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How accurate measurement of power supply ripple

In Electronic Infomation Category: H | on April 14,2011

The example shown in Figure 1, a junior engineer completely wrong to use an oscilloscope. His first mistake is the use of an oscilloscope with a long ground wire probe; his second error is to probe the formation of the loop and ISO103 datasheet and grounding wire are placed near the power transformer and ISO103 price and switching devices; his last error is to allow the oscilloscope probe and ISO103 suppliers and excess inductance between the output capacitor. The problem manifested in the high-frequency ripple waveform picked up. In power, there are a large number of probes can be easily coupled with high-speed, large-signal voltage and current waveforms, including the coupling of the magnetic field power transformers, electric field coupling from the switching nodes, and each transformer winding capacitance by the common mode current.


Figure 1 error less ripple measured measurements

Using the correct measuring method can greatly improve the results of the measured ripple. First, commonly used to specify the ripple bandwidth limit to pick does not really exist to prevent high-frequency noise. We should be used to measure the bandwidth of the oscilloscope to set the correct limits. Second, by removing its probe "cap", and constitute a pickup (see Figure 2), we can eliminate the formation of the long ground wire antenna. Probe the ground in a short line wrapped around the connection point and the ground connection to the mains. This can reduce the exposure to electromagnetic radiation of high power near the end of the first length, thereby further reducing the pickup.

Finally, in isolated power supply, will produce a lot of ground connection points of the probe through the common mode current. This ground connection point in the power ground connection points and oscilloscopes between the pressure drop, thus the performance of the ripple. To prevent this problem from occurring, we need to pay special attention to power supply design of the common mode filter. In addition, the oscilloscope leads wrapped around the ferrite core also helps to minimize this current. This forms a common mode inductor, which does not affect the differential voltage measured at the same time, also reduce common mode current is caused by measurement error. Figure 2 shows the same circuit of the ripple voltage, the use of improved methods of measurement. Thus, high-frequency peak was truly eliminated.


Figure 2 the four minor changes would greatly improve the measurement results

Fact, integrated into the system after the power supply ripple performance is even better. In between the power supply and other components of the system is almost always some inductance. This inductance may be present in wiring, or only exist in the PWB on the etching. In addition, there will always be around the chip an additional bypass capacitor, which is the power of the load. This is the two together constitute a low-pass filter, further reducing the power supply ripple and / or high frequency noise. In extreme cases, the current flows through short 10 F 15 nH inductor and bypass capacitors of an inch conductor, the filter cutoff frequency of 400 kHz. In this case, it means that high-frequency noise will be greatly reduced. In many cases, the filter cutoff frequency in the power supply ripple frequency of the following, which could greatly reduce the ripple. Experienced engineers should be able to find in his testing process, ways of how to use this method. Power output ripple About

Ideally, the power output of the DC voltage should be a fixed value, but many times it is rectified by an AC, filtering can be accomplished, more or less there will be exchange of the remaining components, which includes periodic and the random component of the clutter signal is called ripple. Large ripple affect CPU and GPU to work, the lower, the better.

determine the ripple of the standard

Intel ATX12V 2.31 specification requirements in the +12 V output ripple should not exceed 120 mV, +3.3 V and +5 V shall not exceed 50 mV ripple, the amount of power for most brands are very well-off, I tested Most power supplies will not exceed this value, but almost all of the cottage will supply excessive ripple at full load, the internal use of materials design can be imagined.

In fact, we can put the power of the ripple pattern and sound spectrum linked together. When the sound vibration frequency is very high, often appear messy and even low spectrum sounds obvious deviation from the situation. This is the performance and power ripple is relatively the same.

Intel ATX12V 2.31 the provisions of the power supply ripple


PBzone output ripple test settings

Power load per output ripple value of the current value of the road has a lot to the general power supply ripple at light load is never exceeded, we record the ripple under three conditions: 100 % load, +12 V combined output full load, +3.3 V output full load, +5 V output full load. Full output in the three-way test, we put all the way by which the nominal nameplate full load, the other two currents are set to 2A.


Figure 3 Test ripple using digital oscilloscope


Figure 4 +12 V ripple records screenshots

Ripple test result is not difficult to understand, the two pictures above are screenshots of high and low frequency, ripple value of the sum of the two is the final result. Please click on an image to enlarge, you will find the bottom line figure has two values, the vertical scale value 20.0mV, and horizontal division value 10.0us. We only need to pay attention mV this value, 20.0mV on behalf of Y-axis of each grid cell is equal to 20mV, the first map of the peaks and valleys separated by roughly a grid, it means that the high frequency ripple 10.0us peak - peak is about 20mV * 1 = 20mV. Also note on behalf of low-frequency 10.0ms, that is right, the right to remove burrs after the high-frequency ripple peak - about a grid that is 20.0mV. The sum of high frequency and low frequency ripple of the output value, the two together as 40mV, far less than Intel provided 120mV, so the test results can be very good.

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