In today's intelligent switch-mode power supplies, an internal microprocessor or DSP is typically used for internal monitoring and communication. The microprocessor chip has stringent requirements for the power supply, demanding a stable amplitude and absolutely no significant spikes or harmonics to avoid electromagnetic interference. Additionally, 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 function properly first, conducting self-checks and various status checks to determine if the rectifier can be turned on. In the event of high or low AC voltage, even if the rectifier has stopped working, the monitoring section must continue to operate normally, maintaining regular monitoring and communication.
Certain power supply products have experienced unintended resets during operation. During the design of auxiliary power for high-power switching power supplies, an analysis was conducted and revealed numerous issues with the auxiliary power supply under various AC input voltages and load conditions: narrow AC adaptability, low load capacity, unstable and extremely asymmetric working waveforms, bias magnetization, and extremely severe electromagnetic interference.
The working principle of a general switch-mode rectifier auxiliary power supply is: The input alternating current is rectified into high-voltage direct current, then transformed into low-voltage high-frequency square waves through a conversion circuit. These square waves are then passed through a rectifier and filtering circuit to become the stable low-voltage direct current required by the system. It is typically regulated by a three-terminal voltage regulator and provides a high-frequency switching drive pulse control loop's voltage feedback signal through a single DC output. 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.
Note: The communication low voltage is the actual measured value of the low input voltage when the auxiliary power supply starts to operate. Aviation ground power 2, 36V aviation power supply 1, 400Hz variable frequency power supply 7, 115/200V power supply 2, 27V DC power supply 5, 400Hz power supply 12, aviation power
Under lower communication input voltages and without current feedback, auxiliary transformers are unable to operate normally. The pulse widths of their waveforms vary, some being wide and others narrow, with oscillations occurring, making it impossible for the oscilloscope 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 that auxiliary power supplies can operate stably at both the upper and lower limits of AC input voltage, as well as maintaining stable and normal operation across the entire range from no load to overload, presents significant challenges. This involves several technical hurdles: the voltage and overload capabilities of power devices; the design of high-frequency transformers; and the selection of parameters for the drive pulse control circuit.
