Solar Battery Charger in CMOS 0.25 um Technology

Solar Battery Charger in CMOS 0.25 um Technology

Tao Wang*, Chang-Ching Huang, and Tian-Jen Wang

Department of Electronic Engineering, Chang Gung University, Taiwan

(Received 26 December 2013; Accepted 3 March 2014; Published on line 1 September 2014)
*Corresponding author:
DOI: 10.5875/ausmt.v2i2.107

Abstract: A solar cell powered Li-ion battery charger in CMOS 0.25um is proposed. The solar battery charger consists of a DC/DC boost converter and a battery charger. The voltage generated by a solar cell is up converted from 0.65V to 1.8V, which is used as the VDD of the battery charger. In this way, the solar battery charger automatically converts solar energy to electricity and stores it directly to a Li-ion rechargeable battery. In this system, a super capacitor is needed as a charge buffer between the boost converter and the charger.

Keywords: solar energy; battery charger; super-capacitor


Concerns for peak oil and climate change have focused global attention on renewable energy resources. Solar energy is widely seen as an promising future solution because of its high energy intensity, which reach 15mW/cm2 [1, 2]. However, this energy density is dependent on stable and fair weather conditions. In real world conditions, solar energy extraction can be quite unstable and can only be generated during daylight hours. Thus, maximizing solar energy potential requires methods for efficient transduction and storage.

Power consumption of CMOS integrated circuits has been reduced to milliwatts, thus CMOS ICs can be powered by button-sized Li-ion rechargeable batteries. Therefore, a battery charger able to convert energy from a solar cell to a Li-ion battery would be very useful [3]. Figure 1 illustrates the architecture of the proposed charger. A solar battery charger stores energy directly to the battery without an external control system. The energy is first stored in a super capacitor, which is used as an electricity buffer. When the super capacitor has stored enough energy, the charger starts to function and begins energy conversion.

To topology of a small-size solar battery charger is quite different from that of a conventional battery charger which uses a complex control circuitry to meet the accuracy requirements of the battery charging procedure. The disadvantages of this complexity are accentuated as the size of solar cell shrinks because the expense in power and area becomes non-negligible in a low power system, thus reducing the charger’s efficiency. As a result, a simpler topology is preferable for a small-size solar-energy powered charger.

Another issue raised by small battery chargers lies in the limited power available from the solar cell. For a solar cell with a large light receiving area, the power available from the cell is high enough to continuously supply the battery charger. However, when the size of solar cell shrinks to a size suitable for use in a portable device, the amount of power the solar cell can acquire is too limited to maintain continuous charging and battery charging will be easily interrupted. The battery charger switching on and off frequently resulting in increased dynamic power consumption. The following section discusses adding a capacitor between the solar cell and battery charger help to reduce this interruption effect.

Solar Battery Charger Design

DC/DC Boost Converter

Figure 2 shows the schematic of the DC/DC boost converter in the proposed system. A diode-connected mosfet M1 is connected to inductor LD , RL, CL and switch M2 [4]. Switch M2 is controlled to a clock. When M2 shorts, the current passes from the solar cell to the ground through M2. At this time, energy is stored in the inductor as current. When M2 suddenly opens, the conducting current in LD cannot respond in time, so the current is forced to flow into the diode-connected mosfet M1. As a result, the charge is stored in Cload. In steady state, the output voltage of this converter is constant as shown in Figure 3. By adjusting the clock frequency and capacitance value CL , one can control the final voltage of this stage.

The switching signal of the DC/DC boost converter is generated from an inverter-based ring oscillator followed by an inverter-chain buffer. Note that the DC/DC boost converter is supplied by the solar cell, providing a voltage as low as 0.65V. The loading of M2 can be too large for the clock generator, thus a buffer is needed between the clock generator and the boosting stage.

Battery Charger

Li-ion battery offer high electricity capacity but this advantage is based on the assumption that the charging procedure is precisely controlled. An ideal Li-ion battery charging procedure is shown in Figure 4. The process entails three charging modes: trickle charging mode (TC), constant current mode (CC), and constant voltage mode (CV).

When battery voltage (VBAT in Figure 4) is low, the inner resistance of battery is large. During this period, a large charging current (IC in Figure 4) will cause thermal damage to the electrodes and shorten the battery lifetime. Therefore, a mode with a small charging current, namely trickle charging mode, is required as illustrated in Figure 4. Once the battery voltage reaches a certain value, the inner resistance is reduced, thus the charge current can maximized to shorten the charging time. The charging mode with the maximum charging current is defined as the constant current mode (CC mode). As shown in Figure 4, the battery voltage rises rapidly due to the high charging current in CC mode. Finally, when the battery voltage approaches a nominal voltage set by a reference voltage Vref, the charger starts to enter the constant voltage CV mode. In CV mode, the charging current drops dramatically, and VBAT will eventually be locked with Vref.

A conventional charger uses a microprocessor as a control unit to manage the three charging modes. However, our proposed system uses analog circuits instead [5]. The block diagram of battery charger is shown in Figure 5. It comprises a low voltage battery VBAT detector, a current driver, an amplifier and a reference voltage (V-Ref). The low VBAT detector is adopted to sense the battery voltage VBAT for TC to CC mode switching, while the CC to CV mode switching is accomplished by the nonlinearity of the amplifier (Amp in Figure 5). Figures 6(a)-(d) show schematics of the circuit blocks in this solar charger.

Note that this analog circuit makes use of the saturation and linear regions of an amplifier as shown in Figure 7. In the beginning of the CC mode, the amplifier operates in the saturation region, in which the output current of this amplifier is constant, leading to a constant charging current as needed by CC mode. As VBAT approaches Vref , the amplifier enters the linear region and its gain rises, forming a negative feedback loop that locks VBAT with Vref. Through this step the charger moves from CC mode to CV mode.

To maximize power efficiency, it is better to bias the transistors in sub-threshold region. The transconductance over bias current (gm/ID) versus VGS is shown in Figure 8. It is obvious that one can have a higher gm in sub-threshold region with power constraints. In the battery charger design, the battery is simply modeled by a capacitor. The simulated time response is shown in Figure 9. Three modes are found to exist in this transient response. However, different curves of the CC mode between Figures 4 and 9 can be observed because the battery model is simplified by a small capacitor for faster simulation results, whereas an enormous capacitor would be needed to accurately model the battery, but this would result in unacceptably long simulation times.

System Simulation and Preliminary Measurement

The DC/DC boost converter is connected to battery charger for system simulation as shown in Figure 10.

In this system, the capacitance (CB) between the boost converter and battery charger is essential since it is used to save sufficient charge to keep the battery charger functional. It is well known that if CB is small (e.g. 0.4pF), the output voltage can be easily boosted to a high voltage. However, the charge stored in CB will be insufficient to sustain the battery charger, because the charge stored in CB is quickly drained. As illustrated in Figure 11, the battery charger is periodically turned on/off because of a lack of charge. To solve this problem, we use a super-capacitor to store enough charge before turning the charger on. Figure 12 shows the time response when a 40 mF super capacitor is used. With a large capacitor, the battery charger keeps charging without interruption. The drawback is that it requires more time to first charge the super-capacitor.

The stand-alone DC/DC boost converter and battery charger have been taped out in CMOS 0.25µm technology, with the layout shown in Figure 13. Integration of both circuits are also to be taped out to CMOS 0.18 µm technology as shown in Figure 14.

Measurement results of the DC/DC boost converter in 0.25µm technology is done by loading the output with a resistor with 1MΩ as shown in Figure 15 . The relationship between the output voltage and clock frequency is measured and shown in Figure 16, where the input DC is 0.65V. A clock frequency needed to obtain the maximum output DC voltage can be observed. When the switching frequency is low, charge pumping time will be reduced, leading to a lower averaged dc voltage output. On the other hand, if the clock frequency is too high, the MOS switch cannot catch up to the clock speed, thus reducing dc voltage output.


A solar battery charger consisting of a DC/DC boost converter and an analog battery charger is proposed for an SOC powered by a rechargeable battery. The proposed system is able to charge a 1.8V Li-ion battery from a 0.65V solar cell. This system requires a super capacitor, and future work will focus on refining the integration.


This work was supported by the National Science Council of Taiwan (NSC 102-2221-E-182-025). The authors are grateful to TSMC and CIC for chip fabrication and tapeout support, respectively.


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