Noise susceptibility of precision temperature sense architectures in highly integrated power converters

The design specifications for power converters are getting continuously tighter, keeping pace with the ever-increasing power requirements of the loads they supply. A step-down DC-DC converter used in modern computing applications must deal with higher load currents and lower output voltages, provide a faster transient response and lower output ripple, and do so while maintaining the maximum possible efficiency. The result of these design constraints is that the sources of noise on a power converter, related to the inherent high di/dt and dv/dt switching events, are increasing in severity. In addition, converter circuits must also be designed with as small a footprint as possible, and be used on ever more tightly packed PCB layouts, meaning that other circuits are much more susceptible to the increased noise. In recent years, companies such as Intel and IBM have presented several cases where their server platforms have suffered from signal integrity issues caused directly by such converter noise. In the work published, the affected signals were digital in nature, for example high-speed differential signals. In the research to be discussed in this thesis, an analog circuit is the victim, namely a high-precision low-voltage VBE temperature sensing circuit. The output signal of this circuit was seen to become grossly inaccurate as the DC-DC load current was increased, meaning that the circuit was unusable. The work presented in this thesis, then, concerns the investigation of this issue. First, experimental data collected from the original power module and then a dedicated test-board are presented. A plausible hypothesis and model are next proposed, where it is considered that (1) switching noise from the DC-DC converter switching loop is inductively coupled to the VBE circuit; and (2) if the noise is severe enough, it is effectively rectified by the NPN transistor in the circuit, resulting in a VBE error. Further experimental data is then collected, and it will be seen that there is a strong correlation between simulation and measurement. From the work done, the following design guidelines are proposed: (1) The bias currents used to generate the VBE signal should be increased to reduce susceptibility to the rectification effect. (2) The value of the noise capacitor used in typical VBE circuits should be kept as low as possible, as it has a significant effect on the error.