[Technology Report]
Optimized Power Supplies Beget Superior Data-Center Efficiency
Data centers are conspicuous consumers of power. Power-supply design that maximizes efficiency can deliver big payoffs.
Also, the ability to move the bulk capacitors that store the energy needed to handle the load’s transient current demands from the output of the POL upstream to the input reduced the amount of capacitance needed by the square of the POL’s step-down ratio. Finally, it provides the ability to precisely control the load voltage through the isolation barrier without long, noise-sensitive feedback lines or opto- or magnetic couplers.
Factorized power introduces its own terminology. Isolated voltage transformation modules (VTMs) are at the load, while preregulator modules (PRMs) can be found upstream. Load regulation is performed using feedback to the upstream PRM. The PRM adjusts the factorized bus voltage to maintain the load voltage in regulation.
The key to this is the VTM’s function as a current transformer. It multiplies the current (or divides the voltage) by a “K” factor. This takes place with essentially a 100% transformation duty cycle, so there’s no loss of efficiency at high values of K. Thus, the bus voltage can be (and is) greater than 12 V. In fact, it’s limited only by safety concerns.
The bulk capacitance at the VTM input reflects itself at the POL with a gain equal to the square of the VTM current gain. Only very small amounts of ceramic bypass capacitance, effective over a short time scale of less than a microsecond, are needed at the load.
PRMs employ a ZVS buck-boost control architecture. One can operate with input voltages from 1.5 to 400 V, and step up or step down over a 5:1 range, with a conversion efficiency up to 98%. In normal configurations, the output voltage is approximately equal to the input voltage: 48 V unregulated to 48 V regulated. One PRM can put out up to 300 W, and VTMs and PRMs may be paralleled for higher output power.
A VTM employs a zero-voltage switching and zero-current switching (ZCS/ZVS) topology that Vicor calls a sine amplitude converter, or SAC (Fig. 5). The power train is a low-Q, high-frequency, controlled oscillator with high spectral purity and common-mode symmetry, resulting in practically noise-free operation.
The control architecture locks the operating frequency to the power-train resonant frequency, optimizing efficiency and minimizing output impedance by effectively canceling reactive components. ROUT, an equivalent resistance that summarizes all losses in the VTM, can be as low as 0.8 mO from a single VTM. If that isn’t low enough, or if more power is required, VTMs can be paralleled for current sharing. The SACbased VTM is, for the most part, a linear voltage/current converter with a flat output impedance up to about 1 MHz.
The secondary current in a SAC VTM is a virtually pure sinusoid. The very low, essentially non-inductive output impedance of the VTM allows an almost instantaneous response to a 100% step change in load current. Because there’s no internal regulation circuitry in a VTM, and none of the attendant loop delays and stability issues, no internal control action is required to respond to the change in load. The internal ASIC controller simply continues controlling and synchronizing the operation of the switches to maintain operation at resonance.
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