Switching Power Supply Noise
Typical Standard Switching Power Supply Noise
Generally speaking, noise is “Acoustic noise (audible noise)”, but in electronics, “noise” is an unwanted signal that causes interference with a load and the environment.
Switch mode power supply generates the following noises: ripple noise, conducted emission noise, radiated emission noise, common mode noise. Ripple noise might affect the behavior of the DC load. Conducted emission is conducted back to the AC supply and effects other circuits. Radiated Emission is an RF transmitted signal to the environment.
Conducted emission is conducted back to the AC supply and effects other circuits.
The conducted emission exists not only at the switching frequency but also at harmonies of the switching frequency.
In order to overcome this noise, designers place filters at the AC power input. Most Medical Devices need to meet FCCI class B level.
Many standard switching power supplies don’t meet the standard.
The diagram on the left shows the conducted emission level of a 50W standard switching power supply at a frequency range of up to 30 MHz.
It is clear that at some frequencies the noise level exceeds the required standard for medical devices.
As a result of the switching an AC signal appears on top of the DC output and this is called Ripple Noise.
Typically, the ripple noise level of a standard switching power supply is 1% of its output voltage.
The Diagram shows a wave form of 50W standard switching power supply DC output.
At the shown case the Ripple Noise exceeds 120mV PtP.
This Ripple Noise level might cause distortions in analog circuits, imaging cameras or sensor signals.
There are very strict standards for the electromagnetic signals that electrical instruments are allowed to transmit.
The radiated emission of electrical instruments causes interferences with radio and other electronic devices.
IEC 61000-4-3:2020 is a common standard for noise immunity in electrical and electronic equipment.
Common Mode Noise
Common Mode is a path connecting Line to Ground or Neutral to Ground.
Standard Switching power supplies send surges to the ground line, resulting in voltage rises which can disrupt audio, video, data and communications signals and also damage interconnected equipment.
The chart shows a wave form of Common Mode Noise of a standard switching power supply.
Line to Earth Leakage current
Line to earth leakage current causes an electric shock to users who touch an electrical instrument.
In order to prevent electric shock, the leakage current has to be minimal. Power supply designers add coil-based filters at the AC input circuits of switching power supplies in order to reduce the Common Mode Noise and the Radiated Emission Noise.
By adding these filters, they increase the leakage current and reduce the safety of the power supply. This becomes critical when the power is supplied to a medical device.
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