The best of both worlds: combining batteries and supercapsHybrid capacitors deliver the power density and energy that today’s electronic systems need. By NORBERT PIEPER, From home automation to industrial robots, our networked world demands that electronic systems offer constant availability, resulting in the need for permanent — often network-independent — energy supplies. Rechargeable batteries are practical and reliable for these systems until they reach the end of their working life, when they must be replaced or thrown away. 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 offer high power density and long working life, but low dielectric strength. Electronic systems require a compromise between these technologies, solutions that combine the advantages of classic batteries and dual-layer capacitors without the limitations. Refer to the Table for comparison among the dual-layer capacitor, the typical battery, and the Hybrid ENYCAP capacitor. Table: Basic comparison of the capacitor and battery systems. The physical single cells of ENYCAP 196HVC devices have a
nominal maximum voltage of 1.4 V and can be interconnected in
series without special balancing measures, thus achieving higher
nominal voltage. Currently, voltages of 8.4 V and capacitance
values between 4 F and 90 F can be obtained. Fig. 1: ENYCAP structural form variations. Hybrid systems can reach very high energy densities of >13
Ws/g (>3.6 Wh/kg) and are, therefore, a viable alternative to
the classic battery. Furthermore, these energy storage
capacitors are characterized by very low stray current and
self-discharge. Fig. 2: Balance of power chart for capacitors, batteries and hybrid capacitors. As already implied in the examples above, specific parameters must be tuned to the respective application to select the correct component:
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. Fig. 3: Dual-layer capacitor as a backup source. Fig. 4: ENYCAP as a backup source. The good charging and, where applicable, fast charging
behavior of dual-layer capacitors can be observed from the
voltage plot. It can be seen that precautionary measures must be
taken against extreme current charge peaks due to the low ESR.
Furthermore, with constant backup power, the linear drop of the
discharge voltage results in a disproportionate increase of the
discharge current and constant back-regulation of the dc/dc
converter as well as high current peaks. In some cases, higher
ripple currents at the capacitor have to be taken into account. Fig. 5: Charging and backup block diagram using an LTC3355. An example is a reference design that 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.
Moreover, this system can be operated using ENYCAP, dual-layer
capacitors, classic capacitors, and batteries, and realizes
automatic switching between primary supply plus charge function
and energy source backup. For this purpose, charging voltage
(1.3 V to 3.2 V), charging current (35 mA to 600 mA),
discharging voltage (>1 V), and discharging current restriction
(100 mA to 5 A) can be varied in several ranges and can easily
be adapted to the customer’s circumstances. The charging process
can use either constant current or constant voltage regulators. Fig. 6: Evaluation design kit (MAL219699001E3). ENYCAP energy storage capacitors have a higher power density
than a battery and higher energy density than classic electric
dual-layer capacitors (EDLC). Therefore, faster charging and
discharging times are possible than those for a battery. We
suggest impulse charging for applications such as backup
systems. The idea behind this is that the ENYCAP is initially
charged up to the maximum of its energy storage capacity and
then the charge status is maintained in a trickle mode by
individual electrical impulses. This compensates for
self-discharge and the capacitor is kept under ideal operating
conditions. If charging were to take place constantly and only
be switched off when the supply is interrupted, the working life
of the product would be reduced dramatically. The maximum charge
voltage should be set somewhere between 2% and 3% higher than
the specified nominal voltage (rated voltage). This compensates
for the voltage drops at the product’s internal resistor. Cell
voltage is always lower than charge voltage. It is very
important that the energy storage capacitors are loaded up to
full nominal voltage. Higher voltages should not be used. It
must be taken into account that the product of charge
current*charge time is equal to the maximum charge. Only then is
the maximum degree of effectiveness and the highest possible
energy storage achieved. Learn more about Vishay Intertechnology |