Efficient holdup circuit design to meet power quality requirements for single or dual isolated input bus

Abstract

Reliability, size and cost are essential parameters that are considered for airworthy hardware. The primary function of an avionics grade power supply is to provide the desired regulated output voltages and comply with the various power quality and switching bus transients (also called holdup time) [1]–[3]. These switching bus transients call for energy storage components like capacitor banks, which can store energy during nominal operation and deliver the energy back during these power interruptions. The conventional way to achieve the holdup time is by having the holdup circuitry on the primary side. There have been multiple primary side holdup methodologies for extending the holdup time, like boosting the holdup voltage circuitry. [4]–[11]. There are many drawbacks and limitations associated with conventional design for holdup solutions where the holdup circuit is based on the primary side of the power supply. The holdup capacitors need to be designed to take care of the efficiency of both the step-down buck converter and the DC-DC converter, which are used to regulate the output from the input bus. In the case of a dual input bus system, the galvanic isolation between the two input buses dictates the use of two separate holdup circuitries for the two input buses. Another major drawback with primary side holdup circuitry is that the higher boosted voltage will result in the implementation of safety requirements which will call for a bleeder circuit to dissipate the holdup capacitor energy to reduce the boost voltage to a safe limit. The design solution explained in this paper overcomes the limitations and drawbacks of conventional holdup design solutions with less form fit, lower cost, and higher reliability. The solution is implemented and tested in a dual input bus airworthy system where electrical isolation is needed between the input power buses and the input power bus to output. The holdup circuit architecture discussed in the paper is based on the secondary side and is referenced to the secondary return (chassis). This unique architectural change ensures that only one set of holdup circuitry is sufficient to meet the redundant bus power interrupt requirements without compromising the isolation requirements and higher efficiency.