Fundamentals of Power Electronics 1 Chapter 19: Resonant Conversion Reduction of power converter size through increase of switching frequency Increasing switching frequency reduces value and size of filter inductances and capacitances Up to a point, increasing switching frequency reduces transformer size Increasing switching frequency increases switching loss: P sw = f sw ∆v ds Q sw Much R&D effort has been devoted to increasing the switching frequency and reducing the loss in high- density power supplies Approaches to achieve these goals include use of resonant converters and soft switching techniques

Fundamentals of Power Electronics 2 Chapter 19: Resonant Conversion 4.3. Switching loss Energy is lost during the semiconductor switching transitions, via several mechanisms: Transistor switching times Diode stored charge Energy stored in device capacitances and parasitic inductances Semiconductor devices are charge controlled Time required to insert or remove the controlling charge determines switching times

Fundamentals of Power Electronics 3 Chapter 19: Resonant Conversion Transistor switching with clamped inductive load Buck converter example transistor turn-off transition Loss:

Fundamentals of Power Electronics 4 Chapter 19: Resonant Conversion Efficiency vs. switching frequency Add up all of the energies lost during the switching transitions of one switching period: Average switching power loss is Total converter loss can be expressed as where P fixed = fixed losses (independent of load and f sw ) P cond = conduction losses

Fundamentals of Power Electronics 5 Chapter 19: Resonant Conversion Efficiency vs. switching frequency Switching losses are equal to the other converter losses at the critical frequency This can be taken as a rough upper limit on the switching frequency of a practical converter. For f sw > f crit, the efficiency decreases rapidly with frequency.

Fundamentals of Power Electronics 6 Chapter 19: Resonant Conversion Soft switching: Zero-voltage and zero-current switching Conduction sequence: D 1 –Q 1 –D 2 –Q 2 Q 1 is turned on during D 1 conduction interval, without loss Soft switching can mitigate some of the mechanisms of switching loss and possibly reduce the generation of EMI Semiconductor devices are switched on or off at the zero crossing of their voltage or current waveforms

Fundamentals of Power Electronics 7 Chapter 19: Resonant Conversion Soft switching in a PWM converter Example: forward converter with active clamp circuit Forward converter Switching transitions are resonant, remainder of switching period is not resonant Transistors operate with zero voltage switching Beware of patent issues

Fundamentals of Power Electronics 8 Chapter 19: Resonant Conversion Analysis of resonant converters Series resonant dc-dc converter example Complex! Small ripple approximation is not valid Need new approaches: Sinusoidal approximation State plane analysis

Fundamentals of Power Electronics 9 Chapter 19: Resonant Conversion Outline of course 1. Analysis of resonant converters using the sinusoidal approximation Classical series, parallel, LCC, and other topologies Sinusoidal model Zero voltage and zero current switching Resonant converter design techniques based on frequency response 2. Sinusoidal analysis: small-signal ac behavior with frequency modulation Spectra, beating, and envelope response Phasor transform method 3. State-plane analysis of resonant, quasi-resonant, and other soft-switching converters Fundamentals of state-plane and averaged modeling of resonant circuits Exact analysis of the series and parallel resonant dc-dc converters

Fundamentals of Power Electronics 10 Chapter 19: Resonant Conversion Outline, p Resonant switch and related converters Quasi-resonant topologies and their analysis via state-plane approach Quasi-square wave converters Zero voltage transition converter Soft switching in forward and flyback converters Multiresonant and class E converter 5. Server systems, portable power, and green power issues Modeling efficiency vs. load, origins of loss Variable frequency approaches to improving light-load efficiency –DCM –Burst mode Effects of parallel modules DC transformers