A brief discussion on the noise test method of module power supply

At present, the design of module power supply is becoming more and more standardized, and the control circuit tends to adopt digital control. The growth rate of non-isolated DC-DC converters (including VRM) is faster than that of isolated ones. With the improvement of semiconductor process and packaging technology, and the extensive application of high-frequency soft switching technology, the power density of module power supply is getting higher and higher, the power conversion efficiency of module power supply is getting higher and higher, and the volume is getting smaller and smaller, and chip-level module power supply has appeared. Module power supply is widely used in communication fields such as AC equipment, access equipment, mobile communication, microwave communication, optical transmission, routers, automotive electronics, aerospace, etc. Its characteristics are that it can provide power for application-specific integrated circuits (ASICs), digital signal processors (DSPs), microprocessors, memories, field programmable gate arrays (FPGAs) and other digital or analog loads. The noise test of module power supply is mainly divided into three points, as follows:

1. Test conditions: module power supply AC input voltage 220V, output full load (for multiple outputs, each channel is fully loaded)

(the AC input voltage can be set to 90V, 220V, 265V as needed, and the test is carried out under full load output conditions).

2. Test method: During the test, the oscilloscope TIME/DIV is set to 10uS/div, the bandwidth is set to 20MHz, and the output voltage peak-to-peak value displayed on the oscilloscope is the output ripple voltage (the peak-to-peak value including the burr is the 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) The ripple of the module power supply is usually expressed in peak-to-peak value. The main test method.

The ripple and noise of the module power supply are the AC components superimposed on the DC output voltage. The measurement of ripple and noise is carried out at rated load and room temperature. For a switching AC/DC module power supply, the output ripple voltage is a small pulse with a high-frequency component 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 value of the 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 must be further considered.

Because the measured ripple contains high-frequency components, special measurement techniques must be used to obtain correct measurement results. In order to measure all high-frequency harmonics in the ripple peak, an oscilloscope with a bandwidth of 20MHz is generally used.

Secondly, when measuring ripple, you must be very careful to prevent the introduction of erroneous signals 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. The measurement method using a probe with a ground ring eliminates interference. This is

a method of using a 50Ω coaxial cable to measure the output ripple voltage of the module power supply. The coaxial cable is directly connected to the oscilloscope. To reduce noise, an aluminum or copper grounding plate should be used during measurement. The measured value is 1/2 of the actual value.

This is another measurement method using a double-stripe line.

The module power supply is placed 25mm above the grounding plate, which is made of aluminum or copper. The output common terminal of the module power supply and the AC input ground terminal are directly connected to the grounding plate. The grounding wire should be very thick and no longer than 50mm.

Use 16AWG copper wire to make a 300mm long twisted pair, one end is connected to the power output, and 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 be careful not to connect the polarity in reverse. The "ground wire" of the oscilloscope probe should be connected to the ground ring as much as possible. The bandwidth of the oscilloscope should not be less than 50MHz, and the oscilloscope itself should be grounded in AC.

3. Output noise test (optional test content, divided into peak-to-peak noise, telephone weight 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 categories, series, specifications and varieties of module power supplies produced by various companies are difficult to count, their functional characteristics and physical characteristics are not the same, so the installation, use and maintenance are also different. With the extensive use of semiconductor processes, packaging technologies and high-frequency soft switches, the power density of module power supplies is getting higher and higher, the conversion efficiency is getting higher and higher, and the application is becoming simpler and simpler.