Up till the previous section, we have explained the selection of major components and calculation of component values. This time, we shall indicate measures for reducing EMI.
EMC can be described as one important issue for electronic equipment in recent years. Different countries have various regulations, and these must be satisfied in the equipment design. Prior to this, if a switching power supply spews noise all around, the S/N of the equipment is lowered, and there are cases in which the equipment specifications cannot be satisfied. Hence noise countermeasures can be said to be essential.
On occasion there is some confusion about EMC, EMI, and other terms. Below we have summarized the related terminology.
As EMI, in relation to pathways there are conductive noise and radiated noise; conductive noise is further classified as differential noise and common noise according to the manner of propagation.
As indicated in the explanation of terminology, EMI affects other circuits, and so the gist of countermeasures here is preventing this noise from being released. Points that emit noise are nodes and lines at which large currents are switched. Measures to address this essentially involve addition of capacitors and resistor/capacitor circuits for impedance matching and to serve as bypass or filter circuits. We will look at an entire circuit once again; please examine the countermeasures used.
・Adding a filter to the input section
The input voltage is a high voltage with a ripple component, which rapidly switches an internal MOSFET on and off. By adding an input filter, noise can be reduced.
・Adding a capacitor across the drain and source of the internal MOSFET
This is the capacitor C8 in the circuit diagram. A static capacitance of about 47 to 100 pF and a rated voltage of at least 500 V are necessary. This reduces surges during turn-off arising from high-speed switching; the capacitor is one type of snubber. However, losses are increased, and so rises in temperature must be considered.
・Adding an RC snubber to the output rectifying diode D4
The capacitor C9, 500 V/1000 pF, and the resistor R10, approx. 10 Ω/1 W, are added in parallel to D4. This reduces the spikes that occur upon on/off switching. The approach is the same as that for an input snubber. The component values are reference values; actual values should be adjusted while observing the noise level.
・Adding an LC filter to the output
The circuit diagram on the right is an example of addition of an LC filter circuit to the output. L2 is 10 μH, and C10 is approx. 10 to 100 μF.
The output voltage has a ripple component that depends on the switching frequency, and in addition there is noise that depends on harmonics, inductances, and capacitances. When these kinds of noise pose problems, it is effective to add an LC filter to the output.
The above are the main countermeasures to noise. It will be necessary to measure noise itself, or at the least, confirm the effect of noise on the equipment. To make accurate noise measurements, a measurement environment and measurement equipment are essential. When such quantitative measurements cannot be made, it may still be possible to ascertain whether noise has an effect on performance, through the S/N ratio for the equipment, say.
The measures described here are noise countermeasures for a power supply circuit configuration. The occurrence of noise is also related to the PCB layout, component arrangement, component performance, and so on. In some cases, it may be necessary to expand an LC filter from a simple L-type to an π-type and a T-type, or provide a shield for PC board, or otherwise modify the design.
Moreover, depending on the equipment specifications, for example the standards instituted by the International Special Committee on Radio Interference (CISPR) or some other noise-related standards should be satisfied. When conformance to standards is necessary, it is extremely important to keep such matters in mind from the design stage.
・EMI countermeasures involve adding input filters, capacitors at switches (across drain and source), and snubbers at output rectifying diodes.
・LC filters are added to the output to address output noise.
・The board layout also has a considerable impact, and should be studied as well.