Determining the Maximum Allowable Heatsink Voltage to Ensure Compliance with a Given Radiated Emissions Specification Xinbo He, Haixin Ke, and Todd Hubing Clemson Vehicular Electronics Laboratory Clemson, SC, USA Abstract—A model to estimate the minimum voltage between a heatsink and a PCB that is required to generate a given radiated electromagnetic field strength from a PCB with attached cables is introduced. The model is based on a previously introduced technique for calculating the maximum possible radiated field due to a known voltage driving a heatsink against a circuit board with attached cables. A closed-form expression is derived that can be used to determine if a measured voltage on a heatsink is capable of generating a field strong enough to exceed an FCC or CISPR radiated emissions limit. Keywords—heatsink, PCB, cable, radiated emissions, FCC limit I. INTRODUCTION Common-mode currents induced on cables attached to printed circuit boards (PCBs) are a well known source of unintentional radiated emissions. The mechanisms by which intentional signals induce common-mode currents on attached cables are generally divided into two categories: current- driven and voltage-driven. The former describes coupling from the signal’s magnetic field and the latter from the signal’s electric field. PCB radiation due to currents induced on attached cables by a heatsink is usually voltage-driven and the source parameter of interest is the voltage between the heatsink and the top of the circuit board [1] [2]. The amplitude of this radiation depends on the level of the differential-mode voltage and the geometry of the PCB, cable and heatsink. Calculation of the exact radiated fields from any specific geometry is complicated and generally requires sophisticated numerical modeling techniques. It is helpful to have a simplified model that calculates the maximum possible EMI level attainable from a given heatsink and PCB cable geometry. A model like this is described in [4]. Conversely, suppose the maximum radiated field was the known quantity and the voltage between the heatsink and PCB was the object of the calculation. With such a model, one could estimate the maximum allowable differential-mode voltage between the heatsink and the PCB subject to a known radiated emissions limit. Such a model is introduced in this report. With this model, the maximum allowable differential-mode voltage between a heatsink and a PCB (that guarantees the heatsink is not the source of a given radiated emissions problem) can be calculated. II. VOLTAGE ESTIMATION MODEL A. Simplified heatsink-PCB-cable model. Most circuit board heatsinks have resonant frequencies above 1 GHz. At frequencies below the heatsink’s resonance, the EMI is more likely to be dominated by the resonances of the attached cables. If the PCB is electrically small and the self-capacitance of the heatsink is smaller than that of the PCB, a simplified model can be used to estimate the radiated EMI due to the heatsink. In such a model, the original heatsink-PCB-cable configuration is replaced with an equivalent PCB-cable configuration, where the geometry of the board and the cable remains the same. In the new model, an equivalent voltage source is placed at the junction where the cable is connected to the board. The amplitude of the equivalent voltage source is calculated using the following formula [2], sin heat k CM DM board C V V C = (1) where, C heatsink and C board are the self-capacitance of the heatsink and the board, respectively; V DM is the differential mode voltage between the heatsink and the PCB in the original configuration; and V CM is the equivalent common-mode voltage between the cable and the board in the new configuration [3]. The self-capacitance can be calculated with 3-D static field solvers or estimated with closed-form expressions [2]. Fig. 1. Heatsink-PCB-cable model and equivalent PCB-cable model. B. Maximum radiated electric field calculation The simplified model is actually an unbalanced monopole with the source located some distance away from the ground VDM VCM 978-1-4244-1699-8/08/$25.00 ©2008 IEEE