In today's smart switch-mode power supplies, there is an internal microprocessor or DSP for internal monitoring and communication purposes. The microprocessor chip has stringent requirements for the power supply, necessitating a stable amplitude and absolutely no significant spikes or harmonics that could cause electromagnetic interference. Moreover, the auxiliary power supply must have a wider AC adaptability range than the rectifier's normal operating range. When the rectifier is connected to AC input power, the monitoring section must operate normally first, conducting self-checks and various status checks to determine if the rectifier can be powered on. In the event of high or low AC voltages, even if the rectifier has stopped working, the monitoring section must continue to operate normally, maintaining regular monitoring and communication. Avionics Power Supply
Certain power supply products experienced unintended resets during operation. During the design of auxiliary power for high-power switching power supplies, an analysis revealed numerous issues with the auxiliary power under different AC input voltages and load conditions: narrow AC adaptability range, low load capacity, unstable and highly asymmetric working waveforms, magnetic bias, and severe electromagnetic interference.
The working principle of the auxiliary power supply for general switching rectifiers is as follows: the input AC power is rectified into high-voltage DC power, then converted into low-voltage high-frequency square waves by the conversion circuit, and further rectified and filtered into the stable low-voltage DC power required by the system. This is typically stabilized by a three-terminal voltage regulator and provides a DC output for high-frequency conversion drive pulse control loop voltage feedback signals. Current feedback signals are taken from the main power conversion circuit by串联 resistors, and the drive pulses for the power conversion tubes are generated by control chips like the UC3844 and their peripheral circuits. Aviation Power
(Note: Low voltage during communication refers to the actual measured low input voltage when the auxiliary power supply starts to operate)
Under lower communication input voltages and without current feedback, auxiliary transformers are unable to function properly. Their waveform pulse widths vary, some being wide and others narrow, with oscillations occurring. The oscilloscope is unable to stabilize the waveform capture. With current feedback, the pulse widths also vary, reaching a duty cycle of 47%. However, the UC3844's high duty cycle is only 50%. If the load is increased, the output voltage will decrease.
Ensuring stable operation of auxiliary power supplies at both upper and lower limits of AC input voltage, as well as maintaining stable and normal operation over the entire range from no load to overload, presents significant challenges. This involves several technical hurdles: the voltage and overload capacity of power devices; the design of high-frequency transformers; and the selection of parameters for the drive pulse control circuit.
