Attention should be paid to check whether the magnetic flux is close to saturation in the condition of minimum input voltage and maximum pulse width. Due to the high input voltage, pulse width will be narrowed, so the core is far from saturated.

In the transient load, when the input voltage is higher, and the load current is relatively small, if the load current suddenly increases, then the control circuit will immediately increase the pulse width in order to provide additional power. In this way, the input voltage and pulse width are both changed to the maximum, even if it is only a short period of time, the transformer will be saturated, and cause runaway and failure. Here are a few measures to avoid out of control.

Transformer (1) in accordance with high input, voltage, wide pulse design, the core work in low magnetic flux density and wound more primary winding. The disadvantage of this method is to reduce the transformer efficiency and increase the volume weight of the transformer.

(2) to improve the performance of the control circuit, so that the switch tube can quickly respond to the control in the current operating cycle. UC1846 / UC1847 current mode control chip, can automatically achieve this limit action.

(3) the control circuit should work under the condition of high sensitivity, when the high input voltage appears, it can limit the pulse width very quickly, so that it is still in the safe value.