I built a boost/inverting converter using TPS65130. The output voltage is +15V and - 15V. Below is a picture of the schematics:
I know the schematics shows a zener diode symbol, but that's just a mistake. I'm using a schottky diode. Also, the base of U37 is not connected because I have a zero ohm resistor across R50. U41 is an LT3094 and U42 is an LT3045, they are both linear regulators used to filter noise. What I'm trying to do is to measure the noise of the power supply. Here is the procedure that I followed to measure the noise of this power supply:
- used a 1:1 probe that has a max bandwidth of 35Mhz.
- used a bandwidth limit of 20Mhz on the oscilloscope.
- used a ground spring instead of the long ground lead. I also made sure there is nothing else connected to the oscilloscope picking up common mode noise.
- I measured directly at the output capacitor C152 for the boost and c179 for the inverting converter.
- the load current is 80mA.
- AC coupling.
The problem is I noticed some weird overshoot and undershoot. The first picture shows what I see when I measure directly across C152 which is the positive supply:
The next picture is what I see when I measure across C150 which is after the linear regulator: The Next picture is what I see when I measure across C179 which is the negative supply:
The final one is if I measure across C1 Which is after the linear regulator:
I have checked everything including the inductor and output capacitors. Checked things like saturation current, rating current, etc. These waveforms repeat and when I measure the frequency between two repeating waveforms, I get 6Hz. This is the same for both the positive and negative supplies.
Any Idea on what this could be?
Edit: I noticed that the problem disappear if I lower the input voltage to 4V. Not sure why does it work with 4v better than 5V. Edit2: zoomed picture of the ripple on the positive power supply.