Factorised power architecture (FPA) enhances power system flexibility by separating the three classic converter functions into two modules: pre-regulator module (PRM) and voltage transformation module (VTM). This architecture can be flexibly deployed, so that if designers want to combine the PRM and VTM, they can; if they want to have the PRM off the board with only the VTM at the point of load, they can do that also. This is because the VTM can provide very low voltages with very high step-down ratios, allowing relatively high voltages to be distributed with minimum I²R losses and freeing the PRM to be mounted away from the load or even on a different board.
Power designers now do not have to make radical choices or compromises; they simply make the right choices, depending on the application.
Figure 1 shows the FPA modules in a basic arrangement. The PRM and VTM can be operated alone, together, open loop, local loop, adaptive loop, remote loop, colocated, separated, paralleled, or combined with conventional power conversion devices (eg, DC-DC converters, point-of-load converters, charge pumps) to achieve the desired power solution.
The recently introduced VICBricks, shown on the cover of this issue of Dataweek, are high-performance DC-DC converters that will introduce V·I Chip technology to many manufacturers of telecom and IT equipment. VICBricks conform to industry-standard packages and functions, while offering unparalleled power density, efficiency, transient response, and cost.
Factorised power architecture, while still in rollout mode, is delivering unprecedented flexibility in structuring power systems at low system cost. Examples of certain existing power configurations are given, and elements of the enabling technology are described.
Application examples
The VTM may be applied to provide an isolated low output voltage at high current from a controlled 48 V source. The overall efficiency of a power system based on FPA, using VTMs and PRMs, exceeds the efficiency of power systems based on distributed power architecture, using DC-DC converters, or the intermediate bus architecture, using intermediate bus converters and non-isolated POL converters. Using a 48 to 1,5 V at 100 A VTM as the basis, let us explore a typical example:
In an 'adaptive' loop implementation as shown in Figure 1, the VTM is connected to the output of a PRM - the 'factorised bus' -and a feedback connection is made between control ports on both devices. The no-load output of the VTM is the voltage at its input multiplied by its transformation ratio, the 'K factor'. The VTM in question, the 48 to 1,5 V model V048K015T100, has a K factor of 1/32; hence, at 48 V d.c. from the PRM, the VTM's output is 1/32 · 48 V = 1,5 V d.c. The output of the VTM can be set over the range of 0,8 to 1,7 V d.c., at no load, and 0,8 to 1,5 V, at full load (100 A), by controlling the output of the PRM from 26 V to 55 V.
The VTM has a very low output resistance, ROUT. This will cause the output voltage to change slightly with load current unless ROUT is compensated by a control loop. Without compensation, the output voltage of the VTM can be expressed as follows:
VOUT = (K · Vf) - (ROUT · ILOAD)
where Vf is the factorised bus input voltage to the VTM.
For the V048K015T100 with an ROUT of 1 mΩ, this equation becomes:
VOUT = (1/32 · Vf) - (0,001 · ILOAD)
In some applications, compensation for this voltage variation may not be necessary. However, the VTM provides a feedback signal to the PRM that will adjust Vf to compensate for this voltage droop. This signal is taken from the input side of the VTM and therefore does not cross the VTM's isolation barrier.
A variation of the VTM called the bus converter module (BCM) can be used open loop to provide an isolated intermediate bus voltage for use with point-of-load converters. The BCM is a high-performance bus converter available in outputs from 3 to 48 V d.c. It is a high efficiency (>96%), isolated, fixed ratio converter operating from an input voltage of 38-55 V d.c. BCMs can provide up to 300 W of DC to power non-isolated point of load converters or other loads and may be directly paralleled for higher power or redundancy.
V·I Chips - the building blocks of FPA
The building blocks of factorised power are called V£I Chips, their name deriving from their ability to multiply currents and divide voltages while preserving the V·I power product essentially constant.
All of the above V·I Chips - PRM, VTM and BCM - are in an SMD 'full-VIC' package that takes up only 6,9 cm² of board area and can be populated onto the board as SMD devices. Only 5,9 mm high, V£I Chips achieve power densities of 74 W/cm³ and current densities of 25 A/cm³.
VTM capabilities are without precedent in terms of performance and breadth of applicability: broad input voltage capability - 1,5-400 V d.c; wide input range - 2:1; available output voltages - 0-400 V d.c.; current multiplication - 200:1-1:200; high current and power capability - 100 A and 300 W; high efficiency - to 97%; and high operating frequency - to 4 MHz.
The high performance of the VTM results from a class of proprietary zero-voltage switching and zero-current switching (ZVS/ZCS) 'sine amplitude converter' (SAC) topologies. The powertrain is a low Q, high frequency, controlled oscillator, with high spectral purity and common mode symmetry, resulting in essentially noise-free operation. The control architecture locks the operating frequency to the power train resonant frequency, optimising efficiency and minimising output impedance by effectively canceling reactive components. ROUT can be as low as 0,8 mΩ from a single VTM. If that is not low enough, or if more power is required, VTMs can be paralleled ad infinitum with accurate current sharing. Quiet and powerful, the SAC-based VTM is an essentially linear voltage/current converter with a flat output impedance up to about 1 MHz.
The performance of the PRM is as impressive as that of the VTM. Based on a patented zero-voltage switching (ZVS) buck-boost regulator control architecture, PRM features include operation with input voltages from 1,5 to 400 V; up to a 5:1 input voltage range; a step up/step down range up to 5:1; full-VIC output power up to 310 W; and conversion efficiency up to 98%.
The PRM topology and its soft switching, ZVS control architecture, maximise conversion efficiency with the output voltage approximately equal to the input voltage and operate at a typical fixed operating frequency of 1 MHz. Like VTMs, PRMs may be paralleled to achieve increased output power. A unique feature of the PRM control architecture is that the switching sequence does not change in either buck or boost mode - only the relative duration of phases within an operating cycle need be controlled.
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