Switch mode power supply problems.

[and101]

I am trying to design a 12V smart USB charger based around the TPS54394 step-down switcher from TI and I have run into a problem.  I based the switching circuit off of a design from TI using their power supply workbench tool and it is supposed to work between 9V and 16V input.  Up to 12V it works fine with a stable 5V output but above 12V the power output starts to get erratic with glitches like the ones in the screen shot I have attached from my scope.  The higher the voltage gets the worse the output becomes.  It also increases as the load on the output increases, with no load the output is smooth.

The circuit also includes a 3V supply using the other half of the TPS54394 and this side of the supply is working properly but that could be due to the fact that it is only powering a microcontroller and bluetooth module so the current draw is only a few milliamps and possibly not enough to cause the erratic behaviour that the 5V side is showing.

I don't have much experience with designing switching supplies, I normally use off the shelf modules so I am not sure if it is a problem with the circuit or the PCB layout. I have attached the schematic for the power supply part of the circuit as well as the PCB layout.  Hopefully someone with a bit more experience in switching PSU design can tell me where I have gone wrong.

[rbola35618]

Hi,

What frequency is the PWM running, and what are the full load currents?

Robert

[rbola35618]

Hi And101,

On page 15 of the datasheet, it says that a 1 uF ceramic capacitor must be connected between the VREG5 and GND pins for proper operation. It is recommended to use a ceramic capacitor with a dielectric of X5R or better.

Robert

[and101]

Hi Robert

Thank you, I added a 1 uF capacitor onto the VGREG5 and ground pins and it is now smooth on the 5V output. 

Next time I will read the datasheet instead of just believing that TIs webbench designs are correct.   

[diyaudio]

I am trying to design a 12V smart USB charger based around the TPS54394 step-down switcher from TI and I have run into a problem.  I based the switching circuit off of a design from TI using their power supply workbench tool and it is supposed to work between 9V and 16V input.  Up to 12V it works fine with a stable 5V output but above 12V the power output starts to get erratic with glitches like the ones in the screen shot I have attached from my scope.  The higher the voltage gets the worse the output becomes.  It also increases as the load on the output increases, with no load the output is smooth.

The circuit also includes a 3V supply using the other half of the TPS54394 and this side of the supply is working properly but that could be due to the fact that it is only powering a microcontroller and bluetooth module so the current draw is only a few milliamps and possibly not enough to cause the erratic behaviour that the 5V side is showing.

I don't have much experience with designing switching supplies, I normally use off the shelf modules so I am not sure if it is a problem with the circuit or the PCB layout. I have attached the schematic for the power supply part of the circuit as well as the PCB layout.  Hopefully someone with a bit more experience in switching PSU design can tell me where I have gone wrong.

TPS54394

Fixed Soft Start : 1 ms, The erratic waveforms shows the controller resting itself, a short maybe ?
700 kHz Switching Frequency, what's your frequency ?
You show a 3D model, is your design on an actual PCB? if so post your rig.