2.5W low power RCC switching power supply production

The AC adapter circuit of a commercially available portable CD/VCD player is shown in the attached figure. The adapter has a nominal output of 5V, 500mA, a volume of 7×4×1.8cm, and a weight of about 180g. Its power to volume ratio is significantly better than that of ordinary power frequency transformer adapters. When the mains power is 220v input, the output voltage is about 5.2V when no load is used and the VCD player is playing normally, with no significant change. The adapter is sold randomly without drawings, and the printed board does not have component numbers. The component values ​​in the picture are measured by the author, and the circuit is drawn based on the actual product. Although the Q16 and Q17 logos of the power supply have been removed with sandpaper, it can be inferred that Q16 is MJE13003 and Q17 is 8050 based on the circuit structure and tube volume shape.

This adapter is different from the general pulse width modulation switching power supply. Q16 is the switching tube, R84 is the starting resistor, R83 and C15 are positive feedback RC components. D5 is the discharge path of C15. Q17 is a pulse control tube, and the voltage drop of its base R82 forms the overcurrent protection circuit of the switching tube Q16. R81 and C12 serve as isolation circuits to prevent the voltage stabilization control signal input to Q17 from being short-circuited by R82. Optocoupler IC1 and Q17 form an oscillation suppression type (also known as RCC type) voltage stabilizing control circuit. The special feature of RCC is to stabilize the output voltage by changing the duty cycle of the switch tube on/off through the process of suppressing self-oscillation. Instead of controlling the pulse width of each oscillation cycle.

When the mains power is turned on and Q16 is started, the conduction current of Q16 stores magnetic energy in the pulse transformer. With the positive feedback process, the charging current of C15 gradually decreases and the switch tube enters the cut-off area. T releases the magnetic energy. L4 generates an induced voltage to charge C30 through D7. , after several oscillation cycles, the charging voltage of C30 rises to above 5V. The output voltage passes through R17 and the optocoupler light-emitting diode causes the voltage regulator tube D8 to break down. The primary and secondary optocoupler are turned on at the same time, causing L2 of T to be The rectified output voltage of D6 is added to the base of Q17, Q17 is saturated and turned on, and the switching tube Q16 stops oscillating. At this time, C30 discharges to the load circuit. When the discharge voltage of C30 is lower than 5V, the voltage regulator tube D8 is cut off, Q17 is cut off immediately, and the switching tube Q16 starts to oscillate again and repeats the above process. As a result, the output voltage is kept stable through an oscillation-suppression process.

The difference between this voltage stabilization process and pulse width modulation is that Q17 does not control the conduction time of each oscillation cycle of Q16. As long as the charging voltage of C30 does not reach the D8 setting value, Q17 will not act. Therefore, the RCC method is called non-conducting time. Cycle power on and off. In this voltage stabilization method, in addition to the output ripple caused by the oscillation of Q16, there is also a lower frequency ripple caused by the suppressed oscillation of Q16. Therefore, a filter circuit composed of L10 (82 uH) C32 needs to be connected to the output end. RCC has a simple circuit and also has the function of automatic load current regulation. It is the preferred circuit for small switching circuits below 50W. The most convenient thing about an AC adapter for portable electrical appliances is that it will not damage the power supply even if it is no-load. When there is no load, the discharge process of C30 is extended and only 50mA of discharge current flows through R4. Q16 is in an oscillation state with a very small duty cycle and consumes very little power. Small.

The key to making an RCC power supply is the winding of the pulse transformer T. No matter what kind of switching power supply pulse transformer, the calculation is extremely tedious and extremely inaccurate. It is more practical for amateur manufacturers to determine it through estimation and experimental adjustment. Even the estimation involves the influence of core material and structure, which is limited by the discrete nature of product parameters in the current market, making it difficult for amateurs to determine. Only some calculation results are listed here for reference. The production parameters of the small power switching power supply pulse transformer (refer to the attached drawing for winding functions) are as shown in the attached table.

 Note: Used for 220V mains input rectifier filter power supply. The self-oscillation frequency is about 22~24kHz. The primary inductance can be adjusted by adjusting the air gap to reach the above value.

Reasonable adjustment of component data can make up for the errors in pulse transformer parameters. This article will omit the inspection of faults that cause no output due to errors in the assembly process (T phase reverse connection, unqualified components, open circuits, short circuits during assembly, etc.) and discuss the selection and debugging of components. . After the assembly is correct, connect the power supply to the mains through the 25W bulb. At the same time, connect a 25Ω, 2W dummy load resistor to the output end. There should be voltage at the output end. Then replace D8 between 3.3~4.5V to make the output voltage 5V± 0.2V. The relationship between malfunctions during adjustment and component values ​​is as follows.

1. There is no output voltage, indicating that the power supply has not started to oscillate. If the phase of the pulse transformer is correct, try replacing R84 with a 270kΩ/0.5W resistor. If there is still no output, the β value of Q16 is too low, replace it with a switching tube with β greater than 40.

2. The output voltage is always low. Replacing D8 is ineffective because the positive feedback amount is too low. It can be proved by removing the external 25Ω dummy load and increasing the voltage. The reason is that the number of L2 turns of T is too small and the leakage inductance is too large. You can try reducing R83 to 160Ω, 120Ω...68Ω, 32Ω. If the improvement is not significant, rewind L2 and increase the number of turns appropriately. The reduction limit of R83 is limited when the voltage starts to rise. If the resistance value is too small, the switching tube loss will be increased.

3. The output voltage is on the high side. If D8 switches to 3.1V and still outputs a voltage above 5.5v, the possible reason is that the saturation voltage drop of Q17 is too large. Choose an 8050 with a small saturation voltage drop. If the forward voltage drop of the optocoupler light-emitting tube exceeds 2V, PLC817 can be used to try it.

4. The power supply makes a "squeaking" sound. First, use 4700pF or 6800pF polyester capacitors for C15, and use 0.47 uF non-polar capacitors for C12. If the inductance of L1 still sounds "T" and is too large, try increasing the air gap of the magnetic core. It is 0.2~0.3mm. The method is to insert enameled wire of the same diameter into the core column interface and press it firmly.

5. When the switching power supply pulse transformer is used in a single-ended circuit, an air gap must be left. For the magnetic core of the new cylindrical core column, an air gap of about 1mm has been left during production. The E1 type magnetic core can be separated by 0.6mm enameled wire on the end faces of the side columns on both sides. Adjusting the air gap can fine-tune the primary inductance. Regardless of the power, the single-ended switching power supply cannot be energized without leaving an air gap. Once the magnetic core is firmly absorbed and the magnetic flux is saturated, it will produce great noise and damage the switching tube.

6. The maximum output power of the circuit in the attached picture is 20W, and the pulse transformer is wound according to the data in the attached table. Change Q16 in the original picture to BUT11A, change R82 to a 0.5Ω resistor, and change C11 to a 68~100 uF/400V electrolytic capacitor.