In the research and application of flyback switching power supplies, a noteworthy phenomenon is that the auxiliary winding voltage increases with the increase of secondary output power. A thorough understanding of the principles behind this phenomenon is crucial for optimizing the design of flyback switching power supplies, improving their performance, and ensuring their stable operation.
A flyback switching power supply, a common switching power supply topology, mainly consists of a switching transistor, a transformer, an output rectifier and filter circuit, and a control circuit. Its operation can be divided into two main stages: the transistor turn-on stage and the transistor turn-off stage.
During the conduction period of the switching transistor, the input voltage is applied to the primary winding of the transformer, and current flows through the primary winding. Electrical energy is stored in the transformer in the form of magnetic field energy. At this time, due to the same-name terminal relationship of the transformer, the voltage induced on the secondary winding and auxiliary winding causes the rectifier diode to be in the off state, and no energy is transferred to the load.
When the switching transistor is turned off, the current in the primary winding drops rapidly. According to the law of electromagnetic induction, the magnetic field energy stored in the transformer begins to be released. The voltage induced on the secondary and auxiliary windings changes direction, the rectifier diodes in the secondary winding turn on, and energy is transferred to the load through the secondary winding to provide electrical power to the load; the auxiliary winding provides the necessary power for the control circuits, etc.
The intrinsic relationship between auxiliary winding voltage and secondary output power
(I) From the perspective of energy conservation
From the perspective of energy conservation, when a flyback switching power supply is operating, the input electrical energy is ultimately transferred to the secondary load and the circuit connected to the auxiliary winding through the transformer. When the secondary output power increases, it means that more energy needs to be transferred from the primary winding to the secondary winding through the transformer. In this energy transfer process, the transformer acts as the medium for energy transfer, and there is a close energy connection between its various windings.
Since the core characteristics and turns ratio of a transformer are fixed, the primary winding needs to store more energy during the energy storage phase to meet the increased output power demand of the secondary winding. This requires a larger current in the primary winding during the switching transistor's conduction period. According to the law of electromagnetic induction, an increase in the primary winding current will inevitably lead to an increase in the change of magnetic flux in the transformer core.
The auxiliary winding and primary winding are wound on the same magnetic core and are interconnected through magnetic coupling. When the change in magnetic flux in the core increases, according to Faraday's law of electromagnetic induction (where is the induced electromotive force, is the number of turns in the winding, and is the magnetic flux), the induced electromotive force in the auxiliary winding will also increase accordingly. Ignoring the influence of the auxiliary winding's own resistance and the equivalent resistance of the load on the voltage, the voltage on the auxiliary winding will also increase.
(II) Load Effect and Feedback Mechanism
Flyback switching power supplies are typically equipped with feedback control circuitry to stabilize the output voltage. When the secondary output power increases, the secondary output voltage tends to decrease. To maintain a stable secondary output voltage, the feedback control circuitry responds by increasing the on-time of the switching transistor, allowing the primary winding to store more energy within a single switching cycle.
As the primary winding's on-time increases, the peak current in the primary winding increases, further increasing the change in magnetic flux in the transformer core. Similarly, based on the principle of electromagnetic induction, the voltage induced in the auxiliary winding also increases. In this process, the load effect (the influence of secondary output power changes on the output voltage) and the feedback mechanism (adjustment of the switching transistor's on-time to stabilize the output voltage) work together, causing the auxiliary winding voltage to increase with the increase of secondary output power.
(III) Influence of Transformer Parameters
Transformer parameters, such as the turns ratio, significantly influence the relationship between the auxiliary winding voltage and the secondary output power. Assume the transformer has *n* turns in the primary winding, *n* turns in the secondary winding, and *n* turns in the auxiliary winding. According to the transformer voltage transformation formula (where *n* is the primary winding voltage and *n* is the secondary winding voltage) and (where *n* is the auxiliary winding voltage), with a fixed primary winding voltage and turns ratio, when the secondary output power increases, causing changes in the primary winding current and magnetic flux, the auxiliary winding voltage will change accordingly based on the turns ratio.
If the auxiliary winding has relatively few turns, the induced voltage change may be larger for the same change in magnetic flux; conversely, if it has more turns, the voltage change will be relatively smaller. However, the overall trend is that the auxiliary winding voltage increases with the increase of secondary output power.
The phenomenon that the auxiliary winding voltage of a flyback switching power supply increases with the secondary output power is the result of a combination of factors, including its operating principle, the law of conservation of energy, load effects and feedback mechanisms, and transformer parameters. A deeper understanding of this phenomenon helps engineers better optimize circuit parameters, improve power supply performance and stability, and meet the needs of various practical application scenarios when designing and applying flyback switching power supplies.
