Analysis of the solution to interference problems faced by IDEALPLUSING high-voltage DC power supply

With the development of power supply technology, high-voltage DC power supply control has gradually evolved from early analog circuits to highly integrated control devices such as microprocessors and DSPs. These devices are small and very precise, but the switching power supply generates electromagnetic interference and radiation, the working environment is more rugged than other communication equipment, and the demand for auxiliary power is also high. Therefore, today we use auxiliary power for high-voltage DC power supplies, it is necessary to explain its working characteristics and waveforms, and pay attention to the analysis, problems and parameter selection of high-voltage DC power supplies based on experimental data.

Analysis of solutions to interference problems faced by high-voltage DC power supplies

   1. Interference Problems of High-Voltage DC Power Supply

        Today's intelligent switching power supplies have internal microprocessors or DSPs for internal monitoring and communication. Microprocessor chips have very high power requirements, the required amplitude is very stable, not to mention the large spikes and glitches that can cause electromagnetic interference, and the AC adaptability of the auxiliary power supply is greater than the normal operating range of the rectifier. It must be wide. When the rectifier is connected to the AC power supply, the monitoring part must first operate normally, perform self-tests and various conditions to see if the rectifier can be turned on. If the AC voltage is too high or too low, the rectifier will stop working. However, the monitoring part must continue to operate normally and maintain normal monitoring and communication.

        During operation, some power products reset for no reason. The design analysis of the auxiliary power supply of large-capacity switching power supply shows that the auxiliary power supply has many problems under different AC input voltages and different load conditions. Common problems include AC adaptation range, low load capacity, unstable and asymmetric working waveform, magnetic bias, severe electromagnetic interference, etc.

        The general working principle of the switching rectifier auxiliary power supply is to input AC power, rectify it into high-voltage DC power supply, and then convert the circuit into a low-voltage high-frequency square wave, and then convert the rectifier filter circuit into the stability required for the system to convert to low-voltage DC power supply. The voltage is controlled by a three-terminal regulator, and the DC output provides a voltage feedback signal for the high-frequency conversion drive pulse control loop. The series resistance sample in the main power conversion circuit is used as a current feedback signal, and the power conversion tube drive pulse is generated by the control chip and its peripheral circuits.

(Note: AC low voltage is the minimum input voltage measurement value when the auxiliary power supply starts working.)

     It can be seen that when the AC input voltage is low and there is no current feedback, the auxiliary transformer cannot work properly, the pulse width of the waveform is different, there is jitter, and the oscilloscope cannot capture the waveform stably. For current feedback, the pulse width of the waveform is wide and narrow, and the duty cycle is as high as 47%. Increasing the load will reduce the output voltage.

     It is generally difficult to stably operate the auxiliary power supply at the upper and lower limit voltages of the AC input, and it is difficult to stably operate the auxiliary power supply normally in the entire load range from idling to overload. Technical issues: dielectric strength and overload capacity of power devices, design of high-frequency transformers, parameter selection of control pulse control loop.

   2. About the anti-interference solution of high-voltage DC power supply

     Through specific theoretical analysis and experimental research, the technicians improved the auxiliary transformer and control loop and finally solved the problem. The solution is to adjust the turns ratio of the auxiliary transformer, change the side turns Np, reduce the ratio of the secondary side turns, and reduce the duty cycle at low voltage. This is far below the 45% limit specified in UC3844. The RC filter network of UC3844 matches the parameters. After many experiments, we finally got the ideal parameters and the number of filter capacitors. The hl increase. Test the same secondary winding of the auxiliary transformer again under the same conditions.

     When the AC input is too high or too low (and the starting working voltage is lower than the starting working voltage before enhancement) or there is no load or heavy load, these four waveforms will help you identify the enhanced auxiliary power supply. Compared with before the improvement, the working waveform is more stable, the pulse width is symmetrical and balanced, and the load capacity is significantly improved. Compared with the low input voltage, the duty cycle after improvement is reduced by 7% compared with the duty cycle before improvement. Even if the load increases, the output voltage of the auxiliary power supply is stable, and the load capacity is high. It clearly shows that it is strong. Improvements to previous auxiliary power supplies have produced noticeable results.

   III. Experience Summary

     In terms of improving the auxiliary power supply, engineers started from multiple aspects, such as: changes in the PI setting parameters of the voltage feedback loop, changes in the pulse frequency and the increase of the filter capacitor after the secondary side rectification. However, the cause of the problem could not be found. Under high and low AC input voltage, light load and overload conditions, the waveform still fluctuates and the DC output voltage is unstable. When adjusting the RC filter network parameters of the UC3844 current feedback link, many experiments have been conducted to find better experiments. Engineers found that after theoretical analysis, they needed to verify the improved results through continuous experiments.

     The above conclusions are useful for other low-power switching power supplies using the same circuit. By changing the RC filter network parameters of the current feedback link on the control chip, the method also obtained obvious results, and the specific parameters depend on the differences of each circuit. The differences are different, but the direction of improvement is the same.