DATA CENTRES ANALOG DEVICES
step-down conversion in data centres and telecom systems
Ya Liu, Jian Li, San-Hwa Chee and Marvin Macairan, all with Analog Devices, highlight that there has been a shift in data centre and telecom power system design.
K
ey applications manufacturers are replacing complex, expensive isolated 48V/54V step-
non-isolated, high density step-down regulators (Figure 1). Isolation is not necessary in the regulators’ bus converter since the upstream 48V or 54V input is already isolated from hazardous ac mains. For a high input/output voltage application (48V
to 12V), a conventional buck converter is not an ideal solution because component size tends to be larger. That is, a buck converter must run at a low switching frequency (for example, 100kHz to output voltage. The power density of a buck converter is limited
by the size of passive components, especially the bulky inductor. The inductor size can be reduced by increasing the switching frequency, but this reduces losses and leads to unacceptable thermal stress. Switched capacitor converters (charge pumps) size over conventional inductor-based buck converters. In a charge pump, instead of an inductor, energy from input to output. The energy density of capacitors is much higher than inductors - improving power density by a factor of 10 over a buck regulator. However, charge pumps are fractional converters – they do not regulate the output voltage – and are not scalable for high current applications. An LTC7821-based hybrid converter has the charge pumps: output voltage regulation, scalability, regulates its output voltage with closed-loop control just like a buck converter. With peak current-mode control, it is easy to scale the hybrid converter up for higher current levels (for example, a single-phase design for 48V to 12V/25A to a 4-phase design for 48V to 12V/100A). All switches in a hybrid converter see half of the
input voltage in steady state operation, enabling the use of low voltage rating MOSFETs to achieve good converter are lower than a conventional buck converter, enabling high frequency switching.
14 September 2024 Irish Manufacturing
Figure 1. A traditional telecom board power system architecture with an isolated bus converter. The isolated bus converter is not necessary in systems where 48V is already isolated from the ac mains. Replacing the isolated converter with a non-isolated hybrid converter significantly reduces complexity, cost, and board space requirements
above 97% at full load is attainable with the LTC7821 switching at 500kHz. To achieve similar LTC7821 would have to operate at a third of the frequency, which results in a much larger solution size. Higher switching frequencies allow the use of smaller inductances, which yield faster transient response and smaller solution size (Figure 2). The LTC7821 is a peak current-mode hybrid
converter controller with the features required for high density step-down converter for an intermediate bus converter in data centres and telecom systems. The LTC7821’s key features include:
• Wide VIN range: 10V to 72V (80V abs max) • 1.5MHz
• Integrated quad ~5V N-channel MOSFET drivers
• RSENSE or DCR current sensing • Programmable CCM, DCM, or Burst Mode® operation
• CLKOUT pin for multiphase operation • Short-circuit protection • • Monotonic output voltage start-up • 32-lead (5mm × 5mm) QFN package
Figure 2. Size comparison of a non-isolated buck converter and equivalent 48V to 12V/20A hybrid converter
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