MICROTEK scanner switching power supply - Power Circuit - Circuit Diagram

After the power is turned on, the mains is rectified by the rectifier bridge and C5 filtered to obtain a DC voltage of about 300 V. This voltage is applied to the gate of Q1 through the starting resistor R11 to provide a starting bias for Q1, and the +300 V voltage is passed through the switching transformer. The primary winding of T1 is applied to the drain of Q1, which turns Q1 on. The induced potential on the positive feedback winding of T1 is applied between the gate and source of Q1 through C10 and R9, which quickly saturates Q1. IDS of Q1 When linearly increasing, T1 stores energy. When the T1 flux is saturated, the induced potential on the positive feedback winding charges C10, causing the gate potential of Q1 to gradually drop, causing Q1 to exit saturation. The drain current begins to drop, and the positive feedback winding induces The potential polarity is reversed and Q1 is quickly turned off. After Q1 is turned off, the energy stored by T1 during Q1 conduction is released to the load and the filter capacitor. When the T1 energy storage is released, the T1 secondary winding is in a high-impedance state. When Q1 is cut off, the primary winding of T1 is also in a high-resistance state. Therefore, the parallel resonant circuit composed of T1 primary winding inductance and C4 and distributed capacitance is free to oscillate. After half a cycle of oscillation, the induced potential polarity of each winding of T1 is flipped again, and positive feedback is passed. Bring Q1 back on again. The above process is repeated. Switch into oscillation. The power FET used as a switch, so that the dynamic range of the power increase, but also greatly reduces switch losses.

D1, R2, C4 are spike absorption loops. Its function is to absorb the peak high voltage generated when the switch tube is cut off, to protect the field effect tube from being broken down, and to make the opening loss of Q1 small. D5 is an overvoltage protection component. The access shortens the conversion time of Q1 from saturation to cutoff, thus reducing the power consumption of Q1.

The function of the capacitor C15 connected to the photocoupler PC14 is to provide working power for the phototransistor in PC1. During the Q1 off period, the induced potential on the T1 positive feedback winding is filtered by D3 and C15 to establish a voltage on C15. C15 has a large capacity, so the voltage on C15 is basically unchanged during the normal operation of the switching power supply, which is equivalent to a DC power supply.

Second, the circuit voltage regulator working process voltage control circuit consists of optocoupler PC1, pulse width pulse frequency modulation tube Q2, Q3, IC1 and its peripheral circuits. The circuit uses a voltage reference circuit that HA431 can set the voltage regulation value ( Note: The HA431, a settable voltage regulator with temperature-compensable characteristics, has the same performance as the popular component TIA31.) IC1 has three electrodes: cathode (K), anode (A), and reference (R). The anode of IC1 is grounded, and the LED of PC1 is connected in series in the voltage dividing circuit of IC1. The external resistor of IC1 reference pole is changed, and the output voltage will change accordingly.

When the input voltage drops, the output voltage will also drop, the PC11 pin potential will drop, the internal light-emitting diode of PC1 will become smaller, and the light intensity will decrease; the equivalent resistance between 4 and 8 will increase, and the base potential of Q3 will decrease, Q3 The degree of conduction is reduced; the potential of Q3 collector is increased, the conduction degree of Q2 is also decreased, the emitter potential of Q2 is increased, the gate shunting effect of Q1 is reduced, the IDS of Q1 is increased, and the Q1 switching pulse is occupied. The air ratio rises (the saturation conduction period is extended and the deadline is shortened). The output voltage is lowered to a stable value by correcting the decrease of the input voltage, and the output voltage is raised to a stable value. If a certain factor causes the output voltage to rise, there is The process of changing the process in reverse.

R23 is a feedback resistor. When the negative feedback voltage increases, it will cause the potential of PC13 to rise and the conduction of Q3 to increase. When the collector potential of Q3 decreases, the base potential of Q2 decreases. The degree of conduction of Q2 increases. Q1 gate potential decreases, resulting in a decrease in the conduction level of Q1, a decrease in the duty cycle of the Q1 switching pulse, and a decrease in the output voltage.

Third, maintenance,
Since the user does not disconnect the power supply in time after use, the scanner is in working state for a long time, and the heat is severe, causing a malfunction. In most cases, the breakdown of Q1 is damaged, and it is possible to damage R11, Q2, Q3, and PC1, and the value of R23 is changed. It may also induce damage to the above components. The surrounding components should be carefully inspected during maintenance. Special reminders should be disconnected from the power supply after using the scanner.

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