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 10Ω resistor before being filtered by a 10µF capacitor. The 10Ω resistor serves as a protective element. If a fault occurs in the circuit, causing excessive current, this resistor will blow, preventing further damage to the rest of the system. On the right side, a 4007 diode, a 4700pF capacitor, and an 82kΩ resistor form a high-voltage snubber circuit. When the switch transistor (MJE13003) turns off, this circuit helps absorb the induced voltage from the coil, thus protecting the transistor from breakdown due to high voltage.
The MJE13003 is the main switching transistor, rated for 400V, with a maximum collector current of 1.5A and a maximum power dissipation of 14W. It controls the on/off cycle between the primary winding and the power source. As the primary winding switches on and off, a varying magnetic field is created in the transformer, inducing a voltage in the secondary winding. However, since the dot notation (same polarity end) is not shown in the diagram, it's unclear whether this is a forward or flyback configuration.
From the circuit structure, it can be inferred that this is likely a flyback topology. The 510kΩ resistor on the left provides the initial base current to start the switching process. Below the MJE13003, a 10Ω resistor acts as a current-sensing resistor. The voltage drop across this resistor (10 × I) is fed into the base of a C945 transistor via a 4148 diode. When the voltage exceeds 1.4V, which corresponds to a current of around 0.14A, the C945 turns on, clamping the base voltage of the MJE13003. This limits the collector current, preventing overheating and potential damage to the switch.
The voltage from the sampling winding at the lower left of the transformer is rectified by a 4148 diode and filtered by a 22µF capacitor. For analysis purposes, we assume the emitter of the C945 is grounded. The resulting sampling voltage is negative (around -4V), and the higher the output voltage, the more negative this signal becomes. After passing through a 6.2V Zener diode, this voltage is applied to the base of the MJE13003. When the voltage becomes sufficiently negative, the Zener diode breaks down, reducing the base voltage of the switch transistor, which either turns it off or delays its conduction. This regulates the energy input into the transformer, controlling the output voltage and ensuring stable operation.
The series combination of a 1kΩ resistor and a 2700pF capacitor forms a positive feedback loop. This loop takes the induced voltage from the sampling winding and applies it back to the base of the switching transistor, helping to maintain oscillation. On the right side, the secondary winding is rectified by a diode (likely a fast recovery type such as RF93) and filtered by a 220µF capacitor to produce a 6V DC output. The RF93 diode is expected to have a high-frequency response suitable for switching applications, possibly a Schottky diode like 1N5816 or 1N5817.
Due to the high-frequency operation, the transformer must be a high-frequency switching transformer, typically using a ferrite core with high resistivity to minimize eddy current losses.
Analysis of Each Part:(1) Relationship Between Primary and Secondary Windings: After rectifying and filtering the 220V AC with a 10µF capacitor, the resulting 300V is applied to the C and E terminals of the MJE13003 through the transformer. The 510kΩ resistor provides the initial base current to turn on the transistor. Once the feedback winding (acting as a secondary winding) activates the C945 transistor, it forces the MJE13003 to turn off. This creates an oscillating state, allowing the secondary winding to generate a voltage output.
(2) Common Faults:
A. The 4007, 82Ω resistor, and 4700pF capacitor form a peak absorption circuit that protects the MJE13003. The 510kΩ resistor is the starting resistor, and the 10Ω resistor acts as a fuse.
B. The 4148 diode is used for feedback rectification, and the 4148 also provides clamping protection for the C945. The 6.2V Zener diode offers regulation and protection for both the MJE13003 and C945. The 1kΩ and 2700pF components help provide positive feedback to the MJE13003. The 10µF and 22µF capacitors are used for filtering.
C. This transformer not only steps down the voltage but also provides critical isolation from the mains, ensuring user safety and preventing electric shock.
(3) Determining Full-Load Current: The system detects full-load condition based on the load current rather than the output voltage. When the battery is fully charged, the current drops close to 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 MJE13003 acts as the main oscillator, drawing power through the 80T primary winding. The secondary winding (12T) induces a lower voltage, which is rectified by the RF93 diode and filtered by a 220µF capacitor to produce a low DC voltage (6V in the example). Another set of 12T windings supplies the drive current for the MJE13003. A small transistor, C945, is used for current limiting. The 10Ω resistor at the emitter of the C945 samples the current, and when it exceeds a threshold, the C945 turns on, reducing the base voltage of the MJE13003 to limit its collector current. This helps stabilize the output voltage.
This is a switching transformer commonly used in power supplies. The circuit is simple, and the rectifier and filter on the right side of the transformer are straightforward. The left side has two windings: one 80T main winding and a 12T feedback winding. The feedback winding ensures the circuit remains positively excited and oscillates. Under normal conditions, the voltages from these windings should be similar. It’s estimated that the AC at the left end of the 4148 diode is around 6V, creating a negative voltage that stabilizes the 6.2V Zener regulator at 0V, ensuring the MJE13003 receives the correct bias voltage. Too much voltage could cause the transistor to overheat and fail.
The C945 transistor functions as both a protection circuit and a negative feedback mechanism. When the current through the MJE13003 increases, the voltage across the 10Ω resistor rises above the diode’s threshold, turning on the C945 and lowering the base voltage of the MJE13003 to prevent overcurrent. The 510kΩ resistor serves as a starting resistor, and the reverse voltage protection circuit also contributes to the overall safety of the system.
One end of the 12T secondary winding is effectively grounded, while the other end is connected to both positive and negative feedback loops, forming a self-excited oscillation. This oscillation controls the switching transistor, creating a changing magnetic field that induces a voltage in the secondary coil.
(5) MJE13003 as the Switching Transistor: The MJE13003 is an NPN-type switching transistor, often used in electronic ballasts and charger circuits. It has a maximum current rating of 1.5A and a breakdown voltage of up to 700V. Similar models include 1301, 1302, and 1303, making it a versatile component in various power supply designs.
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