Analyzing a power supply typically begins with the input. In this case, the input is 220V AC. One side of the AC goes through a 4007 diode for half-wave rectification, while the other side passes through a 10-ohm resistor before being filtered by a 10uF capacitor. This 10-ohm resistor serves as a protective component. If an overcurrent occurs due to a fault in the circuit, the resistor will blow, preventing further damage. On the right side, the 4007 diode, a 4700pF capacitor, and an 82KΩ resistor form a high-voltage snubber circuit. When the switch transistor 13003 turns off, it absorbs the induced voltage from the coil, protecting the transistor from breakdown caused by excessive voltage. The 13003 is a switching transistor (MJE13003), capable of withstanding up to 400V, with a maximum collector current of 1.5A and a maximum power dissipation of 14W. It controls the on-off state between the primary winding and the power supply. As the primary winding switches on and off continuously, a varying magnetic field is created in the transformer, inducing a voltage in the secondary winding. However, since the dot notation is not shown in the diagram, it's unclear whether the configuration is forward or flyback.
Based on the circuit structure, it’s reasonable to assume this is a flyback topology. The 510KΩ resistor on the left provides the initial base current to start the switching action. Below the 13003, a 10Ω resistor acts as a current-sensing resistor. The current flowing through it generates a voltage (10 × I), which is then applied to the base of a C945 transistor via a 4148 diode. When the sampled voltage exceeds 1.4V, corresponding to a current greater than 0.14A, the C945 transistor turns on, reducing the base voltage of the 13003. This clamping effect limits the collector current, thus protecting the switch transistor from overcurrent conditions. Essentially, this forms a constant-current control loop, limiting the maximum current to around 140 mA.
The voltage from the sampling winding at the lower left of the transformer is rectified by a 4148 diode and filtered by a 22uF capacitor to create a feedback voltage. For easier analysis, we can consider the emitter of the C945 transistor grounded. In this case, the sampling voltage becomes negative (approximately -4V), and the higher the output voltage, the more negative this voltage becomes. After passing through a 6.2V Zener diode, the voltage is applied to the base of the 13003. When the sampling voltage becomes sufficiently negative, the Zener diode breaks down, lowering the base potential of the switch transistor. This causes the 13003 to either turn off or delay its conduction, controlling the energy input into the transformer and regulating the output voltage accordingly.
The series combination of a 1KΩ resistor and a 2700pF capacitor on the lower part forms a positive feedback path. The induced voltage from the sampling winding is fed back to the base of the switching transistor, helping to sustain oscillation. On the right side, the secondary winding is rectified by a diode RF93 and filtered by a 220uF capacitor to provide a 6V DC output. Although the exact specifications of RF93 are not available, it is likely a fast recovery or Schottky diode, such as 1N5816 or 1N5817, suitable for high-frequency switching applications.
Given the high operating frequency, the transformer must be a high-frequency switching type, typically using a ferrite core with high resistivity to minimize eddy current losses.
Analysis of each part:(1) Relationship between the primary and secondary windings: After the 220V AC is rectified and filtered by the 4007 and 10uF capacitor, approximately 300V is applied to the collector and emitter of the 13003 via the transformer. The 510KΩ resistor provides the starting bias to turn on the 13003. Once the feedback winding (acting like a secondary winding) activates the C945, it forces the 13003 to turn off. This creates an oscillating state, allowing the secondary winding to generate a voltage output.
(2) Common fault analysis:
A. The 4007, 82Ω resistor, and 4700pF capacitor form a peak absorption circuit, protecting the 13003. The 510KΩ resistor is the start-up resistor, while the 10Ω resistor acts as a fuse. B. The 4148 diode is used for feedback rectification, and also provides clamping protection for the C945. The 6.2V Zener diode stabilizes the base voltage of the 13003. The 1KΩ and 2700pF components provide positive feedback. The 10uF and 22uF capacitors are filtering components. C. The transformer not only steps down the voltage but also provides essential electrical isolation, ensuring safety by preventing direct contact with the mains voltage.
(3) Determining full-load current: The system doesn’t monitor the output voltage directly but instead detects the load current. When the battery is fully charged, the load current drops to nearly zero. The LED display reflects changes in the load current, indicating charging status.
(4) Working principle of the transformer: This is a standard high-frequency self-oscillating step-down circuit. The 13003 transistor acts as the main oscillator. Power is supplied through the 80T primary winding. The secondary winding, with 12T turns, induces a lower voltage, which is rectified by the RF93 diode and filtered by a 220uF capacitor to produce the final 6V DC output. Another 12T winding provides the drive signal for the 13003. A small C945 transistor is used for current limiting, with the current sampled across a 10Ω resistor. When the current exceeds a threshold, the C945 turns on, reducing the base current of the 13003 and maintaining a stable collector current. This helps regulate the output voltage.
This is a typical switching transformer used in power supplies. The circuit is simple, and the rectifier and filter circuits on the secondary side are straightforward. The left side has two windings: the 80T main winding and the 12T feedback winding, which ensures proper excitation and oscillation. Under stable conditions, the voltages on these windings are similar. It’s estimated that the AC voltage on the 12T winding is around 6V, creating a negative voltage at the left end of the 4148 diode. This allows the 6.2V Zener to stabilize the base voltage of the 13003, keeping it above 0.7V to prevent unnecessary turn-on. The C945 transistor also acts as a protection and negative feedback mechanism, turning on when the current through 13003 increases, thereby reducing its base voltage and preventing overcurrent.
One end of the 12T secondary winding can be considered grounded, while the other end connects to both positive and negative feedback paths, forming a self-oscillating circuit. This controls the switching transistor, creating a changing magnetic field that induces a voltage in the secondary winding.
(5) 13003 is the switching transistor, also known as MJE13003
It is an NPN-type switching transistor commonly used in ballasts and charger circuits. It can handle up to 1.5A of current and withstands voltages up to 700V. Similar models include 1301–1303.
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