UPS designs with SuperCapacitor

2019-07-10 [15:25]
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SuperCapacitor can solve the problems in designing UPS(Uninterruptible Power Source).

As all systems become more intelligent, Always-on Devices are becoming normal. Disconnecting or removing power from a circuit, such as removing power supply or system battery, results in a power outage. At the time, the system makes desperate efforts up to a millionth of a second (microsecond) to back up critical data.

Supercap or supercapacitor(EDLC, electric double layer capacitor), which functions similar to insurance service, contributes to preventing failures by providing temporary backup power over a short period of time. Some features of the supercapacitor(ultracapacitor) may constrain smooth recovery without the help of the power converter.

Ultracapacitors(EDLC) provide more capacitance in the same range. Lithium-ion batteries are sufficient as a rechargeable backup power source, but they are heavy, take a long time to recharge and have a short lifetime. It also requires special circuits or algorithms for charging and discharging, which are costly for short-term power backups.

This article will explain the function of supercapacitors and introduce how supercapacitor is actually used as a charging and discharging solution in power backup applications.

Supercapacitors are high-capacity capacitors that are available at lower rated voltage ranges and have very high capacitance values. High energy density, low DC Equivalent Series Resistance (DC-ESR), and linear voltage charging/discharge compared to current.

Comparing supercapacitor with standard capacitors and batteries, Supercapacitor capacitance is hundreds of times higher than the capacitors, which makes it easier to store large amounts of energy.

Energy storage means the amount of energy a capacitor or battery can store. The energy storage density is usually expressed in mWh/g. The mid-level energy storage density supercapacitor can be used as a short-term power in many applications.

The energy storage of an EDLC (Electrical Double Layer Capacitor) or supercapacitor corresponds to the mid-range of the battery and capacitors (such as tantalum, ceramics, film, silicon, electrolyte, etc).
Figure 1. Comparison between Supercapacitors, Tantalum capacitors and Batteries
Capacitors or battery ESR measurements can occur near DC and at high frequencies such as 100 kHz. Typically the required ESR value of capacitors has a test frequency on the higher side. The DC-ESR approximation is important for most supercapacitors and battery applications. This is because the charging/discharge current is usually near a DC event. The ESR generates a low voltage error with the charging/discharge current of the supercapacitor.

Both of batteries and supercapacitors store electrical energy.
Widely used batteries have a higher energy density. However, the high-power density supercapacitor is capable of fast charging/discharging. Supercapacitors do enough work to narrow the gap between standard capacitors and batteries, and can also be used as appropriate temporary backup power because of their high capacitance and medium energy density.

Supercapacitors only can deliver a limited amount of energy to circuits in case of power interruptions when they are supplied with sufficient charge. Consider portable applications that batteries are the main source of power and supercapacitors are the backup.

In Figure 2, time is the most important element in supercapacitors because it is limited. If the supercapacitor is selected correctly, the circuit's power is maintained for a long time so that the backup recovery operation can take place quickly. Figure 3 shows a typical timing diagram for the block diagram in Figure 2.   

Figure 2. When the battery is removed, the circuit supports the backup of the millisecond system by providing sufficient current using a supercapacitor.

In Figure 3, the SuperCapacitor (CSC) temporarily collects charge insurance while the battery voltage level is reached with the insertion of the battery into the circuit. Less than 1% of battery charge storage is lost.

The system is powered by battery voltage (VBAT) and current (IB_SYS). When the supercap voltage (VSC) reaches the battery voltage (VBAT), the supercap (CSC) enters the period of rest and becomes ISC=0 to maintain the charge.

The system will keep the supercapacitor in a state of rest throughout the operation of the device until the battery is removed. Depending on the device's power usage, the pause time can range from days to months or years.

Figure 3. When a battery is installed, the supercapacitor is charged and the system battery is removed, it supports limited power.

When the battery is removed, a system backup begins. During this short period of time, the SuperCapacitor (CSC) additionally guarantees the system current (ISC) and supercapacitor voltage source (VSC) for rapid backup recovery. This connection continues until the system IC begins to collapse by reaching the minimum power voltage.

Supercapacitors temporarily maintain power voltage, but there are also some disadvantages such as unused energy and maximum voltage limits. The system minimum power supply leaves unused energy in the supercapacitor, for example, if VBAT=3.3V and VSYS(MIN)=2.7V, 66% of the available energy is left.

Currently, the average voltage for supercapacitors is between 2.5 and 2.7 volts. This voltage limits the type of system available if the supercap is not stacked or placed in series.

For example, the two laminated 2.7V supercapacitors could provide 5.4V. However, the total capacitance of the serial capacitor decreases by CSC = (C1 x C2) / (C1 + C2), which is C1=C2. The total capacitance shall be 50% less than C1 or C2 and therefore C1 and C2 shall be twice the supercapacitors value of the design.

The use of supercapacitor increases the PC Board layout geometry and application costs. This is because size of two supercapacitors increases by four times compared to one. This is because the two large capacitors (C1 & C2) are twice the size of a single small (CSC) capacitor. In the case of laminated EDLC, the serial capacitor needs a cell balancing circuit to eliminate the allowable deviation of capacitance, resistance and leakage current.
Source : ITBIZNEWS 2019.07.05 & Wrote by Bonnie Baker, Technical Editor
Summarized by Klaus Hyeon(Klaushyeon@vina.co.kr)


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