices, which provide both advantages and drawbacks depending on the type, are naturally used in applications that leverage their strengths. For example, lithium ion batteries – used in a wide range of consumer products, including smartphones, tablets and notebook PCs – feature excellent energy density (amount of charge per unit area). This makes them suitable for use as main batteries for hybrid vehicles and electric vehicles as well. (Figure 2)

Figure 2. Power Storage Device Classes

However, in recent years, supercaps, or EDLCs (Electric Double Layer Capacitors) are emerging as a superior alternative. Although inferior to lithium ion cells in terms of energy density, supercaps feature superior power density (amount of power available per unit time). This results in excellent charge-discharge efficiency, making it possible to momentarily supply large amounts of power.

However, until now, the supercap market has mainly consisted of capacitances on the order of several F, suitable only for compact electronic devices or low-power applications that backup the load supply to CPUs during power loss. But recent advances have allowed higher capacitances to be achieved, in the range from several hundred to even several thousand F, expanding market applicability significantly. (Figure 3)

Figure 3. Large Capacitance Capacitor Market Trends and Projections

* Source: ‘2013 Large Capacitance Capacitor Market Survey Results’ announced by the Yano Research Institute, Inc. on July 3, 2013

Large capacitance capacitors are primarily used as power supply backups during instantaneous voltage drops and during power supply instability in renewable energy systems, as well as for energy recovery in industrial equipment and construction machinery such as cranes. And although far less common, supercaps are starting to see increased adoption in automotive systems due to their superior power density and charge-discharge characteristics compared with other power storage devices. In regenerative braking systems supercaps, which feature good charging efficiency, are used to store energy generated over a short period. This energy is then used to help supply power to the vehicle’s electrical system (that conventionally was provided through engine generation and lead-acid batteries).

Supercaps also feature additional advantages over other power storage devices in terms of longevity, safety and environmental footprint, by the fact that they ensure minimal performance degradation due to repeated charging and discharging, provide low fuming and ignitability, and contain no toxic heavy metals.

Given these benefits, new applications and possibilities are being considered by combining with the advantages of secondary cells such as lithium ion batteries.

3. Cell Balancing Circuit Required for Supercaps

The voltage of a typical supercap cell is normally around 2.5V. For example, when used as backup for a 12V power supply line 5 or 6 cells are connected in series to achieve approximately 12V. Here, a cell balancing control circuit is required to ensure that the voltages of all the cells are uniform. This important because if the cell voltages are not uniform, individual cells may be subjected to high voltages, which can lead to cell degradation. And since supercaps themselves feature long life, achieving balanced cell voltages in this manner is essential for realizing maximum performance.

4. Supercap Cell Balancing IC (BD14000EFV-C)

ROHM’s dedicated cell balancing IC not only integrates cell balancing functionality, but a variety of monitoring functions as well, making it possible to build safer and more reliable EDLC systems.

The primary features are explained below.

1) Integrated cell balancing functions provide high reliability and reduces the number of external parts

One BD14000EFV-C IC can control 4 to 6 supercap cells in series. A simple shunt system is utilized that allows the shunt current level to be set with an external resistor. Plus, built-in MOS switches are turned on/off by the IC itself to maintain uniform cell voltages, allowing cell balancing operation to be easily achieved with a very simple configuration. (Figure 4)

  Figure 4. BD14000EFV-C Block Diagram

In comparison, when constructing a cell balancing circuit using discrete components it is necessary to create a complicated balancing circuit for each cell, requiring a large number of parts. And incorporating an overvoltage detection circuit to improve safety entails additional components, leading to larger mounting area, more complex parts management, and higher costs. Furthermore, the wide variety of parts used make it difficult to ensure reliability due to the inherent variability of individual components.

In contrast, ROHM’s cell balancing IC not only consolidates required parts, but also increases functionality through integration and makes it possible to standardize cell balancing circuits composed of different products with varying cell voltage specifications. As a result parts management is significantly improved. (Figure 5)

Figure 5. Simple design achieved through chip integration

2) Easy Scalability

It is important to set the cell balancing voltage to an optimal value based on the element breakdown voltage, application, charge/discharge frequency, temperature environment, and other factors. The BD14000EFV-C allows the cell balancing voltage to be set between 2.4V and 3.1V by configuring the three VSET pins (0 through 2) to either High or Low, enabling support for a variety of EDLC applications.

A detection voltage accuracy of ±1.0% (max.) is guaranteed at Ta=25°C and ±2.0% (max.) within the operating temperature range of –40°C to 105°C .

It is also possible to connect multiple BD14000EFV-C ICs in series to support higher voltage applications (i.e. backup power supplies, construction equipment). (Figure 6)

Figure 6. Supports a variety of supercaps

3) Safety design with two-stage overvoltage monitoring

Two-stage overvoltage monitoring of supercap cell voltages is possible. The VO_OVL01 pin detects overvoltage at the first stage and outputs a flag to a microcontroller. Similarly, a flag is raised from the VO_OVLO2 pin when overvoltage level is detected at the second stage. This makes it possible for the system to know when degradation of any given cell has progressed, providing an effective indication for cell replacement.

The overvoltage value can be selected from two cell balance voltage patterns: +0.15V/+0.25V for the first stage or +0.3V/+0.5 V for the second stage. This can be set by switching the OVLOSEL pin to High or Low.

4) Built-in self-diagnostic function for monitoring cell balance (patent pending)

The BD14000EFV-C includes a proprietary self-diagnostic-type monitoring function. When the internal shunt switches for all channels are working normally, a flag is output to the VO_OK pin, making it possible to easily confirm proper cell balancing operation.

In contrast, for cell balancing circuits without such a function it is necessary to check whether cell balancing for all channels is operating properly for each cell during the inspection process. This will result in significantly longer inspection time and higher costs.

5) Hysteresis-less design reduces wasteful current consumption

A hysteresis-less comparator is utilized in the cell balancing detection circuit. (Figure 7)

Figure 7. Current Consumption Comparison With/Without Hysteresis Function

In the case of a hysteresis type, even when the release voltage becomes lower than the detection voltage and the cell voltage at the time of detection cancellation decreases below the detection voltage the cell balancing switch will remain on, resulting in unnecessary shunt current consumption. On the other hand, with a hysteresis-less comparator the detection and release voltages are the same, and when the cell voltage during detection cancellation becomes lower than the detection voltage the cell balancing switch turns off, preventing unwanted shunt current consumption. Therefore, ROHM uses a hysteresis-less comparator to ensure high-efficiency cell balancing operation. 

6) Automotive-grade quality

The specifications for BD14000EFV-C comply with the international quality standard AEC-Q100, enabling worry-free use in automotive applications.

5. In Conclusion

The BD14000EFV-C achieves superior levels of reliability and safety by integrating all required functions for supercap cell balancing operation on a single chip. This also translates into reduced design load and development time. Going forward ROHM will continue using its analog design technology, cultivated over many years, to promote wider adoption of more environmentally friendly power storage devices.