SLYT830 June   2022

 

  1. 1Introduction
  2. 2Selecting CBulk capacitance
  3. 3Baby boost converter design considerations
  4. 4Design implementation and test results
  5. 5References

1 Introduction

In the year 2000, server front end power-supply units (PSUs) – with an AC input to a 12-V/48-V DC rail – achieved around 10 W/in3 of power density, with about 85% peak efficiency [1]. Today, so many server PSUs can meet 80 Plus Platinum (94% peak) and 80 Plus Titanium (96% peak) requirements [2] that the latter requirement, along with very high power density (>90 W/in3), are both now becoming minimum requirements.

One reason why server PSUs can achieve higher power density levels is because of technological innovations in the semiconductor industry. New semiconductor fabrication processes enable devices to have lower parasitics and better figures of merit [3], significantly improving power dissipation and facilitating higher power density.

Topology and architecture innovations are also behind PSUs with high power density. Applying a totem-pole bridgeless power factor correction (PFC) circuit to the AC/DC rectifier stage of new server PSUs – along with wide bandgap devices such as gallium nitride (GaN) and silicon carbide (SiC) (Figure 1-1) – achieves the best converter efficiency over other bridge or bridgeless PFC topologies [4] [5]. Although higher efficiency does minimize the area needed for heat dissipation, a bulk capacitor (CBULK in Figure 1-1) is still required to hold the output voltage in regulation after AC dropout. In order to accomplish this for more than 10 mS, a 3-kW server PSU would need a total capacitance of over 1.3 mF, which would consume at least 30% of the overall space. To further improve power density, you must reduce the bulk capacitance.

Figure 1-1 Server PSU block diagram.

In this article, the concept and operational principles of a “baby” boost converter (a compact boost converter that only operates during AC dropout events) is introduced to reduce the bulk capacitance. Test results on a PFC reference design [6] with a baby boost converter show that a 910-µF bulk capacitor (as opposed to a 1.3-mF capacitor) is enough to hold the output voltage above 320 V for more than 10 mS after AC dropout with a 3-kW load.