Canon FAX-450 fax machine power circuit diagram and principle analysis

The power circuit of Canon FAX-450 fax machine is shown in Figure 1.

Figure 1 Power circuit schematic diagram of Canon FAX-450 fax machine

As shown in Figure 1, the power supply system mainly consists of surge current suppression, main switch conversion, DC voltage output, voltage stabilization control and automatic protection circuit. The specific circuit working principle and process are as follows:

1. Working principle and working process of the surge current suppression circuit The surge current suppression circuit is mainly composed of CR1, R2, ⑤~⑥ winding of T1, D3, R3, R4 and other components. Its function is to suppress the large charging surge current of capacitors C7 and C8 at the moment of power-on. At the moment of power-on, since the capacitors C7 and C8 are equivalent to an instant short circuit, the larger charging current is suppressed when passing through the resistor R2. When the voltages across C7 and C8 stabilize, the voltage induced by the ⑤~⑥ windings of T1 is added to the control electrode of the triac CRl through D3, R3, and R4, triggering CRl to turn on and short-circuiting the resistor R2. In this way, the main current no longer passes through the current suppression resistor R2, so that R2 no longer affects the normal current of the power supply after completing the suppression of the surge current at startup.

2. Working principle and process of the main switch conversion circuit The main switch conversion circuit of the fax machine is mainly composed of switching tube Q1, integrated circuit ICl, switching pulse transformer T1, capacitor C12 and other components. Its function is to convert the rectified, smoothed and filtered 300 V pulsating DC voltage into a pulse voltage and couple it to the secondary circuit through T1. The specific working process is as follows.

When the AC power supply is turned on, one path of the rectified, smoothed and filtered 300 V pulsating DC voltage is added to the drain of the switching field effect transistor Q1 through the primary winding ①~③ of the pulse transformer T1; the other path is supplied to the integrated circuit through the resistor R8 The pin of ICl is powered by the Vcc terminal. When the power supply works normally, the ICl roller pin power supply is provided by the DC voltage output after the voltage induced by the ⑦~⑥ winding of T1 is rectified by D4 and filtered by C12. After the ICl pin obtains normal voltage, its internal circuit starts to work. After the pulse generated by the oscillator is processed by its internal amplification and shaping tube, its pin outputs a switching pulse with adjustable width and is added to the gate of Q1. , controlling the intermittent conduction of Q1. This generates alternating current in the secondary of T1, and after rectification by RC2, RC3, etc., various required DC voltages are obtained. The level of the output voltage can be controlled by changing the conduction time of the switch Q1.

3. The working principle and working process of the DC voltage output circuit are shown in Figure 1. The DC voltage output circuit of the fax machine is composed of T1 secondary high-frequency rectification, filtering and voltage stabilizing circuits, which respectively output stable +5 V, ±12 V and +24 V DC voltages. Among them, +24 V is the main power supply, which supplies the heating sensor head, stepper motor, etc. as the working power supply and is the controlled output voltage.

The pulse voltage induced by the secondary winding ⑧~ of T1 is rectified by RC2, filtered by L3, C19, C20, C21, C22, L4, etc., and then a 24 V DC voltage is output. Transistor Q3 in this loop is used as a control switch for the +24 V power supply and is controlled by the main control circuit. When the main control circuit sends the power-on signal VRC, Q3 turns on and outputs +24 V DC voltage; when the main control circuit does not send the power-on signal VRC, Q3 turns off and there is no +24 V power voltage output.

At the same time, after rectification and filtering, the output +24 V DC voltage is also stabilized by T2, Q5, etc., and outputs 5 V DC voltage after π-type filtering composed of C29, L, C31, etc.

The pulse voltage induced by the secondary winding of T1 is rectified by D12, D15, D13, D14, filtered by C35 and C36, and then stabilized by two three-terminal regulators IC3 and IC4 respectively. Output ±12 V DC voltage.

4. Working principle and working process of the voltage stabilizing control circuit The pulse width modulation voltage stabilizing circuit of the fax machine switching power supply consists of the integrated circuit ICl and the photoelectric coupler PC1. By controlling the conduction time of switch Q1, the automatic stabilization of the 24 V and 5 V output terminal voltages is achieved.

When the voltage at the 24 V output terminal increases, the voltage applied to the b-electrode of transistor Q2 increases after being divided by the potentiometer RVl, resistor R18 and resistor R19, and the e-electrode voltage of Q2 passes through R16 and zener diode D5. Voltage stabilization is fixed. This increases the c-pole current. When this current passes through the light-emitting diode in the photocoupler PC1, the brightness of the light-emitting diode is enhanced. Then the current of the phototransistor in PC1 increases, the output impedance decreases, and is added to the integrated circuit ICl⑥ pin (connected to the internal The voltage at the non-inverting input terminal of the error amplifier decreases, and after passing through the ICl internal pulse width control circuit, the pulse width output by its pin is narrowed, the Q1 conduction time is shortened, the energy storage of T1 when Q1 is turned on is reduced, and the coupling As the voltage to the secondary winding decreases, the output DC voltage naturally decreases, thereby achieving the purpose of voltage stabilization.

In addition, the 5 V voltage loop is also equipped with a voltage stabilizing circuit, which is composed of integrated voltage regulator IC2, resistors RV2, R31, R32, transistor Q6 and other components. When the 5 V output terminal voltage rises, the current flowing through the cathode-anode (KA terminal) of IC2 increases, Q6 is turned on, and the b pole of the switch Q5 is short-circuited to the ground through D19, Q5 is cut off early, and its e The output voltage of the pole decreases, and the voltage of the 5 V DC output terminal naturally decreases to achieve the purpose of voltage stabilization.

When the voltage at the 24 V and 5 V output terminals drops, the working principle and process of the voltage stabilizing circuit are opposite to the above.

5. The working principle and working process of the automatic protection circuit. The switching power supply system of the fax machine is equipped with overcurrent and overvoltage protection circuits. The specific protection process is as follows:

(1) Working process of overcurrent protection circuit: The overcurrent protection circuit is mainly composed of overcurrent detection resistors R5, R6 and integrated circuit ICl. When the current passing through the D and S poles of the switch Q1 is too large, the voltage drop formed on the resistors R5 and R6 increases. This voltage is added to the ICl⑩ and pins, and is compared with the standard voltage in ICl. The result is processed so that the ICl pin has no pulse output, thereby turning off the switching tube Q1 to achieve over-current protection.

(2) Working process of the 5 V overvoltage protection circuit: When the voltage stabilizing circuit fails and the DC voltage at the 5 V output terminal is too high, the voltage stabilizing diode D11 is broken down, and the breakdown current passes through the light-emitting diode in the photocoupler PC2. When the light-emitting diode is made to emit light, the output resistance of the phototransistor in PC2 decreases, and the voltage applied to the ICl pin 8 increases. After passing through the pulse width control circuit in ICl, the ICl pin is forced to output no pulse, and Ql is turned off to achieve over-passing. pressure protection purpose.