[Reprint] Power supply test series: Output ripple noise (Output Ripple noise Test)

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[Reprint] Power supply test series: Output ripple noise (Output Ripple noise Test) [Copy link]

Original address: Power supply test series output ripple noise (Output Ripple noise Test) Author: Renco

Purpose of the test The output voltage/current of the power supply and the ripple noise on the output signal are important factors that cause system instability or interference, so they must be limited. Usually we need to examine the following factors on the output voltage/current and output signal of the power supply: - The amplitude of the power frequency signal (which can cause power frequency interference between systems), - The ripple frequency and amplitude, - The high-frequency noise amplitude, - The total amplitude of the ripple + high-frequency noise;

Therefore, the following indicators are usually proposed in the specifications of general power supplies:

1. Maximum ripple noise value: The magnitude (absolute value) of the ripple noise (including power frequency signal) on the output voltage (current or output signal) under input/output conditions within the specification range, usually expressed as peak-to-peak value or effective value;

2. Ripple coefficient (%): The ratio of the ripple effective value Vrms on the output voltage (or output signal) to the output DC voltage (or output signal) Vo under the input/output conditions within the specification range, that is: Ripple = V rms/Vo × 100%, for current: Ripple = I rms/Io × 100% ;

3. Ripple voltage suppression ratio: Under specified conditions, the ratio of the ripple voltage Vr-i in the input voltage (or signal) to the ripple voltage Vr-o in the output voltage (or signal), that is: ripple voltage suppression ratio = Vr-i/Vr-o; for current: Ripple = Ir-i/Ir-o.

Test conditions and schematic diagram
- Input: minimum, rated and maximum input AC/DC voltage, minimum and maximum AC frequency defined in the specification
- Output: minimum and maximum output load defined in the specification and the minimum capacitance and type allowed by the specification
- Temperature: minimum operating temperature, normal temperature and maximum operating temperature

Test requirements
1. Oscilloscope settings:
- Bandwidth: set according to the minimum bandwidth requirements in the specifications. For power supplies with switching frequencies below several hundred kHz, 20MHz can meet most test requirements;
- Sampling mode: Sample, Peak or Envelop mode. Sample mode can better observe the output ripple (Ripple) and measure the noise (Noise) frequency, while Peak and Envelop modes can better measure the output power frequency and the overall level of ripple and noise;

- Timebase: The setting of Timebase needs to meet the observation needs of the local information of the measured signal (such as amplitude-frequency characteristics), and generally it is sufficient to display 3 to 5 cycles. Therefore, for the observation of power frequency signals, Timebase can be set to 10mS; and for the observation of ripple, it is generally set to 5uS (generally the switching frequency is 30~80KHz); but for the observation of high-frequency noise, it needs to be adjusted according to the actual measurement conditions.

2. Probe and measurement circuit:

Test steps
1) Set the minimum ambient operating temperature, minimum input voltage/frequency and maximum load according to the specification requirements;
2) Observe the changes in output ripple and noise after 1~2 minutes of power on;
3) If the oscilloscope display waveform has no obvious changes within 1~2 minutes, record the measurement data and save the waveform;
4) Otherwise:

* Change the minimum time resolution (time base) of the oscilloscope to display more waveforms.

* Check whether the test circuit has poor contact;
5) Change the test conditions (load, input voltage/frequency and ambient temperature) in sequence, repeat steps 3 and 4, measure and record the corresponding data.

Note: If there is a standby auxiliary output, the ripple noise value of the auxiliary output should be measured when the main output is ON and OFF.

Judgment conditions
- Ripple noise is less than the specified value
- The output ripple noise does not contain power frequency noise signal

PS: For measures to improve output ripple noise, please refer to another blog post "Reference Measures for Improving Output Ripple of Switching Power Supplies".

Appendix: Test report form reference

Note: For test report formats under other test conditions, please refer to the above form!

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The Vrms that appears here is explained together with VPK:

Vpk : voltage peak value, vrms: voltage effective value.

The effective value/root mean square value Vrms is obtained by squaring the signal within one cycle, integrating it, and then taking the square root and averaging it.

The meaning of effective value is: the amount of work done in one cycle is equal to the amount of work done by a DC voltage equal to this value. For a sinusoidal signal, the peak value is √2 times the effective value.

Voltage peak value describes the range of variation of the signal value.