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Flyback transformer saturation and transient effects

source:Industry News Popular:adapter release time:2021-06-09 09:43:25 Article author:sznbone

  Note: The magnetic flux level of the magnetic core is selected under the conditions of minimum input voltage and maximum pulse width. It can be seen that the weak point of magnetic core saturation under high input voltage is retained. However, the pulse width required to transmit power under high voltage conditions will be narrowed accordingly, and the transformer will not saturate.

Flyback transformer saturation and transient effects(图1)

  Under transient load conditions, when the power adapter is lightly loaded and working at a high input voltage, if the load needs to be increased suddenly, the control amplifier will immediately widen the driving pulse to provide additional power. As a result, the input voltage and pulse width are both maximum within a short period of time, and the transformer will saturate, which will lead to failure.

  To prevent this, the following points should be considered.

  (1) Design the transformer under the condition of higher voltage and maximum pulse width. This requires lower magnetic flux density and more primary turns. This has the disadvantage of reducing the efficiency of the transformer.

  (2) The control circuit must be able to withstand high voltage conditions and maintain the pulse width at a safe value during transient conditions. Sometimes this is difficult to do because the response time to the current is relatively slow.

  (3) The third point is to provide a double pulse current limit for the drive transistor Q1. The current-limiting circuit will determine the core saturation caused by the sudden increase in the primary current and prevent the pulse width from further increasing. This method has the fastest response time and is the recommended technique. Current-type control automatically provides this function.

  The charger manufacturer Jiuqi Industrial Co., Ltd. gave a quick and practical design method for the flyback transformer. Many examples show that the results obtained with this simple method are often close to the optimal design. This method quickly provides transformer working standards for further design and evaluation of power adapters.

  In this design example, the wire size, wire shape, or winding topology is not specifically specified. These are definitely basic issues to be considered. Designers should refer to Chapter 4 of Part Three, where there is a detailed discussion of these factors. It is important to just fill the effective area of the winding frame with the number of turns of the wire, but high-frequency transformers cannot do this. Due to the skin effect (see Appendix 4. B), the copper loss generated by this method can easily exceed the optimal design value by 10 times or more.


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