The application note below should help designers make their own electric vehicle battery charging solutions. If required, help is available from the company
The popularity of electric vehicles (EVs) is increasing rapidly in India. According to a survey, the EV market in India is estimated to increase from 3 million units in 2019 to 29 million units by 2027 with a CAGR of 21.1 per cent. As a result, demand for AC/DC chargers, the smart chargers for EVs, will also increase.
In order to charge the batteries efficiently, and to ensure their long life, we need a smart battery management or charging system. To realise such an EV charging system, Holtek has come up with smart EV charger solutions based on their low-cost ASSP flash microcontroller (MCU) HT45F5Q-X for charging EV batteries.
At present, three EV charger designs suitable for Indian market – with specifications of 48V/4A, 48V/12A and 48V/15A – are available for rapid development of the product. This semiconductor-based smart charging system can support both lithium-ion as well as lead-acid battery types.
Block diagram of the EV charging solution is shown in Fig. 1. Here, battery charger ASSP flash MCU HT45F5Q-X is the heart of EV charger circuitry, with in-built operational amplifiers (OPAs) and digital-to-analogue converters (DACs) that are necessary for battery charging function.
Specifications of the battery charger flash MCU HT45F5Q-X series are shown in Fig. 2. Designers can choose an appropriate MCU from HT45F5Q-X series according to their application requirement.
The features and working of EV charger solution for 48V/12A specification is briefly explained below. This EV charger design utilises HT45F5Q-2 MCU for implementing battery charging control function.
The MCU incorporates a battery charging module, which can be utilised for closed-loop charging control with constant voltage and constant current for efficiently charging a battery. Internal block diagram of MCU HT45F5Q-2 is shown in Fig. 3.
The battery charging module in HT45F5Q-2 has built-in OPAs and DACs that are needed for charging process. Therefore the design reduces the need for external components like shunt regulators, OPAs and DACs, which are commonly used in conventional battery charging circuits. As a result, the peripheral circuit is compact and simple, resulting in a smaller PCB area and low overall cost.
Working of EV charger
Input power to the EV charger is an AC voltage in the range of 170V to 300V. The EV charger uses a half-bridge LLC resonant converter design, because of its high-power and high-efficiency characteristics, to obtain DC power for charging the battery.
The design utilises a rectifier circuit for converting input AC voltage to high-voltage DC output, and it also has an electromagnetic interference (EMI) filter to eliminate high-frequency noise from input power source. A pulse-width modulation (PWM) controller IC, like UC3525, can be used for driving the MOSFETs of half-bridge LLC converter.
The battery charging process is supervised by the MCU HT45F5Q-2. It monitors the battery voltage and charging current levels and gives feedback to the PWM controller IC. Based on the feedback, the PWM controller varies the duty cycle of its PWM signal and drives the MOSFET circuit to obtain variable output voltage and current for charging the battery.
For better protection, HT45F5Q-2 is isolated from rest of the circuit (i.e., high-voltage components) using a photo-coupler. Battery-level LED indicators are provided for knowing the charging status.
Battery charging process
The change in charging voltage and current during the charging process is graphically illustrated in Fig. 4. If the battery voltage is too low when connected for charging, low charging current (i.e., trickle charge (TC)) will be set initially and charging process will start.
When the battery voltage increases to a pre-defined level (Vu), constant voltage (CV) and constant current (CC) is applied for charging and continued until the battery is fully charged. Battery is considered to be fully charged when voltage reaches VOFF. When charging current drops to Iu, final voltage (FV) is set. The voltage, current and temperature control process in this EV charger are explained below.
(a) Voltage control. The charging voltage is decided based on the initial voltage of battery when it is connected for charging. As the charging progresses, charging voltage changes accordingly and, finally, when battery is fully charged, the final voltage is set. The charging-voltage decision levels for 48V/12A battery charger are explained below.
(b) Current control. Charging current is set depending on the battery voltage. Initially, if the battery voltage is too less, trickle-charge current would be set for charging the battery. Once battery voltage reaches certain level, constant current is supplied for charging, until battery is charged fully. The charging-current decision levels for 48V/12A battery charger are listed below.
(c) Over-temperature protection. The EV charger has a negative temperature coefficient (NTC) thermistor to monitor the temperature and a fan to regulate the heat. When temperature increases, the fan is automatically switched on to dissipate the heat; it gets switched off when the temperature is reduced to the lower set threshold. Also, the fan turns on when charging current is high and turns off when charging current is low.
(d) LED indications for charging status. These are listed below.
(e) Charging duration. When charging duration is exceeded (duration depends on battery capacity), the voltage drops to FV, the current is reduced to TC, and charger repeatedly monitors the battery voltage.
Schematic and PCB assembly
The schematic of Holtek EV charger design for 48V/12A type is shown in Fig. 5 for reference and its PCB assembly is shown in Fig. 6.
The ASSP flash MCU HT45F5Q-2 can also be used for designing higher-wattage solutions. It offers a programmable option for setting parameter thresholds, which makes it very convenient for EV charger designs. Holtek provides technical resources such as block diagram, application circuits, PCB files, source code, etc to help designers in rapid product development and speed up time-to-market.
EV charger development platform for HT45F5Q-X series will also be available soon. Using this software tool, users would be able to easily select the charging voltage/current and other parameters to create a program. This application will also be able to generate a program containing a standard charging process, thereby significantly simplifying the development process.
—Krishna Chaitanya Kamasani is director – India operations at Holtek Semiconductor