Zerovoltage transition converters The phaseshifted full bridge converter

Zero-voltage transition converters The phase-shifted full bridge converter Buck-derived full-bridge converter A popular converter for server frontend power systems Zero-voltage switching of each halfbridge section Each half-bridge produces a square wave voltage. Phase-shifted control of converter output ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 1 Efficiencies of 90% to 95% regularly attained Controller chips available Lectures 39 -40

Actual waveforms, including resonant transitions ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 2 Lectures 39 -40

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 ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 3 Lectures 39 -40

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 FPZVT(J), where PZVT(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 4 Lectures 39 -40

Effect of ZVT: reduction of effective duty cycle ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 5 Lectures 39 -40

Phase-shifted control Approximate waveforms and results (as predicted by analysis of the parent hardswitched converter) ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 6 Lectures 39 -40

Diode switching analysis ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 7 Lectures 39 -40

Diode commutation: intervals 3 and 4 ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 8 Lectures 39 -40

Waveforms: ZCS of D 6 ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 9 Lectures 39 -40

Approaches to snub the diode ringing (a) conventional diode snubber ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 10 Lectures 39 -40

Approaches to snub the diode ringing (b) conventional passive voltage-clamp snubber ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 11 Lectures 39 -40

Approaches to snub the diode ringing (c) simplify to one passive voltage-clamp snubber ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 12 Lectures 39 -40

Approaches to snub the diode ringing (d) improvement of efficiency in voltage-clamp snubber ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 13 Lectures 39 -40

Approaches to snub the diode ringing (e) active clamp lossless snubber ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 14 Lectures 39 -40

Approaches to snub the diode ringing (f) primary-side lossless voltage clamp ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 15 Lectures 39 -40

Another application of the ZVT: DC Transformer Operate at a fixed conversion ratio with high duty cycle, leading to high efficiency—avoids the problems of circulating currents Use other elements in the system to regulate voltage PFC 350 V ZVT 5 V DC-DC 1 V Load AC line isolation ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 16 DC-DC Load Lectures 39 -40

Active clamp circuits Can be viewed as a lossless voltage-clamp snubber that employs a current -bidirectional switch See Vinciarelli patent (1982) for use in forward converter Related to other half-bridge ZVS circuits Can be added to the transistor in any PWM converter Not only adds ZVS to forward converter, but also resets transformer better, leading to better transistor utilization than conventional reset circuit ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 17 Lectures 39 -40

The conventional forward converter • Max vds = 2 Vg + ringing • Limited to D < 0. 5 • On-state transistor current is P/DVg • Magnetizing current must operate in DCM • Peak transistor voltage occurs during transformer reset • Could reset the transformer with less voltage if interval 3 were reduced ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 18 Lectures 39 -40

The active-clamp forward converter • Better transistor/transformer utilization • ZVS • Not limited to D < 0. 5 Transistors are driven in usual half-bridge manner: ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 19 Lectures 39 -40

Approximate analysis: ignore resonant transitions, dead times, and resonant elements ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 20 Lectures 39 -40

Charge balance Vb can be viewed as a flyback converter output. By use of a current-bidirectional switch, there is no DCM, and LM operates in CCM. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 21 Lectures 39 -40

Peak transistor voltage Max vds = Vg + Vb = Vg /D’ which is less than the conventional value of 2 Vg when D > 0. 5 This can be used to considerable advantage in practical applications where there is a specified range of Vg ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 22 Lectures 39 -40

Design example 270 V ≤ Vg ≤ 350 V max Pload = P = 200 W Compare designs using conventional 1: 1 reset winding and using active clamp circuit ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 23 Lectures 39 -40

Conventional case Peak vds = 2 Vg + ringing = 700 V + ringing Let’s let max D = 0. 5 (at Vg = 270 V), which is optimistic Then min D (at Vg = 350 V) is (0. 5)(270)/(350) = 0. 3857 The on-state transistor current, neglecting ripple, is given by ig = Dn. I = Did-on with P = 200 W = Vg ig = DVg id-on So id-on = P/DVg = (200 W) / (0. 5)(270 V) = 1. 5 A ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 24 Lectures 39 -40

Active clamp case: scenario #1 Suppose we choose the same turns ratio as in the conventional design. Then the converter operates with the same range of duty cycles, and the on-state transistor current is the same. But the transistor voltage is equal to Vg / D’, and is reduced: At Vg = 270 V: D = 0. 5 peak vds = 540 V At Vg = 350 V: D = 0. 3857 peak vds = 570 V which is considerably less than 700 V ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 25 Lectures 39 -40

Active clamp case: scenario #2 Suppose we operate at a higher duty cycle, say, D = 0. 5 at Vg = 350 V. Then the transistor voltage is equal to Vg / D’, and is similar to the conventional design under worst-case conditions: At Vg = 270 V: At Vg = 350 V: D = 0. 648 D = 0. 5 peak vds = 767 V peak vds = 700 V But we can use a lower turns ratio that leads to lower reflected current in Q 1: id-on = P/DVg = (200 W) / (0. 5)(350 V) = 1. 15 A ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 26 Lectures 39 -40