如何测试纹波和噪音-ST

APPLICATION NOTE HOW TO TEST OUTPUT RIPPLE AND NOISE OF POWER SUPPLIES AN647/0594 1/3 The switching power supplies (either step-down regulators, or isolated DC-DC converters, or OFF- LINE power supplies) have the fundamental advan- tage of high efficiency i.e. low power dissipation when compared to linear voltage regulation. However the switching technique, that is beneficial to raise the efficiency, has an associated weakness i.e. the output voltage has always an AC content. The typical output ripple and noise of a switching power supply is as shown in fig. 1. Figure 1. Output ripple and noise of a switching power supply Four AC components can be identified: - low frequency ripple at 2f being f the AC mains frequency - high frequency ripple due to Pulse Width Modu- lation (PWM) to obtain the required line and load regulation. - the switching noise that has the same frequency of the switching PWM - the aperiodic random noise that is not related to the AC source frequency and/or the switching frequency. The above mentioned parameters are normally specified by the peak to peak amplitude so that the best method for testing is by an oscilloscope with a bandwidth of 20 MHz. Care must be taken when using the scope probe. Fig. 2 shows a wrong method because the ground wire of the probe can collect radiated noise and the scope display is strongly dependent on the probe position. Fig. 3 shows a better method because the collection of the radiated noise is minimized. 2/3 Power supply + GND Figure 2. Wrong output ripple and noise test Power supply + GND Figure 3. Correct output ripple and noise test The ground ring of the probe is pressed directly against the output ground of the power supply and the tip is in contact with the output voltage pin. A preferred method is shown in fig. 4: the waveform displayed on the scope must be multiplied by a factor 2. Figure 4. Preferred method to test output noise and ripple Power supply + GND 50 Ohm 50 Ohm 220nF Coax cable to AC input of scope How to test the output ripple and noise of power supplies 3/3 Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specification mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. SGS-THOMSON Microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of SGS-THOMSON Microelectronics. © 1994 SGS-THOMSON Microelectronics – All Rights Reserved SGS-THOMSON Microelectronics GROUP OF COMPANIES Australia - Brazil - China - France - Germany - Hong Kong - Italy - Japan - Korea - Malaysia - Malta - Morocco - The Netherlands - Singapore - Spain - Sweden - Switzerland - Taiwan - Thailand - United Kingdom - U.S.A. How to test the output ripple and noise of power supplies