Dcdc power module ripple test method

dcdc power moduleThe ripple index is a very important parameter. A clean power supply is a prerequisite for stable operation of digital circuits and an important guarantee for various parameters of analog devices.

In order to make sure that our power supply is clean, the output ripple of the dcdc power supply module must be measured. However, the ripple value measured by many people is often hundreds of mV, or even hundreds of mV, which is far larger than the maximum ripple value provided in the device manual. This is mainly caused by incorrect measurement methods.

1 Correct measurement method

1. Limit the bandwidth of the oscilloscope to 20MHz (most of the low-end oscilloscope gears are limited to 20MHz, and high-end products also have a 200MHz bandwidth limit option), the purpose is to avoid the high frequency noise of the digital circuit from affecting the ripple measurement, and try to ensure the accuracy of the measurement ;

2. Set the coupling mode to AC coupling, which is convenient for measurement (use a smaller gear to carefully observe the ripple, and do not care about the DC level);

3. Ensure that the probe grounding is as short as possible (the main reason for the measurement ripple of hundreds of mV is that the grounding wire is too long), try to use the original short test needle that comes with the probe. If there is no short test needle, you can remove the ground wire and shell of the probe to expose the probe crust, and wind the self-made ground wire around the probe crust to ensure that the length of the ground wire is less than 1cm;

4. The oscilloscope's ground is suspended, and only the probe ground is shared with the reference point of the test signal. Do not share the ground with the test equipment by other means (this will introduce a lot of ground noise to ripple measurement). For example: when the oscilloscope and other instruments share the power strip, the switches of other instruments may cause noise interference to the test through the ground wire.

Oscilloscope parameter requirements:

Support bandwidth limit function: general oscilloscopes support 20MHz bandwidth limit, such as SDS1102CM.

Probe requirements:

In order to make the grounding wire as short as possible, try to use the original short test needle of the probe. If there is no original short test needle, you must make your own short grounding wire: remove the probe grounding wire sleeve and wind the grounding short wire with a metal wire. Copper wire, moderate strength (still a little soft, please recommend if there is a better one). Other candidates are solder wire and cutters.

Choose the 1X non-attenuation gear. Generally, when the passive probe is in the 1X gear, its bandwidth is limited to 6MHz/10MHz bandwidth, so that the front end can effectively filter out the interference of high-frequency noise and reduce the impact of ripple measurement.