Found Ripple Issue? Look Further.
More Things to Check
Found Ripple, Spike and Noise? Look Further...
Recently, I was asked to evaluate two signle output open frame power supplies. Power supply A is reported to have higher spike when doing ripple and noise test. Therefore, power supply B is the preferred solution.
However, something very interesting was revealed in the evaluation. Let's take a look at both power supplies' performance.
- Power supply A @ Nominal input voltage, full load, and time div is set at 4us. Max Vpp reading is 42mV.
- Power supply B @ Nominal input voltage, full load and time div is set at 4us. Max Vpp reading is 18mV.
Just looking at these two above captured waveforms, Power supply B seems to have better ripple and noise performance (19mV vs. 42mV). But, when the time div on the oscilloscope is changed to 4ms, things look different.
- Power supply A @ Nominal input voltage, full load, and time div is set at 4ms. Max Vpp reading is 42mV.
- Power supply B @ Nominal input voltage, full load, and time div is set at 4ms. Max Vpp reading is 46mV.
Yes, you can see a sine wave on the screen. It looks like the output is oscillate in low frequency. And yes, your suspicion is right again. It is oscillate in 120Hz which is generated by the AC input frequency. In the below picture you can see the Vpp reading reaches its max: 60mV (which is out of spec) when the load is reduced to half load condition.
If you further lower the load to 3A, the low frequency noise on the Power supply B starts to change shape as below.
It is obvious to me that Power supply B is fine-tuned to have a better noise and ripple performance at high frequency. They either sacrifice the low frequency performance or never check ripple and noise at the low frequency range.
With the max Vpp at 42mV (which is still lower than the 50mV max ripple and noise spec), Power supply A turns out to be a winner!