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| TPS5430 buck convertor - oscillates & wouldn't start |
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| Niklas:
What you see at lower load current is discontinuos current (DCM) through the inductor. During the high part of the pwm the inductor is charged and that is followed by a short low part where the inductor is discharged through the load and freewheeling diode. The sinusoidal part is just due to residual charge that make the parasitics oscillate. You can try a 10k resistor from the switch node to GND and see that it will bleed the oscillating charge so the ringing is dampend. You will not need it in your design, but just to see that the energy level is quite low in the sine wave part. When the load current increases, then there will always flow current through the inductor (CCM). That current is typically sawtooth shaped and centered around the load's average DC current. |
| SiliconWizard:
--- Quote from: zserg on December 02, 2019, 05:09:31 pm ---Thanks for all suggestions. It turned out to be dodgy connection on the voltage feedback resitor divider. All measurements were OK, but today I just blew off both resistors with hot air to check them separately, then soldered both of them back and it works. Shame on me. |O --- End quote --- Oh, that can happen to just anyone! The important thing here is hopefully you will have learned one lesson or two! First one, checking assembly after hand-soldering. If you have one, use a stereo microscope, also don't hesitate to check connections with a multimeter when in doubt. The second one, how to debug hardware using a methodical approach. |
| zserg:
--- Quote from: SiliconWizard on December 03, 2019, 01:38:10 am --- --- Quote from: zserg on December 02, 2019, 05:09:31 pm ---Thanks for all suggestions. It turned out to be dodgy connection on the voltage feedback resitor divider. All measurements were OK, but today I just blew off both resistors with hot air to check them separately, then soldered both of them back and it works. Shame on me. |O --- End quote --- Oh, that can happen to just anyone! The important thing here is hopefully you will have learned one lesson or two! First one, checking assembly after hand-soldering. If you have one, use a stereo microscope, also don't hesitate to check connections with a multimeter when in doubt. The second one, how to debug hardware using a methodical approach. --- End quote --- Well, the point it, everything was absolutely fine according to multimeter readings - I've checked all parts' values as well as all the connections when I figured out it wouldn't turn on. So it's a bit of a mystery why de-soldering and soldering back did fix it. |
| SiliconWizard:
Some bad soldering jobs can create poor connections that are hard to spot with a multimeter. Of course, you first have to test from component pin to component pin. But even so, sometimes just pressing on the pins with a probe is enough to make a good connection, so you miss the problem. This is where a microscope will also help. But as you saw, inspection is only one step. From your symptoms, it was clear there was a problem either with the feedback network, or with the converter itself. |
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