A fixed‑ratio converter is a DC‑DC converter whose output voltage is a fixed fraction of the input voltage. The converter provides no regulation and the input‑to‑output voltage range is defined by the “turns ratio” of the device. This turns ratio, referred to as the K factor, is expressed as a fraction relative to its voltage step‑down capability. K factors can range from K = 1 to as low as K = 1/72in point‑of‑load converters.

Typical inputs are low voltage (LV), high voltage (HV) and ultra‑high voltage (UHV). K factors are then selected based on the desired PDN voltage and PoL design.

Fixed‑ratio converters can be isolated or non‑isolated and, as is the case with Vicor converters, capable of bidirectional power flow and voltage conversion. For example, a K 1/12 fixed‑ratio converter with bidirectional capability can be operated as a boost converter with a K of 12/1.

Additional design flexibilities offered by fixed‑ratio converters include ease of paralleling to meet higher power demands and connecting converter outputs in series to provide higher output voltages, effectively changing the K factor by N • K (N = number of bus converters).

What is a Sine Amplitude Converter (SAC™)?

In a “hard switched” converter the output power is proportional to the converter duty cycle, which can be varied to provide more or less power from the secondary. This category of circuits is called Pulse Width Modulated or PWM converters. The switching frequency is practically limited to several hundred kHz by the high levels of power dissipated by the switching devices. Despite its shortcomings, the PWM converter is superior to linear regulators in applications where the input/output voltage ratio was large; the PWM converter started the movement to the widespread use of DC‑DC converters.

The SAC is a transformer‑based series resonant topology. Unlike regulated quasi‑resonant ZCS / ZVS converters, the Sine Amplitude Converter operates at a fixed frequency equal to the resonant frequency of the primary‑side tank circuit.

The switching FETs in the primary are locked to the natural resonant frequency of the primary and are switching at zero crossing points, eliminating power dissipation in the switches (boosting efficiency) and greatly reducing the generation of high‑order noise harmonics (requiring less filtering of the output voltage). The current in the primary resonant tank is a pure sinusoid rather than a square wave or a partially sinusoidal waveform as seen in prior generations of converters. This also contributes to reducing harmonic content and provides a much cleaner output noise spectrum.

The leakage inductance of the primary is minimized in a Sine Amplitude Converter since it is not the critical energy storage element.

The SAC can therefore operate at a much higher frequency allowing for a much smaller transformer, and increasing both power density and efficiency. The Vicor BCM operates at a frequency of several MHz; this frequency is fixed regardless of load. In response to an increased load on the secondary, the Sine Amplitude Converter reacts by increasing the amplitude of the sinusoidal current on the primary resonant tank. This in turn increases the amount of energy coupled into the secondary, countering the increased load. When the load current is reduced, the amplitude of the sinusoid decreases approaching zero under “no‑load” conditions.

The output impedance of the Vicor bus converter is extremely low, reflecting the low output impedance of the resonating tank circuit on the primary side of the transformer, which ideally presents zero impedance at its resonant frequency. This impedance is essentially flat up to approximately two‑thirds of the resonant frequency. This is approximately one half the output impedance of a conventional IBC.

The sinusoidal nature of the current in the primary leads to advantages in the electrical noise profile of the SAC. The output noise spectrum is very narrow with major components at the switching frequency and two times the switching frequency (due to the full wave rectification of the output). Output filtering is easily achieved with small high‑frequency ceramic‑type capacitance.

Selecting fixed‑ratio converters

Vicor fixed‑ratio converter modules offer a large selection of voltage ranges, K factors, isolation or non‑isolation and bidirectional operation. There are four primary families that address different bulk power, intermediate bus and point‑of‑load conversion needs:

1. BCM® isolated bus converters
2. NBM™ non‑isolated bus converters
3. VTM™ isolated and non‑isolated current multipliers, used as PoL converters in conjunction with an upstream regulator (PRM™)
4. MCM™ modular current multipliers, used as PoL converters with an upstream regulator and driver (MCD)

The Vicor Sine Amplitude Converter (SAC™) topology is used in all Vicor converter modules, enabling them to outperform competing fixed‑ratio converters in virtually every relevant metric, including conversion efficiency, power density, package profile, transient response and bandwidth.

Solving the toughest problems in automobiles and data centers

Power delivery networks are undergoing significant changes due to the soaring power demands within many end‑markets and applications. As new features are added and performance levels advance, higher voltages are being used for power delivery as a way to reduce the physical size, weight and cost of the PDN itself.

Two markets undergoing rapid changes to their PDN architectures are automobiles and data centers. The electrification of vehicles to meet new CO₂ emission standards and the move to pure electric vehicles has seen the addition of 800V, 400V and 48V conversion and regulation requirements within vehicles.

The data center market is adding 380V and 48V PDNs to support the addition of artificial intelligence (AI) and high‑performance computing. Processor steady‑state and peak current demands have been on an almost exponential rise. This dramatic increase is causing a PDN dilemma at the point‑of‑load, requiring a complete rethinking of architectures, topologies and packaging to solve the problem.

The high‑density current multiplier module (either VTM or MCM) can be placed extremely close to the processor, either laterally alongside the processor or directly beneath, delivering power vertically into the processor power pins above. At 1000 amps of current a PDN resistance of 1μΩ translates to 1W of power loss. Typical PDN resistances with traditional multiphase synch buck VRs have PDN resistances of 200μΩ (200W power loss) which renders system performance unacceptable. Lateral power delivery or vertical power delivery with a Vicor current multiplier lowers the PDN resistance to 50μΩ and 5μΩ respectively.

The unique attributes of Vicor fixed‑ratio converters are enabling power systems engineers to redefine their power delivery architectures to meet the high power and performance demands of their advanced systems. High power density, efficiency and ease of paralleling, plus operational capabilities as step‑down / step‑up converters and as high‑density current multipliers at the point‑of‑load make fixed‑ratio converters one of the most flexible and highest performing DC‑DC converters. Future‑proof your power delivery architecture with Vicor fixed‑ratio converters.