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Transformer Optimization of DCX for Higher Efficiency and Less Output Capacitors of Sigma Voltage Regulator

Fig. 1. Circuit Topology of Sigma VR.
Sigma voltage regulator is attractive from not only the higher efficiency performance by its special circuit topology but also its less output capacitor configuration by its faster DCX current response as shown in Fig 1. As a critical part of Sigma VR, DCX determines almost the overall performance, especially when DCX handle most of the Sigma power. As a determined component of DCX, the transformer impacts the efficiency (Rout) and transient performance of DCX (Lout). Smaller Rout and Lout are preferred for smaller loss and faster transient dynamic response.

An optimization example of the DCX transformer for smaller Rout and Lout are shown in Fig.2 using the concept of Matrix transformer. Take the 6:1 transformer design for example, the original design of this transformer is just simply composed by a six primary winding and one set of secondary winding rounding the same core. By matrix transformer concept, the primary winding could be divided into two in-series 3:1 winding, and the secondary winding could be two set of winding paralleling together.

By this new structure, the winding resistance of the transformer could be reduced to half of the original design, and the efficiency can be improved. Not only will this structure reduce the winding resistance, but also the leakage inductance of the transformer will be which is reduced by around 50%. Further improvement of this concept can be applied by dividing the primary winding into three 2:1 in-series windings, three set of secondary winding are paralleled. This further reduces the winding resistance as well as the leakage inductance. The efficiency improvement and less output capacitors can be achieved by the smaller Rout and Lout as shown in Fig 3.

Fig. 2. Transformer Optimization for Smaller Lout and Rout.
Fig. 3. Efficiency Improvement and Output capacitor reduction by optimizing DCX transformer.
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