Thermal Management
High heat dielectric films with reduced dissipation losses in high voltage DC-link capacitors
SABIC’s ELCRES HTV150A, a new class of high temperature capacitor films with reduced dissipation losses at 150°C, are well suited for high voltage DC-Link capacitors used in AC-DC inverter applications. As operating temperature and frequency increase, dissipation losses in these films are reduced. This advantageous behaviour is opposite to the behaviour of typical polymer films, explains SABIC. Reduced losses in the dielectric film can lead to overall reduction in heat generation, and potentially overall efficiency gain of the inverter module.
D
ue to their negative impact on AC-DC module efficiency, some high heat materials are limited in how they are used to build high heat capacitors. All
materials have internal losses when subjected to heat and electrical field at high frequency. For example, dielectric losses in films made with PEN (polyethylene naphthalate) become excessive when the film temperature reaches 150°C and a 100kHz electric field is applied. Represented by the dissipation factor Df, the losses reach 0.0115 making the capacitor a heat source. To bring the losses to below 0.01, the threshold established by the industry, the designer is forced to lower the maximum operating temperature to about 125°C, thus giving up potential efficiency gains if new MOSFETs (e.g., SiC) are allowed to operate at higher temperatures reaching 150°C. Controlling internal losses in polymer films relies on a deep understanding of how a particular molecular structure responds to applied frequency fields at different temperatures. Different polymers have different characteristic responses. For ELCRES HTV150A films, material selection is based on the material’s characteristic response curves such that minima (or valleys) of the curve span the desired frequency range at temperatures of interest. The material has been successfully converted into ultra-thin film of gauges down to 3µm. Stable performance in example capacitors has been demonstrated at temperatures reaching 150°C. In the current work, measurements of film Df are used to estimate internal losses. The measurements are performed by means of frequency sweeps at different temperatures on films made with materials of interest.
48 July/August 2023
of a polymer ( relaxation) is particularly important due to its association with main chain relaxation in a polymer. As temperature approaches Tg, dielectric loss will significantly increase as segmental chain motions become activated and long-chain cooperative motion takes place. At higher frequencies and/or lower temperatures, additional relaxation modes are attributed to local motion of chain segments or side chains. While each polymer chemistry will exhibit its own loss profile, relaxation peak width tends to span multiple orders of magnitude versus frequency. Thus, we can expect a polymer’s Df to be affected not only by temperature but also by the operating frequency range of interest.
Technical approach
Power dissipation due to polarization is influenced by the applied AC frequency and electric field magnitude as well as Df of the
Applying an electric field to a dielectric material will result in polarization of its atoms and molecules. The macromolecular structure of a polymer will experience multiple polarization mechanisms that take place over very different time and length scales. Therefore, the dielectric properties Dk (dielectric constant) and Df of a polymer have a response to both applied frequency and temperature. Each polarization process is identified by a peak in Df and a step-like change in Dk. Dk monotonically increases when either decreasing frequency or when increasing temperature near one of these polarization events. This behaviour is schematically shown in Figure 1. The glass transition temperature (Tg)
Components in Electronics
Figure 1, Trends for Dk and Df vs. temperature and frequency, for a material with multiple polarization modes.
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