First Oscillation

One of the things that this project is doing is pushing my skills designing hardware. An area I had largely avoided until now was switch mode power supplies (SMPS). For this project, I am planning to use +12V as the base power rail, +5V as the main distribution rail on the backplane for all the cards, and then local regulation down to +3.3V. The +5V to +3.3V is totally doable (and maybe even preferrably done) with a low dropout (LDO) linear regulator. But the +12V to +5V is a bit more as we're talking about 5-6A of current needed.

SMPS are typically very efficient, and I was hoping to use that to my advantage. It's not that I really need the efficiency, but it's like with machining, you always work to tight tolerances so when you need them, you know how. TI has an amazing tool for building out power circuits, and with some knob twiddling, it spit out a design based around the TPS566238, a wee little QFN package (3mm) that would have a target efficiency of 95%. Perfect!

In this post, I'm hoping to dive into a bit of the design and some logic around it all.

The Centerpiece

Before digging into the specific implementation, let's talk about the chip that it is built around. The TPS566238 is a quite modern regulator for an SMPS. On the surface, the implementation is pretty simple:

That's it, right? Well, almost, but not quite as we'll see in a bit.

As I said earlier, part of the driver is the efficiency of the design, which you can see here:

The part can support up to 6A of output current, which makes it perfect for the use case. In addition, it has something like a 50µA quiescent current. Quiescent current being, very roughly, the "overhead" of the chip, or the power that's used that isn't contributing to the switching and outputs. It's more than that, but that's a close enough definition for these purposes.

The Beginning and the End

It all starts with power coming in at +12V, and going out at +5V. Since the project is built around a DIN 41612-based backplane, we need to tie all that together. As you can see from the schematic to the right, we are using quite a few pins for power. The typical DIN 41612 connector is rated at 2A per contact, so with 8 pins (4 at the "top", and 4 at the "bottom"), that gives us a capacity of 16A, and while we'll need to derate the capacity, we won't need to do so more than 15-20%, leaving us plenty of excess current-carrying capacity.

Additionally, there are quite a few ground pins spread out through the connector. I think it's Rick Hartley that recommends almost 50% grounds, I figured that since I was not doing anything truly high-speed, that about 25% would be more than enough.

Now that we have both a source and sink for current, let's dig into the meat of the design.

Theory to Reality

The "typical application" shown in many data sheets is a highly simplified version of the reality, but it's not misleadingly so. What I ended up with, in the first round at least, was this:

You'll see along the schematic, that there's a lot of text boxes with various specs in them. These exist because, quite honestly, I wanted to keep my thought process and decisions localized to the schematic for future me to use. A few things to call out:

1. I upped the inductor from 1.5 to 2.2uH, while keeping the DC resistance (DCR) and saturation current (IDC) roughly the same. The part I chose was Pulse BMQ part. This is a shielded inductor with an DCR of 8.5mΩ and an IDC of 12.5A. The additional inductance allows for a lower minimum input voltage.
2. I used a similar mix of capacitor packages as recommended, although slightly fewer 0402 packages. Smaller packages reduce the loop inductance of the circuit. I also tried to streamline the bill of materials (BOM) a bit by using multiple of certain capacitors rather than one large one. Everything on the board is either Murata or TDK X7R, with the exception of one X5R composition.
3. The feedback (FB) circuit is built with 1% Vishay CRCW thick-film resistors. They're about $0.01, or less, in any quantity.
4. I didn't do anything with the enable (EN) pin. The datasheet is clear that if left unconnected, the internal pull-up resistor will activate the chip.

That's it. Other than the tiny passives, it's not a super complicated circuit.

Blinking (or not) Lights

Everything needs more lights, although in this case, they shouldn't be blinking. The last little bit of the circuit was a test point and some LEDs:

Again, as before, all the math and calculations around current-limiting resistors is included in the schematic. The LTST-C190KGKT are small 0603 LEDs. While I could have gone bigger, I'm trying to push myself to work with smaller components, as that's just the direction the industry is going in.

The Result

I shipped the designs off to JLCPCB, which is the contract manufacturer I use almost exclusively, and after getting the boards back, I did a janky job of soldering up everything, and identified a few design issues:

1. The QFN package is stupidly small. Like, just insane. But, I did manage to get it soldered the first time by applying a tiny bit of solder to the pads and then using a hot air reflow station.
2. The default footprint for the inductor is just barely too small, and required some finessing to get it to actually solder to the PCB.
3. I put an 0402 right up against the inductor, and that was just stupidly hard to solder. I went through 3-4 of them (cheap!) before I finally got something kinda not terrible.

On first power up on a bench power supply with 12V and current heavily limited, there was no smoke, which, given my inexperience here, and my lousy soldering, is quite surprising. It even generated 5V.

It also generated not great output (about 500mV ripple) at around 3kHz and a rather annoying 17kHz audible squeal which I could never quite identify the source of. I think it was some kind of reverse microphonics with the inductor because I could press on it and get the volume to change. Fun! I think the output ripple might be related to not having much load on it, as I couldn't find my programmable load in the basement. SMPS don't really like being unloaded, and a 2W resistor didn't help much there.

One final area I was worried about was whether or not temperature would be an issue, so I let it run with what little load I could generate for a while, and watched it on an IR camera.

As you can see, both the inductor and the regulator/controller both stayed in a very safe range of 34.4-3.45°C. I'll need to verify this under load to have any confidence in the design, but that means finding my programmable load.

Once I've got that, I need to run down those two issues. But, hey, for a first SMPS ever, using tiny freaking parts, there was, in fact, no smoke in the smoke test. So, for now, I'm happy.