Smart Power Bank: Integrating Solar PV, DC Input, and Li-Ion Battery Backup

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Figure 1

   

摘要

A smart power bank charger has been designed using Analog Devices’ products. It features a flexible setup capable of accepting multiple input power sources and intelligently managing battery charging while supplying power to the load. The new design consolidates key functionalities into a compact form factor, making it more suitable for commercial applications while maintaining robust performance and an intelligent power management system.

Introduction

The rise in demand for portable electronics has fueled the need for effective and portable power management systems. Power banks have become indispensable accessories because they offer dependable backup power for smartphones, tablets, and other USB-powered gadgets. Using evaluation demo boards, we first created a modular power bank charger solution as a proof-of-concept. This prototype was assembled by stacking demo boards, and it later evolved into a single-board solution with enhanced performance and improvements. This solution accepts multiple input sources—such as battery, solar, or DC adapters—and intelligently manages the power flow to charge the battery and simultaneously powers the load.

This article aims to explore the intelligent power path management features of Analog Devices’ ICs in a compact design without compromising performance. It outlines the single board solution design considerations, concept, and performance evaluation, and highlights its improvement from the proof-of-concept multiboard.

Design Block Layout

In this layout design, a compact and streamlined architecture has been developed to support dual wide-range input voltages—namely, from a solar panel and an AC-to-DC adapter. Power input is intelligently managed using the LTC4416 power path controller in conjunction with the LTC4162-L power path buck charger. This configuration enables efficient charging of various Li-ion batteries up to 4S1P stack battery configurations.

As illustrated in Figure 1, the system has a buck-boost switching regulator, LTC3115-1, that dynamically regulates the output voltage to the load and ensures a constant maximum output of 5 V and 2 A, as the LTC4162-L monitors the battery’s charge level.

Part Selection and Design Layout

The three main parts optimize the system performance based on the design block setup. These parts were selected to improve system efficiency, minimize power loss, reduce PCB layout space, and reduce overall cost. Their schematic layout is shown in Figure 2.

1. Dual Input Sources Using the LTC4416

A simple OR-gate configuration, using diodes, can be employed to switch between dual input power sources. However, this approach introduces significant power loss due to the inherent forward voltage drop across the diodes—even when using low drop Schottky diodes. The LTC4416 creates seamless switching between two sources with a very low voltage drop, thereby reducing the power loss. By controlling the external P-channel MOSFETs to emulate ideal diodes, this device significantly reduces conduction losses, thereby improving overall system efficiency and reliability.

The LTC4416 operates in six different modes. Each mode of operation is dependent upon the configuration of the E1 and E2 input pins, as stated in the data sheet. In this setup, the selected mode is: V1 is greater than V2, where E1 is set to Sense and E2 is set to 0. This means that the chip gives priority to the V1 power source. Using this mode of operation, the IC is configured in such a way that V1 is prioritized to accept a wide input voltage range of 15 V to 35 V DC power supply while the V2 power supply is a solar panel source (3.6 V to 15 V) acting as a secondary voltage. When V1 is greater than or equal to 15 V, E1 enables the V1 source to be the primary voltage supply and switches off the V2 supply since V1 is greater than V2.

When V1 drops to 13.4 V, V2 becomes the main power supply while V1 is disconnected from the output. Provided that the voltage from the solar panel is within 3.6 V to 15 V, V2 will continue to supply power to the output load until V1 is restored. The restoration point of V1 is set to 15 V, as shown in Figure 2.

The fail and restoration point of V1 can be modified by changing the resistor values of R1, R2, and R3 in Figure 2. This can be done by using the formula from the data sheet as given:

Once V1 has been identified, V2 can be chosen to guarantee the best configuration. If V1 fails or becomes unavailable, the system automatically switches to V2 to maintain the power supply until the restoration point is reached, as long as V1 > V2. Since the output supply follows the higher voltage source, restoration won’t happen if V2 > V1.

2. Intelligent Power Path Management

In applications such as power banks and other devices that require simultaneous usage and charging of the battery, implementing power path charging is an ideal solution. This approach helps optimize battery performance and extend its overall lifecycle by efficiently managing power distribution between the system and the battery. The system intelligently manages the power input by selecting from three sources: the AC-to-DC adapter, the solar panel, or the battery. The AC-to-DC adapter or solar panel is primarily used to charge the battery.

If the AC-to-DC adapter fails and the solar panel voltage drops below the minimum value, the system automatically switches to the charged backup battery to supply power to the load. The output from the LTC4416 power path feeds into the LTC4162-L, which supports input voltages up to a maximum of 35 V.

The LTC4162-L supports immediate operation even with a discharged or absent battery and features integrated maximum power point tracking (MPPT) to enhance solar energy conversion efficiency.

Under bright sunlight, the solar panel operates in two regions: low impedance at constant voltage and high impedance at constant current. This behavior ensures that the device’s control loop remains stable when operating at lower impedance (for example, the higher voltage region). However, as the IC uses input voltage to find the MPPT, the solar panel voltage drops due to higher impedance (for example, the lower voltage region), which makes the control loop become unstable. In the design, the solar panel input is operating at high impedance (<12 V). To address this, the R-C network (R4 and C1), as shown in Figure 2, is used to correct the instability of the control loop, especially under varying sunlight conditions. For low wattage solar panels, a higher capacitance value for C1 (ranging from 100 μF to 1000 μF) is recommended to ensure robust performance of the MPPT.

3. Backup Lithium-Ion Battery

The LTC4162 battery charger supports configurations of up to eight series-connected (8S) lithium-ion cells and is available in multiple variants optimized for different battery chemistries: LTC4162-L for lithium-ion, LTC4162-F for lithium iron phosphate (LiFePO₄), and LTC4162-S for lead-acid batteries. In this design, we have implemented support for up to 4S configurations (1S through 4S) of stacked lithium-ion cells as shown in Table 1.

This configuration is defined using the CELLS1 and CELLS0 pins, following the mapping guidelines provided in Table 1.

4. Switching Regulator

The output of the LTC4162-L is then regulated via a synchronous buck-boost switching regulator. The LTC3115-1 is a high efficiency, monolithic synchronous buck-boost DC-to-DC converter designed for applications requiring a wide input voltage range and low noise. It operates from 2.7 V to 40 V and can deliver up to 2 A continuous current. This switching regulator also features programmable output voltage, seamless transition between buck and boost modes, and robust protection features, making it suitable for industrial and battery-powered applications.

It was selected for this design due to its excellent efficiency and low noise operation. The converter can supply up to 2 A when the input voltage exceeds 6 V, and 1 A for voltages above 3.6 V, making it highly adaptable to varying power conditions. For all the battery configurations (1S, 2S, 3S, 4S), an undervoltage lockout (UVLO) was configured via the connector, as shown in Figure 2 (J5 connector).

5. USB Type-C Output

The output was configured with a USB Type C in non-power delivery (PD) mode to charge any portable devices that require a regulated 5 V output with up to 2 A current. Table 2 describes the choice of resistor values for different current sources for the USB port.

Single Board Performance

The board was specifically designed as a 4-layer PCB to ensure stable, noise-free, and efficient operation as shown in Figure 3. The layout follows a SIG/Power – GND – GND – SIG/Power stack-up configuration, and the recommendations provided in the data sheet for component placement of each part. The board receives power from two inputs, V1 and V2, which are used to charge the battery and power the load. If the primary power source fails, the solar panel takes over during high sunlight intensity and provides power to the load while charging the battery. At night or when sunlight intensity is weak and the solar panel voltage drops, the system automatically detects this and switches to battery power to keep the load running.

Assuming a 1S battery configuration, if the battery voltage drops below 3.3 V, the LTC3115-1 will automatically shut down to protect the battery by activating the UVLO feature. This mechanism helps prevent deep discharge, which can damage the battery or reduce its lifespan. The UVLO threshold can be fine-tuned for each battery configuration by changing resistor values R7, R19, R27, and R21. That can be achieved using the UVLO formulafrom the LTC3115-1 data sheet. This will allow the minimum voltage limit to be set as low as 3.0 V depending on the application requirements.

To safeguard the circuit against incorrect battery connections, reverse polarity protection is implemented using a diode (D3) and a fuse (FUSE1). Additionally, the input is further protected from reverse voltage scenarios by the body diodes of MOSFETs Q1, Q4, and Q3, which act as a barrier against unintended current flow.

The system’s dynamic behavior under varying load conditions is illustrated through its step response and transient response characteristics as shown in Figure 4, showing the performance of the control loop and the effectiveness of the applied compensation network across a range of operating conditions. Figure 5 illustrates the priority switching behavior of the LTC4416 output, as V1 decreases from a higher voltage to 15 V. The device’s output seamlessly transitions to V2 at 8 V, ensuring the output load remains unaffected by the change in voltage. Meanwhile, the V1 restoration point is set to 16.8 V.

Comparison of the Stacked Demo Boards with a Single Board

This section provides a comprehensive comparison between the prototyped multidemo board setup and the newly developed single board solution. In the prototype, the design utilized three separate demo boards: the LTC4416 for ideal diode power path control, the LTC4162-L for battery charging and power management, and the CN0509 USB charger board. The CN0509 was particularly notable for its wide input voltage range of 5 V to 100 V and its ability to deliver a regulated 5 V output at up to 2 A. It combines the LTC7103 buck converter and the LT8302 isolated flyback converter to offer galvanic isolation between input and output.

In contrast, the single board consolidates these functionalities by replacing the LTC7103, and LT8302 with a single component—the LTC3115-1. This transition was aimed at enhancing overall system performance, with improvements in efficiency, reduction in physical size, and a more cost-effective bill of materials. While certain features like isolated output were sacrificed, the trade-off resulted in a more streamlined and practical design suitable for scalable applications.

Physical Board Size

The implementation of a single board solution significantly streamlines the overall system design by reducing the bill of materials (BOM) count by approximately 30% and its size as shown in Figure 6.

Moreover, the compact nature of the single board solution contributes to a more efficient power system architecture. By integrating multiple functionalities into a unified platform, the design becomes more space-efficient, allowing for smaller form factors without compromising performance. This is particularly beneficial in applications where space constraints are critical, such as portable electronics.

Efficiency

One of the most impactful improvements of this board is its ability to deliver power at high efficiency. Optimized power delivery reduces energy losses, which, in turn, supports longer operational times and improved thermal performance. The high efficiency is valuable in battery-powered devices, where conserving energy directly translates to extended battery life cycles. By minimizing power wastage and maximizing energy utilization, the single board solution plays a crucial role in enhancing the system performance. Figure 7 shows that the single board solution performance reaches a peak efficiency of 92.94% at 8 V input and 91% at 10 V. In comparison, the prototype stacked demo board only reached 73.79% peak efficiency at 10 V input. The low efficiency of the prototype stacked demo board is clearly due to the energy losses in the cables used to connect the multiple boards, as well as the losses in the flyback converter section.

When both input sources fail, the battery automatically powers the load. Using a 2S battery configuration with a nominal voltage of 7.4 V, the single board solution achieves a peak efficiency of 94.52%, compared to 77.12% for the prototype stacked demo board. This indicates that the single board design conserves battery power more effectively during system operation as shown in Figure 7.

From the single board optimized solution, the maximum output current of 2 A is achieved from 6 V input voltage, while in the previous prototyped board the maximum current of 2 A is achieved from 12 V, as shown in Figure 7.

Conclusion

A compact and integrated single board power bank solution has been developed utilizing ADI components. This refined design features a streamlined layout that enhances overall efficiency and reduces the physical footprint. The architecture is versatile and adaptable, making it suitable for a wide range of applications involving battery-powered devices. It supports intelligent power path management, which will prolong the battery lifespan.

This concept can be used in embedded automotive systems to combine photovoltaic input with other power supply sources and battery backup in large scale production. In such configurations, high power demand ADI products like the LTC4020 can be used.

Reference

Diarmuid, Carey. “How to Prototype a Power Bank Charger Without Building Any Dedicated Hardware.” Analog Dialogue, February 2023.

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