The Main Source of CM Noise

Usually, we don t have much control over the dV/dt, and nor do we really want to reduce it too much in the interest of efficiency. So to reduce this current, we need to reduce C. But a closer look at the root equations reveals a dilemma. The thermal resistance (Rth — in °C/W) is given by 

Typical values of parasitic capacitance that can be created in a power supply by the insulator are presented in Table 11-1. Here we are comparing a traditional insulator material, mica, with a modern choice, silicone rubber. 

From Table 11-1 we can see that mica creates higher parasitic capacitances despite a lower K, and that is clearly attributable to the smaller thickness of insulator typically required. The same happens when we use some of the modern, expensive, and yet popular polyimide (not polyamide ) insulators which are excellent thermal conductors, but are also very thin. These can be recognized by their typically amber color, and they come in various brand names like Kapton, Kinel, Upilex, Upimol, Vespel, and so on. 

So the question is — should we just put in another layer of insulator to solve our EMI problem In other words, what thickness of insulator do we really need  

Note If we want to know how much thermal resistance is typically attributable to thermal grease, we must remember that without this grease we would have air in the spaces between the device and heatsink, and that is a very poor thermal conductor. Thermal grease lowers this interface resistance significantly by filling the 

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