Abstract: The single event effect threshold of two kinds of pulse width modulation controller is verified by heavy ion irradiation test on facilities of aerospace power module with UC1825L and UC1825AL being used, respectively. It is shown that the robustness of UC1825L against the single event effect is better than that of UC1825AL. Under the single event effect,, the output voltage of UC1825AL will fall in a short time between 40 ms and 50 ms, while no influence is observed in the power supply with UC1825L. The mechanism of the pulse width modulation controller being influenced by the single event effect is analyzed. By comparative analysis of the internal structure of the two PWMs, it is found that the UC1825AL has an extra restart delay latch and an extra fault latch with the function to reset the soft start. Those two latches may account for the sensibility of UC1825AL to the single event effect.