Step-Down Converter Circuit Works on Breadboard but Not PCB

Thread Starter

ryan1

Joined Jun 16, 2020
23
Hello all!

I have a problem where my step-down converter circuit works on my breadboard but not on my PCB. Essentially, I'm using a Teensy 3.2, a stepper motor, a stepper driver, a CANBus driver, and an encoder to be able to control a stepper motor and get position data over CANBus. I have it all wired up on a breadboard and everything works great.

Step-down converter link

I soldered all the SMDs on the PCB with a reflow oven, soldered my Teensy 3.2 straight onto the PCB, and decided to test out my step-down converter with the blink program uploaded on my Teensy. Strangely, the Teensy did not turn on, and a multimeter showed that the Teensy was only receiving around 1.6V instead of the expected 5V (so the Teensy isn't getting enough power). I replicated the same circuit on my breadboard, and the Teensy blinks happily.

I'm stumped. I'm pretty sure my capacitors and diode are in the right direction (particularly my diode--I think my silkscreen is in the wrong direction, and I made sure to match my breadboard exactly). I checked my soldered connections to make sure they were soldered okay with no large resistances, and everything seemed to be soldered fine. I checked my diode with a multimeter, that seems to be working fine. None of my capacitors are shorted. I checked to make sure none of the pins on the IC were bridged--none of them are except for the ON/OFF and GND pins, which was intended (see datasheet). I'm getting an expected 1-2 ohms on my inductor. The step-down converter chip is getting the expected 20-21V.

To avoid confusion, I took a snapshot of the area with my my step-down converter (rather than the entire PCB). Ignore all the traces on the bottom layer, the ground plane, and the large through holes that I attach my stepper driver carrier board to. The purple arrow indicates the 5V line that goes to the Teensy. The feedback line goes from the 5V on the 100uF cap to the respective pin on the step-down converter I apologize if it is still confusing or if the circuit design is bad--I'm relatively new to electronics. Please let me know if seeing more of the PCB would be useful.

I guess my question is: Does anyone have any advice on how I can troubleshoot it? Do you think one of the chips was damaged in the reflow oven? To anyone willing to read through the step-down converter IC documentation, could it be something I am doing wrong in my layout?

Thank you in advance!
 

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dl324

Joined Mar 30, 2015
11,485
Does anyone have any advice on how I can troubleshoot it?
The usual way. You find something that isn't right and work back until you find something that is. You can also go in the reverse order. Start from the beginning and work your way forward until you find something that isn't working.

It would be helpful if you posted a schematic.
 

ronsimpson

Joined Oct 7, 2019
908
The problem is probably not layout related but .............
The schematic might be this: but ?
1599088936214.png
The layout should look more like this:
1599088990491.png
Note the diode is on pin-6 to pin-8. No long traces.
Pin-4 is very short. I can't see where your pin-4 connects.
Capacitors are close.
No vias.

You can make a "ground" just for the PWM and then connect the two grounds together at some points. By having two different grounds you do not have PWM current in data lines.
1599089507715.png
 

Thread Starter

ryan1

Joined Jun 16, 2020
23
Thank you for all the responses! I've definitely gone between the breadboard and the PCB checking for differences multiple times, but I guess it doesn't hurt to do it with a fresh mind tomorrow.

I was a little hesitant to post the schematic because it might be pretty confusing and the symbols are probably not professional at all, but it's attached to this post. The stepper driver carrier board I'm using is the DRV8825 carrier board from Pololu, but it shouldn't be relevant to this problem.

I wasn't too concerned about the layout, mainly because the breadboard and all spaghetti wiring didn't have a similar layout as that in the documentation but the breadboard setup still worked. However, the wire that does go to the pin 4 (the feedback wire) could be an issue because it definitely is pretty long and does pass by the inductor...maybe I'll try cutting some traces and soldering some wires onto my PCB.

I guess the reason I'm confused with troubleshooting is that I don't really know how to test each individual component of the step-down converter circuit. For instance, what should I expect on a multimeter or oscilloscope when I prod at certain parts of the circuit (i.e. between the inductor and IC output pin, across the feedback wire, across the capacitors, etc)?
 

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Marc Sugrue

Joined Jan 19, 2018
149
You look to have a small short between what looks to be the pin 4 Feedback Pin and the ground on the PCB which may be causing an issue with it shutting down. Have you run a DRC on the PCB to check for shorts?

Just some additional advice. I notice that your tracks on the 5V look rather thin. I appreciate you are probably not putting much current through it but unless you need thin tracks going fatter will give you a more robust PCB especially if you are doing development work such as removing components etc.. Thicker tracks will provide some mechanical strength.
 

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ronsimpson

Joined Oct 7, 2019
908
You look to have a small short between what looks to be the pin 4 Feedback Pin and the ground on the PCB
That picture is made my me not ryan1. Using paint. I was trying to show that the grounds are thin. The real layout is in post #1.

The IC is so simple that trouble shooting is not much.
Pin 7 Voltage?
Pin 5,6 = ground? ohm meter!
What is the voltage on pin 4?
Pin 8 should pull up to almost the input voltage and then down to -0.8V.

I am concerned that the load is shorted. Maybe there is too much load and the IC is in current limit. Can you cut loose the load?
trebla's idea of a different inductor is good.
The traces for 5V are too small but should work.
IC in backwards?
 

Thread Starter

ryan1

Joined Jun 16, 2020
23
Great advice, thank you! So I cut the trace from my Teensy to the step down circuit and the step down converter circuit started to output 5V.

4 questions now (sorry, I'm an electronics amateur):
- Does this mean that the IC is in current limit when the Teensy is connected? Doesn't really make sense because the Teensy shouldn't draw more than 100 mA (I designed the circuit to be rated for 200 mA maximum), and I'm running Blink.c off of it, so it shouldn't be drawing too much power... (Edit: I used a multimeter and the Teensy is only drawing 3-4 mA...)
- How would I go about fixing this (thicker traces, higher current rating inductor, etc)? Edit 2: Replaced my cut trace with a jumper wire, and the step down converter circuit output 1.6 V. So maybe not thicker traces....
- Why would the circuit work on a breadboard but not my PCB?
- Could this have to do anything with the fact that I have not yet connected the stepper driver, stepper motor, and encoder to the PCB?

Thank you!
 
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Thread Starter

ryan1

Joined Jun 16, 2020
23
Sweet! Swapped in a different inductor (with a 340 mA current rating), and it worked! I'll connect everything else and see if everything works.

Still don't know why that would make a difference on the PCB, which is the exact same circuit as the breadboard. Does anyone have any thoughts? I'm wondering if it's related to the reflow oven...could the inductor have been damaged? It's rated for 230 C (50 seconds max), but I had the oven at 250 C for probably around 30-45 seconds.
 

trebla

Joined Jun 29, 2019
209
If inductor is working under big saturation current, then its effective inductance is much less and switching regulators may not work. Another problem can badly shielded feedback circuit connection.

Edit: See from datasheet how much is the inductors rated current and 10% derating saturation current. Try not to exceed this values, better give some room for peak current situations.
 
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Thread Starter

ryan1

Joined Jun 16, 2020
23
Interestingly enough, when I swapped out the old inductor (that didn't work) with a new one (but the same device--not the inductor device with a higher current rating), the Teensy powers on just fine. Maybe it's the reflow oven. I guess I did have a longer melting time than suggested on the datasheet, but I didn't think that it would make that much of a difference...
 

trebla

Joined Jun 29, 2019
209
If you use lead-free solder paste, your components must rated to maximum reflow temperature about 248-250C. This temperature can be applied only for few seconds during soldering process. Also make sure that soaking and final ramp-up parts of profile are'nt on too high temperature. After peak temperature, the cooling phase must not too fast, some fragile components, inductors included, may break from fast cooling.

Edit: Inductor in your regulator may work on the edge of efficiency and another inductor of same type may be have tiny bit more inductance then the previous one.
 
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RPLaJeunesse

Joined Jul 29, 2018
110
Under short circuit conditions the LM2594 switch transistor might pass as much as 1.4A before it turns off ("Current limit" rating, p.6). Will your inductor survive that? I always let peak switch current define my inductor current rating, with the inductor never smaller than 50% of peak switch current. Also, inductor current may peak at 140-150% of regulator rated load current, so make sure that is within the inductor rating.

FYI the diode silkscreen looks correct to me. Cathode to IC pin 8 is proper.
 
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