Here it is explained how a single 3.7V Li-ion battery charger circuit works. The circuit is a Switching Mode Power Supply (SMPS) battery charger circuit, specifically called a "Self-Oscillating Flyback Converter" (also known as a RCC - Ringing Choke Converter). This circuit uses a tiny high-frequency ferrite transformer (TR1) to convert 220V AC into a precise 4.2V DC for charging 3.7V Li-ion batteries.
Below is the single 3.7V Li-Ion Battery charger circuit diagam.
How the Circuit Works
Rectification: The 220V AC input is rectified by D1 and smoothed by C3 to create high-voltage DC (~310V).
Startup & Oscillation: R1 and R2 provide a small "startup" current to the base of Q1 (MJE13005). Once Q1 turns on, current flows through the primary of TR1. An auxiliary winding (feedback) then takes over, keeping the circuit oscillating at a very high frequency (typically 20kHz to 60kHz).
Energy Transfer: When Q1 switches OFF, the energy stored in the magnetic field of the transformer "flies back" and is transferred to the secondary side.
Regulation & Cut-off: The secondary side uses the TL431 (U1) as a "brain." It monitors the battery voltage. When the battery reaches exactly 4.2V, the TL431 triggers Q4, which sends a signal back to the primary side (via the feedback loop) to stop or reduce switching.
Component Roles
Primary Side (High Voltage)
J1: AC Power Input (220V).
D1 (1N4007): Half-wave rectifier to convert AC to DC.
C3 (2.2µF / 400V): High-voltage filter capacitor.
Q1 (MJE13005): The main Power Switching Transistor. It does the heavy lifting of switching the high-voltage DC on and off.
Q2 (2N2222): Error amplifier/protection transistor. It pulls the base of Q1 to ground to shut it down if the current is too high or if the output is fully charged.
R1 (4.7MΩ): Startup resistor. It provides the initial "spark" to start the oscillation.
D3 (Zener BZX84): Voltage limiter to protect the base of Q1 from excessive voltage.
Secondary Side (Low Voltage / Charging)
D4 (1N5822): A Schottky Diode. It rectifies the high-frequency AC from the transformer into DC. Schottky is used here because it is fast and has low voltage drop.
C4 (220µF): Filter capacitor for the output DC.
U1 (TL431): Programmable Shunt Regulator. This is the most important part for safety; it ensures the battery never goes above 4.2V (the danger zone for Li-ion).
Q3 (2N3906) & Q4 (2N3904): These act as a switch for the LED indicators (D6/D7). They change the LED color when the battery is charging vs. when it is full.
R11 & R12: These resistors form a Voltage Divider. They tell the TL431 what the current battery voltage is.
R13 (1Ω): Current Sensing Resistor. It limits the amount of current going into the battery so it doesn't charge too fast.
This circuit requires a Ferrite Core Transformer (TR1) designed for high-speed switching.
The following is video shows simulation of how the above single 3.7V Li-ion battery charger circuit works.
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