Eliminate the Guesswork in Controlling EMI

Jan 1, 2008 12:00 PM
By Majid Dadafshar and Jinho Choi, Field Application Engineers, Fairchild Semiconductor, San Jose, C

Using the first equation, the conducted emissions for a power supply with a 110-Vac input, a 150-kHz switching frequency and a 48% duty cycle are calculated and shown in Fig. 4. Comparing these results with Fig. 5 provides a preliminary indication of the levels of attenuation in decibels that will be needed for this power supply to meet the regulatory limits on emissions.

That attenuation of emissions can be achieved through filtering. Additional attenuation can also be obtained by lowering the rise time, but with the penalty of additional temperature rise in the main switching transistor.

To meet the EMI requirements for conducted emissions, the voltage across the line impedance stabilization network (LISN) should be calculated for all frequencies throughout the conducted emissions range. In power-supply products developed for Europe, emissions limits are determined by EN55011 and EN55022, while products designed for North America must meet the limits established by FCC CISPR22. In the case of EN55022, we can translate the emission limits into a voltage level in microvolts (µV) or millivolts (mV), per Fig. 5.

Considering the 500 µV needed across the LISN to meet the emission standard with the maximum voltage across the main switch (155 V), we can estimate an overall needed attenuation:

The above calculation does not account for the effects of leakage inductance, which results in a voltage spike at the front end of the drain pulse when the MOSFET turns off. So some additional attenuation will be required to suppress the noise associated with this spike. If we assume 120 dB of total attenuation and take the anti log of (120/20), we find that the signal associated with the drain voltage must be reduced by a factor of at least 10⁶ from drain to line, before it reaches the LISN. Now that we have the predicted emissions, they can be compared to the limits shown in Fig. 5, to determine the amount of attenuation required across the frequency range of interest (Fig. 6).

In the primary section of the power supply, one problem-generating area is the metal tab of the main switching transistor (Fig. 7). The transistor is usually mounted in close thermal contact to a grounded heatsink through an insulator or layer of Kapton tape. In order to maximize the heat transfer from the MOSFET to the heatsink, the Kapton is made as thin as possible, which creates a parasitic capacitance (C_HS) from the drain to the chassis. Fig. 8 illustrates how the presence of this parasitic capacitance causes a noise current to flow through safety capacitors Y1 and Y2.

Choosing the proper values for Y1 and Y2 and selecting the right thickness of the Kapton tape is crucial (Table 2). Doing these steps correctly ensures that the noise current (I_NOISE) caused by turning on and off the main switch stays in a guided path, so that the expected attenuation is obtained from the filter. If capacitor values and Kapton thickness are poorly chosen, I_NOISE will bypass the filter and flow through the chassis.

Both Y1 and Y2 need to have a specific value to meet the ground safety current requirement mandated by Underwriters Laboratories, to protect the user when the safety ground is not properly connected. The value for the line-to-chassis Y capacitors can be calculated as:

where I is the needed leakage current, F is the line frequency (usually 50 Hz, 60 Hz or 400 Hz) and V is the ac line voltage with a typical value such as 110 Vac or 220 Vac (Table 3).

When the value of these shunt capacitors is limited to meet some stringent requirement for leakage current, we can increase the inductance value of the input filter to gain more insertion loss. But keep in mind that there is a voltage drop (V_L = Ldi/dt) associated with the filter inductor.

The heatsink capacitance C_HS and Y capacitances (Fig. 8) form a high ratio divider. With a proper selection of Kapton thickness, we can obtain both a low thermal resistance and a specific capacitance value for CHS that could be used to reduce the noise current by as much as 1000 times from the drain before it reaches the input filter. Then, given the overall attenuation requirement calculated previously (10⁶), the total remaining attenuation from drain to line should be 10³. A typical C_HS value is in the range of 10 pF to 100 pF.