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High frequency switching power supply interference - How to solve

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With the development of power supply technology, high frequency switching power supply control from the initial analog circuit gradually developed to microprocessor, DSP and other highly integrated controller

These devices are small in size and high in precision, but the electromagnetic interference and radiation in the switching power supply are stronger than other communication equipment in the working environment, which is conductive to electricity

 

The source is demanding more. In this paper, the characteristics and waveform of the auxiliary power supply in high frequency switching power supply are described, and the experimental data are analyzed

 

High frequency switching power supply design should pay attention to the problem and the choice of parameters.

 

First, the interference of high frequency switching power supply

 

In the current intelligent switching power supply, all organic microprocessor or DSP, for monitoring and communication in the machine. Microprocessors have high requirements for power supply

 

, the amplitude is quite stable, but not with a large spike burr, resulting in electromagnetic interference, and the ac adaptive capacity of the auxiliary power supply than the rectifier normal operation

 

Make a wider range. When the rectifier is connected to the AC input power, the monitoring part must first work properly, and perform self-check and various conditions detection to determine the rectifier

 

Whether the device can be turned on; In case of extremely high or very low AC voltage, the rectifier has stopped working, but the monitoring part should still work normally to maintain normal monitoring and communication.

 

Some power products have appeared in the process of operation without cause reset phenomenon, in the design of the auxiliary power supply of high power switching power supply, it is analyzed

 

It is found that the auxiliary power supply has many problems under different ac input voltage and different load conditions: narrow ac adaptation range, low load capacity and working wave

 

Shape instability and asymmetry, magnetic bias, electromagnetic interference is very serious.

 

The working principle of the auxiliary power supply of general switching rectifier is that the input AC is rectified into high voltage DIRECT current, and then becomes low voltage high frequency square wave through conversion circuit

 

, and then the rectifier filter circuit becomes the stable low voltage DC power required by the system. Generally, the voltage is stabilized by a three-terminal voltage regulator and driven by a dc output to provide high frequency conversion

 

Pulse control loop voltage feedback signal. The current feedback signal is sampled by the series resistor on the main circuit of the power converter, and the driving pulse of the power converter is UC38

 

44 control chip and its peripheral circuit generation.

 

(Note: AC low voltage is the measured value of the lowest input voltage when the auxiliary power starts to work)

 

It can be seen that under the condition of low AC input voltage and no current feedback, the auxiliary transformer can no longer work normally, and the pulse width of its waveform is different

 

, some wide and some narrow, and jitter, oscilloscope has been unable to grasp the waveform stably. Current feedback, waveform pulse width is also wide and narrow, duty cycle reached

 

The maximum duty cycle of UC3844 is only 50%. If the load is increased, the output voltage will be reduced.

 

How to make the auxiliary power supply can work stably under the upper limit and lower limit voltage of AC input, and how to make the load of the auxiliary power supply from no-load to overload full norm

 

The stable and normal operation of the enclosure is relatively difficult, which involves several technical problems: the voltage resistance and overload capacity of power devices; High frequency transformer set

 

Meter; Drive pulse control loop parameter selection.

 

Second, solutions

 

Through certain theoretical analysis and experimental exploration, the technicians have made corresponding improvements to the auxiliary transformer and control loop, and finally solved this problem. solution

 

The solution is to adjust the auxiliary transformer turns ratio, change the original side turns Np, reduce the original side turns ratio ratio, so that the duty cycle at low voltage is reduced, far less than UC

 

The upper limit of 3844 is 45%; The parameters of RC filter network in the current feedback link of UC3844 are adjusted, and finally obtained through many experiments

 

More ideal parameter, filter capacitor increase. The same secondary winding of the auxiliary transformer was tested again under the same conditions.

 

From these four waveforms, we can see that the improved auxiliary power supply is either very high or very low in the case of AC input (and the starting working voltage is lower than before the improvement

 

In the case of no load or heavy load, the waveform is more stable than before, the pulse width is more symmetrical and balanced, and the carrying capacity is obviously better than the modified

 

In the former. Compared with the duty cycle before the improvement, the duty cycle after the improvement decreases by 7% under the low input voltage, indicating that the AC input of the auxiliary power is increasing the load

 

Under the condition of, the output voltage is still stable, the carrying capacity is obviously stronger than before, and the improvement of auxiliary power supply has achieved obvious effect.

 

Third, experience summary

 

In the improvement process of auxiliary power supply, technicians have started from many aspects, including changing PI adjustment parameters of voltage feedback loop, changing pulse frequency,

 

However, the root cause of the problem was not found. In the case of ac input high and low voltage, light load and overload, the waveform was still jitter

 

The dc output voltage is unstable. When adjusting the RC filter network parameters of the current feedback link of UC3844, several experiments were conducted to find the relatively high voltage

 

Therefore, it can be seen that engineers still need to verify the improved results through continuous experiments after theoretical analysis.

 

The conclusion above is also useful for other low-power switching power supplies using the same circuit, and the RC filter network of the current feedback link of the control chip can be changed by this method

 

The specific parameters are different for each circuit, but the direction of improvement is the same.