3 Corner Cases and Real-Case Challenges

However, in real case practice, even with global load and one-shot load enabled, some unexpected PWM behaviors have been observed occasionally. Even the behaviors are caused by rare corner cases, and only have very low possibilities to happen, but may still cause sever damages, especially in important power supply applications.

In real case LLC applications, in order to achieve better transient performance, the switching frequency range is selected to be a wide range. For example, 20kHz to 200kHz is the switching frequency normally used in server PSU industry, while 100kHz is the typical control loop ISR frequency. During the transient when LLC works in low switching frequency, there is a possibility that multiple control ISR may happen within the same switching period. Following the recommended one-shot load operation, GLDCTL2[OSHTLD] is set to 1 in the first control ISR. Global load will be responded and GLDCTL2[OSHTLD] will be cleared at next CTR=PRD. However, since the switching frequency is low, the second control ISR happens before switching period ends. Since GLDCTL2[OSHTLD] has already been set, the shadow to active would happen once CTR equals to PRD. Under this condition, one-shot load will not work and PWM disorder may happen as explained in Section 2.1.