LLC resonant converters are becoming increasingly popular in industry power applications. In order to achieve higher efficiency, the rectification diodes are replaced with SR MOSFETs to reduce the conduction losses. And it is common to use digital control for LLC converters due to flexibility and scalability, especially in higher power level applications. The SR control signals usually follow the primary side PWM signals, and the desired SR driving signal under different switching frequency f_S is given in Figure 1-1 [1]. With digital control schemes, SR operation could be divided into two modes: the SR pulse width is equal to approximately half of the switching period if f_S is above the LLC resonant frequency; the SR turn-off edge is determined by approximately half of the resonant period if below or equal to resonant frequency, that is SR clamp mode. However, the present schemes could not ensure the turn-on and turn-off edges are exactly at the SR current zero crossing point, which will vary during different load and input voltage conditions. Turning the SR on/off too early or too late will result in lower efficiency, negative current, or high drain-to source stresses, especially during the load/line transient operation [2].

In order to solve the SR control challenges, a smart gate driver UCD7138 was released with body-diode conduction sensing in the market. The UCD7138 gate driver senses the body diode conduction of the SR and reports to UCD3138A(A version of UCD3138 controllers), to achieve adaptive SR on-time control [3]. However, the turn-off edge optimization is handled by the DTC interface of UCD3138A, which also limits UCD7138’s usage with other MCUs. The application report discusses how to use C2000 devices together with UCD7138 to provide an intelligent SR control scheme, mainly with the below three control targets, which it is challenging for conventional solutions to achieve.

• Minimize the body diode conduction time for high efficiency
• Fast negative current prevention for robust operation
• Automatically compensate the component variations during the SR clamp mode

The proposed control schemes apply for any third-generation of C2000 devices with configurable logic block (CLB) module inside.