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The best of both worlds: combining batteries and supercaps

Rechargeable batteries are practical and reliable for use in electronic systems that offer constant availability. Classic solid electrolyte capacitors provide a more environmentally friendly and cost-neutral alternative, but soon reach their limit with output requirements exceeding 100 mW. Supercap dual-layer capacitors on the other hand offer high power density and long working life, but low dielectric strength. There is a need for electronic systems to compromise between these technologies. A solution that combines the advantages of classic batteries and dual-layer capacitors without the limitations is necessary. Below is a table that presents the basic comparison of the dual-layer capacitor, the typical battery, and the Hybrid ENYCAP capacitor.

ENYCAP 196HVC devices are able to achieve a higher nominal voltage where maximum is at 1.4 V and can also be interconnected in series without special balancing measures. At present, a voltage value of 8.4 V and capacitance values between 4 F and 90 F can be obtained.

Hybrid systems can be considered a practical alternative to the classic battery because of their ability to reach very high energy densities of >13 Ws/g (>3.6 Wh/kg). In addition, Hybrid systems are also distinguished for their very low stray current and self-discharge.

Figure 2 below demonstrates the balance of power for capacitors, batteries and hybrid capacitors. The optimum classification of ENYCAP storage capacitors according to today’s state-of-the-art technology is clearly illustrated in the chart.

As already implied in the examples above, specific parameters must be tuned to the respective application to select the correct component:

  • Backup energy and time interval. Here, the periods between very deep discharging and, in particular, the time up until the first up are particularly critical.
  • Specific peak power and peak current requirements.
  • Output voltage level for the backup solution; in particular, the minimum voltage level.
  • Input voltage range that is available for loading the energy storage device; in particular, the maximum range.
  • Impedance of the primary energy source. For systems with low ESR, extremely high charge current spikes can occur.
  • Protection against complete discharging, short circuits, reverse polarity, overvoltage, and excess temperature.
  • Charging status control of the energy storage device and data-bus-capable signals for advanced power management systems.
  • Cost of the entire system.
  • The parameters above are only a selection of the most important design parameters. To make the complexity clear, the following illustrations (see Figs. 3 and 4) of the principle will show the basic diagrams of charging current, discharge current, and voltage curve over time. The difference in the behavior between hybrid energy storage and classic supercaps can clearly be seen.

Dual-layer capacitor as a backup source.

ENYCAP as a backup source.

The fast charging behavior of dual-layer capacitors can be observed from the voltage plot. This can be considered as a warning that certain precautionary measures must be taken against possible extreme current charge peaks due to the low ESR.

Hybrid dual-layer capacitors on the other hand are characterized by its more gentle charging behavior due to its higher series resistance and optimized construction. . Early on, the nominal voltage on the capacitor is reached. This allows a simpler operation for wake-up switching and low-power sensor applications. The control electronics can certainly be simplified as opposed to dual-layer capacitors. The relatively high initial voltage drop is a disadvantage.

Charging and backup block diagram using an LTC3355.

Provided above  is a reference design of a buck converter utilizing Linear Technology’s LTC3355. The system above integrates an emergency current solution with a charge controller circuit and a backup converter, including the necessary current measurement sensors to evaluate the function as well as all protective functions. ENYCAP, dual-layer capacitors, classic capacitors, and batteries can also be used to  operate this system to realize automatic switching.

The system shown is capable of transforming to the programmed supply voltage for the load and charges the backup energy storage device. Should an the supply voltage be interrupted, the integrated boost converter in the system maintains the output voltage on the load without any interruption until the secondary energy source is exhausted. Moreover, if the capacitor voltage of about 1 V is below minimum, a load switch on the output side turns off the load, thus avoiding complete discharging and constant linear operation of the connected regulators.

LTC3355 performs all functions needed within an intelligent power management system for a server or an IPC. It monitors voltages VIN, VOUT, and VCAP. It also keeps in check the the load performance status of the energy source (CPGOOD), the automatic switchover of the supply to the backup energy storage device (PFOB), and as well as the load regulation of the system.

Evaluation design kit (MAL219699001E3).

ENYCAP 196 HVC series capacitors are specified for normal storage conditions of –40° to +85°C. However, after one year of storage, the parts should be powered and charged. Self-discharge is low. The permanent charge voltage of a fully charged capacitor must be limited to ≤20 µA; otherwise, the product will age early. If higher current flows, it must be ensured that the ENYCAP 196 HVC is removed from the circuit. Complete self-discharge must also be avoided. Most of the energy can be removed from the single cell between 1 V and 1.4 V. Cell voltages under 1 V result in deep discharge. As a result, low discharge currents can already have negative influence on the product’s properties.

When selecting the other components, great care was made to ensure low dc-circuit loss, low temperature factor, and high efficiency inductances to make the available energy as fully usable as possible. Also, input protection circuitry with TVS diodes was implemented to avoid overvoltage events of the complete circuit.

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