Electronic – Debugging a nonsynchronous step-down power supply problem

power supply

I'm a software engineer doing some basic (I thought) hardware design. I have built a 5V power supply around an Analog Devices ADP2303 non synchronous step-down regulator. The schematic is from the top of page 25 in their data sheet.

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I have three PCBs built, the original prototype and two "second generation". The schematics and parts are the same but the PCB layouts are slightly different (I had to cut traces and add two jumpers on the original to get around a design error). My problem is that the original board works perfectly, holding 5 VDC under load, but in both newer boards, the output drops to 2.4V when I connect a load – a Raspberry Pi. In all three cases, I'm using the exact same 12V 5A AC adaptor as a power source.

I have no idea how to debug this problem and would appreciate any pointers. I do not have an oscilloscope, so I hope that's not a requirement.

In looking more closely at my PCB layout, I definitely did not follow AD's layout at all, in either generation of board. But the two big differences are (a) gen-1 punched the ADP2303 ground pad thru to a full ground plane on the back of the PCB, while the gen-2 did not (I don't know why), and (b) in gen-1 the connections between pin 5 (FB) and the output capacitor and between pin 7 and ground are jumper wires instead of traces. So I'm wondering what the likelihood is that these differences are responsible for the fact that gen-1 can support a Raspberry Pi at 5V while gen-2 output drops to 2.4V when connected to a Pi, and whether redesigning this part of the board will solve the problem.

[update]

As there is a clear consensus that my PCB layout is really pathetic, I am redesigning based on AD's recommended layout:

Recommended PCB layout

Thank you all so much for taking the time to explain and recommend. I will update with my results when I get the redesigned board.

Best Answer

If I understand correctly, you have two generations of PCB with the same parts but different layout. And the following result:

  • Gen 1 - no voltage drop
  • Gen 2 - voltage drops from 5 to 2.4 under load

If the components are the same, then the problem must be with connections. You may have an open or a short somewhere, or forgot to connect some component lead. You basically need to perform circuit analysis and determine what is different. Your Gen 1 proves that the circuit and components are capable of working, but then Gen 2 board has a flaw. My hunch is that the task of cutting traces and adding jumpers to correct a design flaw is incomplete: there must be (an) additional design flaw(s) you overlooked.

Debugging it needn't involve an oscilloscope unless you need to look at high speed signals in your circuit.

  • Power off the board and closely inspect all traces and connections. Does the Gen 2 board match exactly (electrically) the Gen 1 board?
  • Verify with a multimeter that traces are connected to places they ought to be, and that they aren't shorted to places they shouldn't. Ideally this would be done before populating the board with components, but you should still be able to confidently say that your ground plane isn't shorted to something else, and so forth.
  • Are you sure the components are the same? In populating the Gen 2 boards, is it possible you used the wrong value component somewhere which disagrees with the Gen 1 version? A transistor mounted incorrectly or a diode installed backwards can cause strange results.
  • With power on, there may be a component overheating because it is drawing excess current (and thus causing your supply voltage to drop). Leaving it powered on could cause further damage, but you might be able to quickly determine what component(s) are overheating (if any). Additionally, you might be able to make some quick voltage measurements with a multimeter to compare the Gen 1 and Gen 2 boards. Finding what nets are the same voltage in both generations could help you isolate a component or board section that is flawed.
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