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Bi-directional converter shrinks battery array to one cell

Case study: Portable X-ray system

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Source for 28V needed to be lighter, reliable and portable

Source for 28V needed to be lighter, reliable and portable

New solid-state X-ray technology allows medical X-ray scanners to be more portable and battery operated, enabling medical professionals to check patients in any location. Medical X-rays typically need a short pulse of high-intensity radiation energy, which can conveniently be stored and supplied to the X-ray diode by a supercapacitor, in this case charged to 28V. Normally the 28V would be sourced from a number of Li-ion batteries in series. However, in a portable application, this type of battery array and charging system is too large and too heavy. The key goals were:

  • Find an alternative to heavy and bulky multiple-cell battery solutions
  • Fast charging of the supercapacitor requires a high current pulse
  • Long battery life to minimize the need for recharging

Bi-directional converter allowed for just one battery array

Bi-directional converter allowed for just one battery array

Use of a bi-directional DC-DC converter reduced the battery array to just one 2.7V Li-ion cell, reducing size and weight significantly compared to a typical boost converter solution. In addition, the bi-directional converter delivered higher efficiencies when charging the supercapacitor from a low voltage source, extending battery life. Key benefits were:

  • Ultra-small, lightweight solution (8.6cm2 footprint, 16g weight) compared to a boost converter solution
  • Unique bi-directional converters (DC-DC transformer) available as standard products
  • Higher efficiency achieved using low-loss Sine Amplitude Conversion topology

Bi-directional VTM, operating in reverse mode, enabled highly-efficient x12 transformation

A Vicor VTM, operating in reverse, stepped-up the 2.7V input to provide a nominal 32V rail with an efficiency of more than 94%. For the supercapacitor charging circuit, a ZVS Buck-Boost converter regulated the 32V and provided the necessary current limiting. This architecture was accomplished in a footprint of just 8.6cm2. To analyze this power chain, go to Vicor Whiteboard online tool.

The power delivery network

Portable Xray
VTM current multiplier

VTM current multipliers

Input: 0 – 60V

Output: 0 – 55V

Current: Up to 115A

Peak efficiency: Up to 96%

As small as 22.83 x 8.52 x 4.9mm

ZVS buck-boost regulators

ZVS buck-boost regulators

Input: 8 – 60V

Output: 10 – 54V

Power: Up to 150W continuous

Efficiency: Over 98%

10.5 x 14.5 x 3.05mm

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