Buck Converter problem

Thread Starter

Bebars

Joined Jan 23, 2021
5
Hello, everyone, this is my first time designing an Stm32 breakout board and it has a buck converter built into it.
3.3v regulator is working fine when I plug the USB cable............ 12V to 5V buck converter is not working and the inductor making noises I think it is a routing problem, but I don't know where. I know the routing is a mess, but this is my first PCB design please help. stm32Routing.jpg
 

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Marc Sugrue

Joined Jan 19, 2018
213
Just out of curiosity when you plug in th e USB whats preventing your onboard 5V backfeeding into the USB of the connected device? Are you sure your not just overloading you 5V when you plug in a USB? Also i'm not sure the purpose of Q1 but it looks to be the wrong way around for its location (pins 3 & 2).
 
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DickCappels

Joined Aug 21, 2008
7,418
I can't find the datasheet for U1. What is it and who makes it?

The best way to troubleshoot something like this is to observe the signals along the power supply. For example, is the pulse width on the cathodes of D2 correct? Is Q1 saturated, etc.
 

Thread Starter

Bebars

Joined Jan 23, 2021
5
Hello,DickCappels, thank you for the information ......this is U1's TPS5430 Data sheet
I just test with an oscilloscope across D2 and it is very confusing .. when I apply voltage after 6v or so the inductor starts to make noises, if I increase the volt it becomes unstable, and the power supply limit current shuts off I am not sure where to look to find the problem, unfortunately, I am still a noob when it comes to electronics. o_OAcross-D2.jpg
 

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Thread Starter

Bebars

Joined Jan 23, 2021
5
Hey, Marc Sugrue, I suppose D5, and D1 should prevent that, but maybe I am wrong, and for Q1 it is just for reverse polarity protection I think 3 and 2 pins are correct, but I could be wrong because I am still a beginner and this is my first PCB design.. thank you
 

Marc Sugrue

Joined Jan 19, 2018
213
Hey, Marc Sugrue, I suppose D5, and D1 should prevent that, but maybe I am wrong, and for Q1 it is just for reverse polarity protection I think 3 and 2 pins are correct, but I could be wrong because I am still a beginner and this is my first PCB design.. thank you
D1 Doesn't as its alway forward biased, I hadn't notice D5 sorry but yes that would prevent it.

Q1 your using the body diode as a diode, i see.
 

DickCappels

Joined Aug 21, 2008
7,418
From the waveform in post #4 and the fact that the inductor starts making a noise, it looks like U1 is in current limit. The spikes you see every few milliseconds and the noise appear to be signs of the circuit periodically checking to see whether the overload has been cleared.

Something is probably wired or connected incorrectly, resulting in too high of a load for your power supply.

I also find myself being suspicious of L1. It could cause symptoms such as this if it is saturating. Do you have a photograph that clearly shows L1? How many amps is this supply supposed to handle?
 

Thread Starter

Bebars

Joined Jan 23, 2021
5
Well, I managed to fix the problem. D2 was assembled in the opposite direction from the manufacturer. fortunately, everything is working fine now.
Thank you very much for the help guys.
 

TechWise

Joined Aug 24, 2018
151
As a piece of advice for any future revisions, @Bebars, you should aim to have the diode D2 as close as possible to the switch node output of the buck converter IC. The reason for this, is that when the internal MOSFET in the IC turns off, the inductor current has to come from somewhere else instead. In fact, it comes from the diode D2. However, as it stands, you have quite a long trace connecting the diode to the switch output which can lead to voltage spikes due to the inductance of the trace. I would place the diode as close as possible to the IC first, then worry about placing the inductor. The length of the trace to the inductor is not as important since any inductance in that trace will add to the inductor which is a good thing.
 
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