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• Designing Isolated Flyback Converter Circuits: Addressing EMI and Output Noise

2016.09.08 AC/DC

Designing Isolated Flyback Converter Circuits: Addressing EMI and Output Noise

Design Method of PWM AC/DC Flyback Converters

In this section, measures to address noise in circuits are explained. Noise evaluation and countermeasures are essential when designing a switching power supply.

To guard against misunderstanding, we begin with a short review of noise-related terms.

・EMI （Electro-Magnetic Interference)
Radio waves and high-frequency electromagnetic waves impacting electronic equipment, and
electromagnetic waves that exert influence.
－Conductive noise: Noise transmitted via cables and circuit board wiring
> Differential (normal) mode noise: Noise that flows in the same direction as a current that
occurs between power supply lines
> Common mode noise: Noise that passes through stray capacitance etc. via a metal case and
－Radiated noise: Noise that is emitted through the air

・EMS (Electro-Magnetic Susceptibility)
The ability to resist or avoid damage even when there has been interference and disturbance by
electromagnetic waves (EMI: conductive noise and radiated noise)

・EMC (Electro-Magnetic Compatibility)
EMI + EMS. Both emission countermeasures and immunity measures

As EMI, in relation to pathways there are conductive noise and radiated noise; conductive noise is further classified as differential noise and common mode noise according to the manner of propagation. The above is somewhat rough-and-ready, but is the bare minimum that one must know in advance.

EMI countermeasures

When the EMI of a switching power supply circuit affects other circuits, EMI countermeasures are taken. In essence, capacitors or resistor-capacitor circuits for impedance matching or to serve as bypass/filter elements are added to nodes and lines where large currents are switched.

1) C12, R17: RC snubber added to an output rectifying diode
As with input snubbers, these components reduce the spikes occurring during on/off switching. Please refer to this for information on input snubbers. C12 is set to 1000 pF at 500 V, and R17 is set to about 10Ω for 1 W.

2) C10: Y-capacitor added between primary and secondary sides
A capacitor called a Y-capacitor is added between ground on the primary side and secondary side. This is one typical method for reducing the common mode noise caused in the secondary side by primary side switching noise via the capacitance across the windings of an isolation transformer. The voltage rating of the Y-capacitor must be equal to the insulation voltage of the transformer. A capacitance of about 2200 pF is chosen.

3) C11: Capacitor added across the drain and source of the MOSFET Q1
There is a method of adding a capacitor across the drain and source of a MOSFET in order to reduce surges at turn-off due to high-speed switching. This is also one kind of snubber. However, greater losses result, and so attention must be paid to temperature increases. Here, a 10 to 100 pF capacitor with a withstand voltage of 1 kV is used.

The above component constants are reference values that should be considered as starting points. They should be adjusted while checking the effect on noise.

Output noise countermeasures

Of course the output voltage of a switching power supply includes ripples, which depend on the switching frequency; in addition, there is also noise which originates in harmonics, inductances and capacitances. When such noise components are a problem, adding an LC filter to the output is effective.

Starting with an inductance L of 10 µH and C10 of approximately 10 to 100 µF, the values are adjusted while observing the noise components.

The above are the main countermeasures to noise. It will be necessary to measure noise and confirm the influence of noise on other equipment. To make proper noise measurements, a measurement environment and measurement devices 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 countermeasures for a power supply circuit configuration. The occurrence of noise is also related to the circuit board 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 a π-type or a T-type, or provide a shield for the circuit 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.

With this section, we conclude our explanation of circuit design with the title {Designing Isolated Flyback Converter Circuits}. Next we will begin [Board Layout Examples].

Key Points:

・Switching power supplies are potential EMI sources, and measures must be taken with respect to both conductive noise and radiated noise.