Digital Control of a Series-Loaded Resonant Converter

Abstract

Primarily because of its low cost and ease of implementation, analogue control has been the dominant control strategy in modern switch-mode power supply designs. The ’on/off’ nature of power switches is essentially digital, which makes it tempting for power electronics engineers to combine the emerging capability of digital technologies with existing switch-mode power supply designs. Whereas an analogue controller is usually cheaper to implement, it lacks the flexibility and capacity to implement the complex control functions which a digital controller can offer. The research presented in this thesis addresses the practical implementation of a digital controller for a Series-Loaded Resonant Converter (SLR). The resonant frequency of the SLR converter is around 60 kHz, and the switching frequency varies up to around 80 kHz to regulate the 12V dc output voltage across a 100W , variable resistive load, from a variable 46.6V –60.2V input voltage. This provides a fair challenge for digital waveform generators as the digital processor needs to have a high clock rate to produce high speed, high resolution and linearly varying frequency square waves, to regulate the output voltage with adequate resolution. Digital compensation algorithms also need to be efficient to minimise the phase lag caused by the instruction overhead. In order to completely understand the control needs of the SLR converter, an analogue controller was constructed using a UC3863N. The feedback compensation consists of an error amplifier in an integrator configuration. Digital control is accomplished with a TMS320F2812 Digital Signal Processor (DSP). Its high throughput of 150 MIPS provides sufficient resolution to digitally generate linearly varying frequency switching signals utilising Direct Digital Synthesis (DDS). Time domain analysis of the switching signals, shows that the DDS generated square