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How to design modular DC-DC systems,
part 5: load considerations Jonathan Siegers
Principal Applications Engineer, Vicor Corporation
&
Vamshi Domudala Application Engineer, Vicor Corporation
The previous tutorials in this series addressed the appropriate filtering, stability, and protection concerns for power modules when used as elements of a complete DC-DC power system. This tutorial is focused on the load-specific aspects of a system designed with power modules. It was once possible to conceptualize the load supplied by a DC-DC module as merely having voltage and current requirements, effectively reducing it to a simple black box. But loads have grown in complexity over the years, and today they have many characteristics relevant to the power delivery network (PDN). Therefore, power system designers must understand the behavior of the load in considerable detail to determine whether it can impact the performance of the DC-DC module. For example, the load might affect the module’s control-loop response and, therefore, its ability to regulate and perform in a stable manner. The load’s behavior might also adversely affect the module’s transient response capabilities.
Every system involves some design considerations that are particular to the load (orange elements of system diagram).
Basic load types and their impacts on control loops, transient response
There are three main load types: resistive, complex, and downstream converters. Resistive loads are the simplest, having no impact on the DC-DC module’s control-loop or transient response at all. A more complex load comprised of resistive, inductive, and capacitive characteristics will certainly impact the module’s control-loop stability and transient response. But the situation becomes less obvious when a DC-DC module is the input to an additional converter module. In this case, the upstream module’s control-loop stability and transient response depend on the control-loop characteristics of the downstream module. In addition to these main load types, some special cases require more careful analysis.
Three basic load types—resistive, complex, downstream power module.
Inductive loads present EMI and performance challenges
Inductive loads present particular challenges for DC-DC power modules at the instance of turn-off. In the following figure, the characteristics of an inductive load are shown for a simple inductor and series switch. Opening the switch breaks the current path from the module to the inductor very quickly, producing a large negative voltage spike as the inductor impedes the change in current passing through it. This action results in a high-frequency noise pulse and, potentially, arcing across the open switch contacts. The voltage spike applied to the DC-DC module’s output (and anything else connected to the bus) can affect the module’s behavior, and the noise pulse will impact the module’s EMI and overall performance. The solution is to add a freewheeling diode connected across the inductive load to provide the required discharge path for the inductor’s stored energy.
Inductive loads have unique features affecting power system design, notably large negative voltage spikes in the instance of switch-off.
Power averaging allows efficient power design for pulse loads Pulse loads, characterized by brief but very high peak load
16 10 | 2021 BISinfotech
