Production and testing of CL-A-15-5 switching power supply

We know that the switching power supply is mainly composed of AC 220V rectifier and filter circuit, switching oscillation circuit, high-frequency pulse rectifier and filter circuit, sampling and voltage stabilization control circuit, etc. The following takes the CL-A-15-5 switching power supply (15W/5V) as an example, as shown in the figure. As can be seen from the figure, it is a universal transformer-coupled parallel circuit. The input voltage is filtered by a line filter composed of C1, C2, C3, and LF1, a bridge rectifier circuit composed of diodes VD1~VD4, and capacitor C6, and becomes about 320V. One direct current is supplied to the power input terminal of IC1 through the self-starting current-limiting resistors R6 and R3. It can be seen that R6 and R3 are the chip's self-starting current-limiting resistors; the other path is supplied to pins 4 and 5 of IC1 (in 1CE2A) through the pulse transformer Tl. The drain of the MOSFET), and a loop is formed by the overcurrent detection protection resistor R1 connected to pin 3 (the source of the MOSFET in 1CE2A). The internal oscillator controls the on and off of the MOSFET. In addition, the auxiliary power circuit composed of the secondary of pulse transformer T1, D6, R5, and C7 supplies power to the PWM controller.

The PWM converter of this circuit is composed of ICE2A (IC1) and pulse transformer T1. Pulse transformer T1 has the functions of energy storage, isolation output and voltage conversion in the circuit. As can be seen from the terminal with the same name in the circuit diagram, this is a flyback switching power supply, that is, when the MOSFET is turned on, electric energy is stored in the primary coil in the form of magnetic field energy, and D7 is turned off at this time; when the MOSFET is turned off, the magnetic field energy is transmitted At this time, the Schottky rectifier D7 is turned on, and a DC voltage output is formed through the output rectification and filtering circuit composed of C12, L1, C13, R9, and C14. R13 and LED1 constitute a lighting indication circuit. In addition, the PWM converter uses ICE2A0565 (IC1), which is a dual in-line 8-pin structure. Its pin functions are shown in the table below.

For switching power supplies, when the power is the same, the higher the switching frequency, the smaller the size of the switching transformer, but the higher the requirements for the switching tube; the secondary of the switching transformer can have multiple windings or one winding has multiple tap to get the desired output.

The output DC voltage of the switching power supply is proportional to the effective value of the pulse voltage, and the magnitude of the pulse voltage is proportional to the conduction duty cycle of the switching tube. On the other hand, the duty cycle control circuit is feedback-controlled through the sampling comparison circuit, so that the duty cycle changes accordingly.

If the output DC voltage drops, the duty cycle becomes larger, causing the output voltage to rise, and vice versa.

The power supply adopts an optocoupler feedback voltage stabilizing feedback control circuit with an adjustable voltage regulator. R12, R11, W1, R8, IC2, and IC3 form a voltage sampling circuit. IC1 is equipped with a reference voltage, a comparison amplifier circuit and a pulse control circuit inside. C10 is the bypass capacitor at the feedback signal input end of IC1. The voltage stabilization process is: when the output voltage rises for some reason, the current of the light-emitting diode in the optocoupler increases, causing the current of the feedback control terminal of IC1 to increase, because the duty cycle of the internal oscillator is proportional to the current. Inversely proportional, so the duty cycle decreases, which forces the output voltage to decrease to achieve voltage stabilization, and vice versa. It can be seen that the feedback control circuit stabilizes the output voltage by adjusting the duty cycle.

In addition, the switching power supply also has some auxiliary protection circuits.

R4, D5 and C8 are buffer protection circuits to prevent the anti-buzzer voltage generated by the pulse transformer from causing the drain voltage of the MOSFET to exceed 650V and damaging the chip when the MOSFET is turned off. The setting of the overcurrent detection resistor Rl can limit the excessive current of the switching tube. The circuit is also equipped with a start-up protection circuit. Since the output DC voltage is not established in time when the power is turned on, the feedback voltage is not established in time, which can easily cause the saturation period of the switch tube to be too long and cause damage. For this reason, a soft-start capacitor C4 is added to pin 1 of the integrated block. , use the charging of the capacitor when starting up: limit the width of the switching pulse, so that the saturation period of the switching tube will not be too long and gradually increase when starting up, so that the voltage stabilizing circuit quickly enters the normal state, and its soft start time is determined by C4. In addition, the C3 capacitor is connected at both ends of the input line to eliminate series-mode interference, and the C1 and C2 capacitors are connected between the input line and the ground line to eliminate common-mode interference.

Component selection and printed circuit board fabrication

The selection of components is detailed in the table below. The production of printed circuit boards is as shown in the figure below.

3. Installation and inspection

According to the selected components and PCB boards, the PCB boards corresponding to the principle circuit are seated, and each node is welded. After the installation and welding are completed, check the circuit again to see if there are any misconnections or missing connections. If any, correct them in time. At the same time, before powering on, use a multimeter R×lk block to measure the input resistance of the switching power supply to determine whether there is a short circuit in the circuit. .

4. Debugging and testing

1. Test the adjustable range of the output voltage

Connect the input terminal and connect the 220V power supply to see if the indicator light displays normally. Adjust the variable resistor W1 so that it is clockwise to the end and counterclockwise to the end, and use a digital multimeter to measure the voltage range of the output terminal at DC20V.

2. Adjustment of no-load voltage

Adjust the voltage regulator to 220V, turn on the power, select the DC20V voltage range on the digital multimeter, and adjust Wl so that the output voltage is 5V. If the voltage is abnormal, check the relevant circuits and eliminate the fault.

3. Measurement of current regulation rate

Adjust the voltage regulator so that the input voltage of the transformer is 220V, and the load current is Io=0 at no load and Io=3A at full load. At full load, connect a sliding rheostat, measure the output voltages U2 and U1, and calculate according to the formula

(U0 is the rated output voltage 5V) current regulation rate. This circuit requires current adjustment rate ≤0.5%.

4. Measurement of voltage regulation rate

At full load, adjust the voltage regulator to raise the input voltage of the transformer to 265V, and measure the output voltage U3 at this time; then, at full load, adjust the voltage regulator to lower the input voltage of the transformer to 185V, and then measure the output at this time. Voltage U4; Finally, substitute the above data into

Calculate using two formulas (Uo is the rated output voltage 5V), and take the larger one from Sul and SU2 as the voltage regulation rate of the regulated power supply. This circuit requires voltage regulation rate ≤0.%.

5. Test of ripple voltage

Use an oscilloscope to measure the output AC ripple voltage, adjust the output DC voltage from the minimum to the maximum under no-load and full load conditions, and observe the changes in the ripple voltage. Record the value of the maximum ripple voltage. This circuit requires ripple voltage ≤80mVp-p.

By making and testing switching power supplies, you can not only become familiar with the working principles and functions of switching power supplies, but also master the assembly and production processes of switching power supplies. You can also become familiar with the debugging and testing technology of switching power supplies. In addition, after successful circuit production and debugging, fault simulation can also be performed, which will also help to master the maintenance skills of switching power supplies.