From the charging profile, it is understood that a controlled current source is required for a single-cell Li-Ion battery charger. The current source output needs to be altered, depending on the battery state. Considering these requirements, the following functional blocks are required for a microcontroller-based implementation:
1. Current control circuit
2. Battery parameter (Voltage, current, temperature) measuring circuit
3. Charging algorithm (for implementing CC-CV profile)
This is shown in block diagram below:
Figure 2: Block diagram of a Li-ion battery charger
The current control circuit can be built using a voltage source and a current feedback. It works like a typical negative feedback control system. The feedback is taken by allowing the charging current to pass through a small resistor, thereby developing some voltage.
The voltage source can be created using two methods:
1. Linear Topology
2. Switching – Buck or Boost Topology
The linear topology uses a series pass element (either BJT or MOSFET) in linear mode as shown in figure 3.
Figure 3: Linear topology
The charging current is controlled by controlling the bias to the series pass transistor Q1. The bias can be controlled using a digital to analog converter (ADC) or pulse width modulator (PWM) with an external RC low pass filter. The linear method is suitable for low charging currents <1A due to power dissipation in series pass element.