Transient Effect of Magnetic Saturation in Transformers

Note that you should check whether the magnetic flux is close to saturation under the conditions of minimum input voltage and maximum pulse width. Since the pulse width will become narrower when the input voltage is high, the magnetic core is far away from saturation.

In case of transient load, when the input voltage is high and the load current is low, if the load current increases suddenly, the control circuit will immediately increase the pulse width to provide supplementary power. In this way, the input voltage and pulse width will both become maximum at the same time, even if it is only for a short time, the transformer will be saturated and cause loss of control and failure. The following are some measures to avoid loss of control.

(1) The transformer is designed according to high input, voltage, and wide pulse, so that the core works at low flux density and has more primary windings. The disadvantage of this method is that it reduces the efficiency of the transformer and increases the volume and weight of the transformer.

(2) Improve the performance of the control circuit so that the switch tube can respond quickly within the current working cycle. The UC1846/UC1847 current mode control chip can automatically achieve this limiting action.

(3) The control circuit should operate under a highly sensitive judgment state and be able to quickly limit the pulse width when a high input voltage appears so that it remains at a safe value.