Digital Implementation of Adaptive Synchronous Rectifier Driving Scheme For High-frequency LLC Converters with Microcontroller
To improve the overall efficiency of an LLC resonant converter, synchronous rectifiers (SRs) should be employed. The SR gate driving signals are important to the efficiency of LLC resonant converters, due to the discrepancy between the driving of primary switches and the driving of secondary switches. Digital controllers have superior advantages over analog controllers. Among digital controllers, cost-effective microcontrollers (MCUs) are preferred in industrial applications. Also, it is important to implement adaptive SR driving by the MCU, instead of using additional SR control chips. The drain to source voltage of the SR is sensed and compared with the threshold voltage to detect the paralleled body diode conduction. For a conventional LLC converter, the SR turn-ON time is synchronized with the primary side switches. The SR turn-OFF time is tuned to eliminate the body diode conduction, based on the output of the comparator, which is connected to the external interrupt of the MCU. This is shown in Fig. 1. For a high frequency LLC converter, if the SR turn-OFF time is tuned every switching cycle, the CPU of the MCU will be occupied most of the time. To release the burden off of the MCU, the output of the comparator is connected to a ripple counter, instead of the external interrupt of the MCU. This is shown in Fig. 2. The SR turn-ON time is still synchronized with the primary side switches, but the SR turn-OFF time is tuned based on the status of the counter output. The counter is cleared every 3rd switching cycle for a 500 kHz LLC converter. The adaptive SR driving for a conventional LLC converter is verified on a 130 kHz LLC converter with a TMS320F2808 MCU. The adaptive SR driving for a high frequency LLC converter is verified on a 500 kHz LLC converter with a TMS320F28027 MCU. Desired performance is achieved in both cases.