DCDC switching power supply voltage ripple measurement method

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DCDC switching power supply voltage ripple measurement method [Copy link]

The following is an explanation of the voltage ripple and inductor current measurement using a buck non-isolated switching power supply circuit.

Voltage ripple measurement:

As shown in the figure above, the 1.2V output voltage must be stable within a certain range, so there will be certain requirements for the voltage ripple, otherwise the ripple will be too large and will affect the operation of the subsequent circuits. Generally, the ripple is about 10% of the output voltage. For circuits with high voltage accuracy requirements, the ripple should be as small as possible.

Then it is very important to confirm whether the voltage ripple is within our requirements. The test method is very important. Different test methods will get different values, which can easily lead to misjudgment.

When measuring, the GND of the oscilloscope probe must be as close to the GND of the output voltage as possible. If it is too far away, it will introduce a lot of unnecessary interference, resulting in a larger ripple value. For example, if you use the grounding clip of the oscilloscope, the grounding clip is clamped on a distant GND (screw hole, mechanical housing, etc.), and point the oscilloscope probe at the output voltage, then the value obtained is not the real voltage ripple.

So how should we measure it?

1. Because voltage ripple is an AC component, you first need to adjust the oscilloscope to AC measurement mode.

2. Because the GND of the oscilloscope probe needs to be in shortest contact distance with the GND of the voltage, you need to use a tool (as shown in the figure). Some probes will come with this tool, if not, you can make one yourself with wire.

3. After the production is completed, the oscilloscope is also adjusted. Taking the above buck voltage diagram as an example, the oscilloscope probe is at the 1.2V position, and the iron wire directly contacts GND. That is, the ripple voltage at both ends of capacitor C401 is directly measured. (The figure below is a more intuitive display)

Finally, you can check the V PK-PK value on an oscilloscope to confirm the voltage ripple, or you can measure the reading manually.

Inductor current measurement:

Current is usually measured using a current probe. Because the current probe is an inductive measurement, you need to disconnect the line first and then connect it with a wire. The wire should be as short and thick as possible.

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It is recommended to measure the inductor current on the side with stable voltage. In the switching power supply circuit, the voltage on one side of the power inductor changes periodically, and the other side has a stable output voltage.

For the buck circuit above, disconnect and reconnect at 2 points in the circuit diagram to measure the current.

Finally, we can get the inductor current. Through the inductor current, we can know whether the switching power supply is in CCM, BCM or DCM mode.

For example, if the current shown in the figure below is obtained, it is in CCM (continuous conduction mode).

吾妻思萌

Current probes are not cheap. This current measuring flying wire is bright. You have to cut the PCB open.

Nubility

The 51mV measurement result may be due to the probe bandwidth limit not being turned on, or the probe contact being poor.

hahajing

If the current wasn't so large, the interference caused by this large coil would be extremely large.

不亦心

hahajing posted on 2023-9-20 20:13 If it weren't for the high current, the interference from this large coil would be very large.

Not only is the loop large, but it also makes a fatal mistake: the loop is covered on the power chip

lkh747566933

Thanks to the host for sharing the knowledge points, the summary is very comprehensive. It's just right for systematic learning!

led2015

Many glitches on the waveform are actually not easy to solve in the design

xjtburden

When testing, it is best to connect a capacitor in parallel with the probe. Generally, I use a twisted pair to lead out from the probe, and connect a PF-level capacitor in parallel with the probe. This will make the loop smaller than using the built-in test probe.