Power Bank - Portable Battery Charger By: Ganesh Raja and Pushek Madaan
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Power Bank – Portable Battery Charger By: Ganesh Raja and Pushek Madaan With an increasing number of features, mobile devices like smart phones tablets etc are requiring more power. To extend operating life, manufacturers try to use bigger batteries but are limited because of the weight and size. To provide users additional power when their devices are running low, many users are learning to rely upon power banks, a portable energy source that can be carried in a pocket or backpack. Power banks store energy in an internal Lithium Ion battery and can charge the mobile devices. In this article, we will discuss the basics of a power bank, various specifications and features and how it can be implemented using a system-on-chip. Figure-1 shows the various parts of a power bank. The power bank has a switch to turn on the output and charge a mobile device. The status LEDs indicate the amount of charge left in the power bank and also indicate charging/discharging operation. A Micro-USB connector is used to connect the charging source to charge the internal battery. The power bank can either be connected to a PC/Laptop or to a wall power source to charge the battery. A USB-A connector is used to connect the mobile device for charging. Figure 1: Power Bank Figure-2 shows the high level block diagram of the power bank. The main component of the power bank is a Lithium Ion battery that stores energy to charge the mobile devices. The battery is first connected to a battery protection circuit that continuously monitors the battery voltage and current and disconnects the battery in case of various events like over charge, over discharge, over load and short circuit.
POWER BANK STATUS LEDs VLOAD / ILOAD SWITCH CONTROLLER ON/OFF VBAT IBAT MicroUSB CHARGING USB-A Wall power or CIRCUIT BATTERY PROTECTION BOOST Load Laptop USB BUCK CIRCUIT CONVERTER CONVERTER BATTERY Figure 2: Block diagram of Power Bank The input voltage from the Micro-USB connector is fed to a charge controller. This usually is a buck converter that converts the 5V input voltage into either a constant current or constant voltage to charge the Lithium Ion battery. The boost converter converts the battery voltage – which can range from 3.0V to 4.2V – to 5V which is used to charge the external mobile devices. A microcontroller performs control functions like turning on the boost converter when the switch is pressed, measuring the battery voltage and controlling the LEDs to indicate state of charge and turn off the boost converter when the external mobile device stops drawing current. Now let us take a look at each of these blocks. Charge Controller The charge controller implements the charging algorithm to charge the internal Lithium Ion battery. It is usually based on a buck converter topology. The charging process passes through several stages to ensure battery is charged to its full capacity; but at the same time the battery is charged safely. During these stages, the battery has to be charged using either a Constant Current (CC mode) or a Constant Voltage (CV mode). Figure-3 shows the various stages involved in charging a Lithium Ion battery.
Figure 3: CC-CV battery charging profile Battery Protection Circuit Lithium ion batteries are extremely hazardous if not handled properly. When the voltage of the battery is allowed to rise beyond 4.2V, the battery gets hot. There have been numerous incidents where lithium ion batteries have caught fire because of poor protection logic. Thus, from safety point of view, battery protection becomes extremely important. The battery protection circuit implements some or all of the following protections. Over charge protection – Disables charging when battery voltage goes above a safe threshold Over discharge protection – Disables discharging when battery voltages goes below a threshold Short circuit protection – Disables discharging if a short circuit is detected Temperature protection – Disables charging and discharging if the battery temperature goes above or below pre-determined thresholds. Boost Converter Lithium ion battery operating voltage typically ranges between 3.0V to 4.2V and almost all of the mobile phones available need 5V for the charging. The boost converter steps up the 3.0V to 4.2V battery voltage to 5V required to charge the mobile devices. Controller The controller in a typical power bank performs basic house-keeping tasks. Some of the tasks performed by controller in a power bank are as follows: Detect if the switch has been pressed and enable output of the boost converter.
Measure the current drawn by the load and turn off the boost converter when the load stops drawing current. Measure the battery voltage and drive the status LEDs to display battery level during charging and discharging. In the following section, we’ll have a look at how to implement power bank using SoC (System On Chip) but before getting into details of implementation, let us have a quick look of some of the advantages offered by SoCs over conventional microcontrollers. SoCs enable integration by providing peripherals like PWMs, Timers, ADCs, Comparators, Amplifiers and many more all within one device. One such very popular SoC available is the PSoC 1 from Cypress Semiconductors. This device provides configurable digital and analog blocks which can be used to implement most of the digital and analog functionalities. Using such SoCs, design of a complicated system like power bank becomes extremely simple. These SoCs can integrate all the smart functions within the controller, leaving only discrete components like MOSFETs, Inductors, Diodes and Capacitors off the chip, thus giving high control over system performance and BOM reduction. We’ll now have a look at detailed implementation of each block discussed in the previous part. SoC Implementation Charger As discussed in the earlier section, a charger should implement both constant current (CC) and constant voltage (CV) control. To implement the Li-Ion charging profile, following functional blocks are required. 1. A switching regulator that can control either the output current or output voltage 2. Battery parameter (Voltage, current, temperature) measuring circuit 3. Charging algorithm (for implementing CC-CV profile) This is shown in block diagram below (figure-4): Switching Regulator Supply charging current + Charging Battery T Algorithm parameter measurement - Battery Pack Figure 4: Block diagram of a Li-ion battery charger
Figure-5 shows the switching regulator implemented using a buck converter topology. The buck converter is formed by the MOSFET T, Inductor L, Diode D and Capacitor C. R1 and R2 form a potential divider to measure the battery voltage and RS is the shunt that is used to measure the battery current. The output of the switching regulator is controlled by the duty cycle of the PWM. To implement constant current output, the PWM duty cycle is controlled based on the current measured through RS. To implement constant voltage control, the PWM duty cycle is controlled based on the voltage measured through R1 and R2. L VSupply T R1 Li-Ion Battery PWM Gate Driver Voltage R2 Feedback D C Current Rs Feedback Figure 5: Switching buck regulator topology Battery parameters measurement circuit: Figure-6 shows the circuit to measure all the battery parameters viz., voltage, current and temperature. R1 and R2 provide the battery voltage and RS provides the battery current. The battery temperature is measured using a thermistor RT. Rref and RT form a potential divider. With Rref being constant, the value of RT can be calculated by measuring the voltage from this potential divider. Once the value of RT is calculated, the temperature can be easily calculated either by using a lookup table method, or by using Steinhart-Hart method. Each of these signals requires a different gain. SoC’s being flexible, make it possible to change the gain of the amplifier during run time. Once the battery voltage, current and temperature are measured, the CPU implements the charging algorithm to either control the output current or output voltage of the switching regulator by controlling the duty cycle of the PWM.
Current control circuit VSupply SoC PWM R1 Voltage Li-Ion Battery Temperature CPU ADC PGA R2 Vbias Vss Rref RT Current Rs Figure 6: Measurement using single ended ADC Charging algorithm: The CPU implements the charging algorithm by reading the battery voltage, current and temperature and implements the following charging profile: 1. During main charging, the CPU measures the battery current and adjusts the PWM duty cycle to keep the current constant. At the same time, the CPU monitors the battery voltage to determine if the battery has reached the full charge threshold. 2. Once the battery reaches the full charge threshold, the CPU measures the battery voltage and adjusts the PWM duty cycle to keep the battery voltage constant. At the same time, the CPU monitors the battery current and terminates the charging when the current goes below the termination current threshold. The constant voltage or constant current control can be implemented using any of the digital control methods like P, PI or PID, depending on the system requirements and the available CPU bandwidth. ADC and PWM parameters: Some of the parameters that need to be considered during the design are the ADC resolution, ADC accuracy and PWM resolution. ADC resolution defines how precisely you can measure the feedback signals. This in turn will have an impact on the ripple on the battery current and battery voltage. ADC accuracy defines how accurately you measure the feedback signals. For Li-Ion batteries, the battery voltage measurement is very critical. The maximum tolerance allowed for the full charge threshold is 4.2V + 50mV. This will require an ADC accuracy of at least 0.5%.
PWM resolution defines how precisely you can control the output voltage or current and this also affects the ripple on the output voltage or current. Higher the resolution, lower the ripple. However, there is a limit to how high the PWM resolution can be. Higher the resolution, lower is the output frequency of the PWM which results in bigger inductors and capacitors. As power banks get more and more compact, size is an important design issue. So, a tradeoff has to be made between the size and PWM resolution. Charging Source Detection Auto detection of charging source – It is possible to charge the internal Lithium ion battery of the power bank through various sources like PC / Laptop or wall powered adaptors. Before we dive further on this, let us have a look at various charging sources as defined in Battery Charging Specifications BC1.2 SDP (Standard Downstream Port): This is the regular USB port present in desktop PCs and laptops. It can provide a maximum current of 500mA with enumeration and 100mA without enumeration. If a device tries to draw more than 500mA of current, this could lead to BSoD(Blue Screen of Death) or port failure. CDP (Charging Downstream Port): This is a special USB port present in some PCs. This USB port can provide a current of upto 1.5A to peripherals. DCP (Dedicated Charging Port): This is an USB port meant only for charging and does not perform any communication functions. This port can supply upto 1.5A of current to a charging device. It is important to distinguish between the type of charging source the power bank has been connected and draw the appropriate power. BC1.2 spec defines a method to detect the type of charging source (SDP, DCP or CDP) that the power bank is connected to. I. Primary Detection: Connect 0.6V on D+ line and measure signal on D- line. If signal on D- line is less than 0.4V, then we are connected to SDP. If the signal is greater than 0.4V then we are connected to either CDP or DCP and we need to perform secondary detection to determine between CDP or DCP. II. Secondary Detection: Connect 0.6V on D- line and measure signal on D+ line, if signal on D+ line is greater than 0.4V then it is DCP else it is CDP. The above logic may be implemented using the DACs and the ADC in SoCs. Figure-7 provides the block diagram for charging source detection. Two DACs are used to generate the 0.6V bias for the D+ and D- lines and the voltage on the other line is measured by the ADC. Once the type of charging source is detected, the maximum charging current drawn by the charger section can be set.
VSupply DAC D- SoC VBus DAC D+ D- D+ Gnd PGA ADC CPU Figure 7: Charging source detection Boost Converter The Boost Converter converts the battery voltage to a constant 5V output to charge the external mobile device. As the efficiency of the boost converter is required to be >85% under full load condition, a synchronous boost converter is preferred in power banks. Figure-8 shows a synchronous boost topology. A brief description of the boost converter is given below. L D T2 R1 Li-ion Load Battery T1 C PWM1 Gate Driver PWM2 Gate Driver Rs1 Current Voltage Rs2 Feedback R2 Feedback Figure 8: Synchronous boost converter topology PWM1 and PWM2 are out of phase signals that drive the low side and high side MOSFETs of the boost converter. When T1 is on, current flows through the inductor L1 and the inductor stores energy. When T1 is turned off (and T2 is turned on), the current through the inductor collapses, which in turn produces a back emf across the inductor. This back emf in series with the battery voltage presents a higher voltage at the drain of T1. At this state, T2 is on and charges the capacitor with the higher voltage. By controlling the duty cycle of T1, the amount of voltage delivered to the capacitor, and hence the output voltage can be controlled. The CPU measures the output voltage through potential divider R1/R2 and adjusts the duty cycle of the PWM to regulate the output voltage.
Boost Converter feedback control: Figure-9 shows the feedback control circuit for the boost converter. The CPU measures both the voltage and load current of the boost converter. Output voltage is measured through potential divider R1/R2. The CPU implements a PI or PID control loop and adjusts the duty cycle of the PWM. The loop response of the control loop should be fast enough to prevent overshoot or undershoot in the output when the load current changes. Synchronous Boost VBat Control Circuit SoC Dead Band PWM R1 Voltage Load CPU ADC PGA R2 Current Vss Rs Figure 9: Boost network circuit The CPU also measures the load current from RS to implement overload cutoff and to stop the boost converter when the external mobile device is fully charged and stops drawing current from the boost converter. Battery Protection As discussed earlier, power bank application needs protection for various parameters like over charge, over discharge, short circuit and temperature protection. SoCs can easily implement all of the above battery protections, as the CPU measures the battery voltage and current during the charging and discharging. However, power bank manufacturers prefer to use dedicated battery protection circuits outside the CPU. This is to ensure that battery is always protected even if the CPU has failed for some reason. This provides two levels of protection in power banks that use SoCs. As mentioned earlier, PSoC 1 is a very popular SoC which provides high integration through configurable digital and analog resources. Figure-10 shows the block diagram of the power bank design implemented using CY8C24423A device from PSoC 1 family.
Buck Converter circuit Boost Converter circuit LOAD VLOAD ILOAD VBAT BOOST PWM P IBAT BUCK PWM BOOST PWM N MUX BUCK Dead Band PWM PWM VSupply PGA VBus D- VBAT SENSE D+ VLOAD SENSE MUX 10 BIT Gnd TEMPERATURE TO SAR ADC USB STATUS LEDS M8C D- DAC D- VREF D+ DAC D- RREF Vref PSoC Vss CY8C24423A THERMISTOR Figure 10: Block diagram of Power Bank using integrated approach Conclusion Following table summarizes the advantages of implementing a power bank solution using a SoC compared to conventional solutions based on discrete devices. System on Chip (SoC) solution Conventional solution Charging source Integrated – automatically detects the Not available. Fixed charging detection charging source and configures the current. charging current. Charging current Configurable Fixed Boost regulator Scalable to different output ratings. Fixed. For changing the output Output Power Requires change to only the external power, the complete switching switching components. regulator circuit has to be changed Battery protection Two levels of protection- SoC + Battery Battery protection IC protection IC Auto-load Auto detects the load and enables the Not available detection output. Stand by current < 60uA – AFE is integrated, thus during 200uA (typical) (System current) sleep AFE can be disabled and even lower current consumption can be achieved.
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