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Modeling and Control for 48V/1V Sigma Converter

Bode Plot of Control
Fig. 1. Bode Plot of Control to Output Transfer Function with Different Control Methods.
  48V voltage regulator modules (VRMs) are critical for telecom power supplies, and are becoming popular for future data centers. By using a novel sigma converter topology, outstanding performances in terms of efficiency (95.2%) and power density (700 W/in3) have been demonstrated. Both the efficiency and power density are much higher than state-of-the-art solutions. However, both modeling and control of the Sigma Converter are challenging due to the quasi-parallel structure and have not been resolved properly yet. The small-signal model of current mode and V2 control are provided and compared with voltage mode control. The V2 control with active droop control is chosen for high bandwidth and constant load line design.

  Fig. 1 shows the bode plot of constant-on-time current mode and enhanced V2 control, as well as the comparison with voltage mode control, where Vin=54V, Vo=1.4V, D=0.07, n=12, Lr=440nH, L=210nH, Cin=10uF, Co=2mF. From Fig. 1, voltage mode has a low frequency double pole and COT current mode has a low frequency RHP pole (Gain decreases and phase increases). As a result, both voltage mode and COT current mode are not suitable for high-bandwidth design and enhanced V2 control is chosen.

  It is well known that CPU VR must achieve adaptive voltage positioning (AVP) to meet Intel load line specification. In order to meet the AVP requirement, the active droop control scheme is adopted as shown in Fig. 2. With this method, constant load line is achieved at all operating regions.
Active Droop Control
Fig. 2. Proposed Active Droop Control Scheme.


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