ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 1 Lecture 37 Zero-voltage transition converters The phase-shifted full bridge converter Buck-derived full-bridge converter Zero-voltage switching of each half- bridge section Each half-bridge produces a square wave voltage. Phase-shifted control of converter output A popular converter for server front- end power systems Efficiencies of 90% to 95% regularly attained Controller chips available

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 2 Lecture 37 Phase-shifted control Approximate waveforms and results (as predicted by analysis of the parent hard- switched converter)

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 3 Lecture 37 Actual waveforms, including resonant transitions

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 4 Lecture 37 Result of analysis Basic configuration: full bridge ZVT Phase shift  assumes the role of duty cycle d in converter equations Effective duty cycle is reduced by the resonant transition intervals Reduction in effective duty cycle can be expressed as a function of the form FP ZVT (J), where P ZVT (J) is a negative number similar in magnitude to 1. F is generally pretty small, so that the resonant transitions do not require a substantial fraction of the switching period Circuit looks symmetrical, but the control, and hence the operation, isn’t. One side of bridge loses ZVS before the other.

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 5 Lecture 37 ZVT Analysis Base quantities: At time t = t 0 :

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 6 Lecture 37 Interval 1

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 7 Lecture 37 Normalized state plane ZVS boundary:

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 8 Lecture 37 Subintervals 2 and 3

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 9 Lecture 37 Subinterval 4

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 10 Lecture 37 Subinterval 5 ZVS: output current charges C leg without requiring J > 1

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 11 Lecture 37 Subinterval 6

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 12 Lecture 37 Subinterval 6 Current i c circulates around primary-side elements, causing conduction loss This current arises from stored energy in L c The current is needed to induce ZVS during next subinterval To maxzimize efficiency, minimize the length of this subinterval by choosing the turns ratio n such that M = V/nV g is only slightly less than 1

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 13 Lecture 37 Subintervals 7 to 11 Subintervals 7 to 11 and 0 are symmetrical to subintervals 1 to 6 Complete state plane trajectory:

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 14 Lecture 37

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 15 Lecture 37 Averaging

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 16 Lecture 37 Phase-shift control

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 17 Lecture 37 Phase shift control

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 18 Lecture 37 Phase shift control: result

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 19 Lecture 37 Effect of ZVT: reduction of effective duty cycle

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 20 Lecture 37 Issues with this converter It’s a good converter for many applications requiring isolation. But… 1.Secondary-side diodes operate with zero-current switching. They require snubbing or other protection to avoid failure associated with avalanche breakdown 2.The resonant transitions reduce the effective duty cycle and conversion ratio. To compensate, the transformer turns ratio must be increased, leading to increased reflected load current in the primary-side elements 3.During the D’Ts interval when both output diodes conduct, inductor Lc stores energy (needed for ZVS to initiate the next DTs interval) and its current circulates around the primary-side elements—causing conduction loss