Noise test of module power supply in circuit design

With the integration and standardization of module power supply design, its power density has shown a linear increase, while the volume of the module power supply is getting smaller and smaller. This makes the various interference tests of the module power supply begin to become important. In this article, the editor will introduce and introduce the noise test method of the module power supply.
    The noise test of the module power supply is mainly divided into three points.
    Test Conditions
    The module power supply AC input voltage is 220V, and the output is fully loaded (for multi-channel output, each channel is fully loaded). The AC input voltage can be set to 90V, 220V, 265V and tested under full load output conditions according to needs.
    testing method
    During the test, the oscilloscope's TIME/DIV file is set to 10uS/div, and the bandwidth is set to 20MHz. Reading the peak-to-peak output voltage displayed by the oscilloscope is the output ripple voltage (the peak-to-peak value including the glitch is ripple + noise).
    1) The input voltage of the module power supply is adjusted to the nominal voltage, and the output current is adjusted to the rated current.
    2) Module power ripple is usually expressed by peak-to-peak value. The main test method.
    Module power ripple and noise are AC components superimposed on the DC output voltage, and the ripple and noise are measured at rated load and normal temperature. For switching-type AC/DC module power supplies, the output ripple voltage is a small pulse with high-frequency components in the system, so the peak-to-peak value is usually measured instead of the effective value (RMS). The measured value is expressed in millivolt peak-to-peak value (mVp-p). For example, when the peak-to-peak ripple of an AC/DC module power supply is 50mV, its RMS value is very low, only 5mV, but whether it can be used in a certain system, it must be further considered.
    Because of the high frequency components contained in the measured ripple, special measurement techniques must be used to obtain correct measurement results. In order to measure all the high frequency harmonics in the ripple spike, an oscilloscope with a 20MHz bandwidth is generally used.
    Secondly, when measuring the ripple, you must be very careful to prevent the wrong signal from being introduced into the test equipment. The probe ground clamp must be removed during measurement, because in a high-frequency radiation field, the ground clamp will receive noise like an antenna and interfere with the measurement results. Use the measurement method of the probe with the grounding ring to eliminate the interference.
    It is a method to measure the output ripple voltage of the module power supply using a 50Ω coaxial cable. The coaxial cable is directly connected to the oscilloscope. To reduce noise, an aluminum or copper grounding plate should be used when measuring. The measured value is 1/2 of the actual value.
    Another measurement method using double lines.
    Place the module power supply at a place 25mm above the grounding plate, which is made of aluminum or copper. The output common end of the module power supply and the AC input ground end are directly connected to the grounding plate. The grounding wire should be thick and not longer than 50mm.
    Use 16AWG copper wire to make a 300mm long twisted pair, one end is connected to the power output, the other end is connected in parallel with a 47μF tantalum capacitor, and then connected to the oscilloscope. The lead of the capacitor should be as short as possible, and the polarity should not be reversed here. The “ground wire” of the oscilloscope probe should be connected to the ground wire loop as much as possible, the bandwidth of the oscilloscope should not be less than 50MHz, and the AC of the oscilloscope itself should be grounded.
    Output noise test (divided into peak-to-peak noise, telephone weighing noise, broadband noise, discrete noise).
    At present, the main suppliers of module power supplies in the domestic market are VICOR, ASTEC, LAMBDA, ERICCSON and POWER-ONE. Since the types, series, and specifications of the module power supplies produced by various companies are difficult to count, their functional characteristics and physical characteristics are not the same.
    Therefore, it is determined that different module power supplies also have some differences in the use, maintenance and installation. And with the extensive use of semiconductors and packaging technologies, module power will inevitably develop toward high density and high efficiency, and application design will become simpler and simpler.