Switch noise on Inverting Buck Boost converter

Thread Starter

HavreHans

Joined Apr 19, 2024
2
I'm currently using a Buck converter as an Inverting Buck Boost and measuring the output voltage ripple. The buck converter is the LM46002 from Texas Instruments. The measured ripple is affected with noise coming from the Switch, se picture below. What causes this noise? Is it some capacitive oscillation between the open Switch and output capacitors? The converter is operating in DCM.
OSC -15V MED Sw.png

I'm grateful for any tips! // HavreHans
 

crutschow

Joined Mar 14, 2008
34,704
Are you using a 10:1 oscilloscope probe with the ground clip connected close to the measurement point?
The picture looks like you are not.
Otherwise the layout is bad as noted by DC, as switching regulators work poorly without proper layout and grounding (usually requiring a ground plane).
 

Thread Starter

HavreHans

Joined Apr 19, 2024
2
Okay thank you, Im using the LM46002PWPWEVM demo board but it is configurated as an IBB with Vout changed to GND of the input supply. I'm measuring on the testpoints installed on the board, the board is also a 4-layer with ground plane installed. Both the input and output capacitors are ceramic and the input capacitors have been moved between Vin and GND instead of Vin to -Vout. The schematic is the same as the simulation model below, with Cin between GND and V5.
Skärmbild 2024-04-17 135112.png
The board:
IMG20240418104555.jpg
 

MisterBill2

Joined Jan 23, 2018
19,052
Moving those bypass capacitors effectively took them out of being able to bypass the noise. And is the "V Sense" connection really open? Or is that just the way it is drawn.
Like I have stated many times, EVERY PART of a switcher supply is part of the system, and has an effect. That includes those capacitrs moved to different parts of the circuit.
 
Top