Can anyone please check my first pcb design for obvious errors? (MPPT) - Page 1
Forum section: electronics / projects

author: cbc02009
author_rank: Regular Contributor
timestamp: August 21, 2018, 05:51:31 pm
content: Hello all,I hope this isn't a lot to ask (I don't really know, considering it's my first design), but I was hoping someone would be kind enough to check out an MPPT controller PCB I've designed and just make sure I'm not doing anything obviously stupid. I have built and tested this circuit on breadboard with ~1A input/outputs, and now I want to move it to actual PCB.The PCB will use a female header to connect to a SAM L21 xplained pro, which will do the computing and controlling for MPPT tracking. It uses an LTC4449 to control the two mosfets in a synchronous buck configuration. There are P-channel mosfets on the input and output to protect against accidental reverse voltage from the load and panel.The circuit uses two allegro hall effect sensors to measure the input and output currents, and voltage dividers to scale the input and output voltages down to the 1V level of the SAML21's internal ADC. All of those signals are buffered using an AD8244 quadruple opamp (this was the one piece I wasn't able to get on the breadboard, but I used other DIP package opamps).the 5V supply needed for the LTC 4449 will be supplied by a external usb power source attached to the bottom header. the 3.3V supply needed for the hall effect sensors will come from the SAML21 pin header.The solar panel I will be using this with is rated at 50W, with Imppt = 2.86A, Vmppt = 17.5V, Voc = 22V, and Isc = 3.14A.The circuit was built and designed in KiCad, since that's what we've used at college, so I'm not sure what format I should use to upload it. Are the Gerber files useful for inspection?The BOM doesn't include the parts I already own (Mosfet driver, through-hole parts, etc).I also have some other questions, if anyone could help me with them.1. Should I put capacitors on the voltage divider outputs, even if they are buffered with the opamp?2. Should I have more vias between the top ground plane and the bottom ground plane? Is there a standard for that sort of thing?3. Having never build a pcb before, I know little to nothing about thermal relief. Do I have enough (or too much) clearance? Should the spokes be wider? Thinner?4. this particular op-amp reccomends guard traces using the output trace around the inputs. Will filled zones do the same thing as actual traces? (I feel like they should, just want to make sure.)5. For the final project I'd like to be able to step up as well as step down the output. I've seen a little about inverse SEPIC controllers. Is it worth it to look into it? Any guides you could recommend for building and calculations? My other thought was to have a buck and a boost and have a mux switch the PWM signal to whichever was needed, depending on weather conditions, etc. That might require a lot of coding, especially if they require different PI constants.Attached are pictures of the front and back copper, and a zip file including the project, BOM, and the gerber files. If there's any other format I should convert them to, please let me know! The images are high rez, so you should be able to view them full size.Thank you kindly!Front CopperBack CopperInputOutputBuck ConverterADC Buffer

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 22, 2018, 07:00:59 am
content: Lots of copper fill -- good!No vias stitching the fills together -- bad!Tim

author: obiwanjacobi
author_rank: Super Contributor
timestamp: August 22, 2018, 07:37:35 am
content: Mounting holes?Why is it so big? Could be half the size (quick look)...?

author: cbc02009
author_rank: Regular Contributor
timestamp: August 22, 2018, 12:24:11 pm
content: Quote from: T3sl4co1l on August 22, 2018, 07:00:59 amThat was definitely one of my questions. Will do. Is there a standard for that, or is it just "have enough to get it done"?

author: cbc02009
author_rank: Regular Contributor
timestamp: August 22, 2018, 12:25:19 pm
content: Quote from: obiwanjacobi on August 22, 2018, 07:37:35 amThe size is only because the manufacturer (seeedstudio) does 100mm x 100mm pcbs for $5. It definitely doesn't need that much space.And thanks, I knew I forgot something!

author: Rerouter
author_rank: Super Contributor
timestamp: August 22, 2018, 12:30:36 pm
content: For Via stitching, the goal is to make sure your signal traces have a nearby ground return, So when a signal goes over a break in the ground, try and provide a stitch across it. When a chip connects to ground, provide it a nearby path to the plane, etc

author: obiwanjacobi
author_rank: Super Contributor
timestamp: August 22, 2018, 01:23:43 pm
content: Quote from: cbc02009 on August 22, 2018, 12:25:19 pmMake it half as big and then duplicate/copy (double) the layout...

author: Siwastaja
author_rank: Super Contributor
timestamp: August 22, 2018, 04:38:20 pm
content: Those thermal reliefs on the power semiconductors kind of defeat the purpose.Of course, if your design doesn't produce nearly the amount heat these semiconductors have been designed to work with (with a proper footprint, directly connecting to a copper pour), then it's not an issue. Have you done the thermal analysis? Power semicon datasheets often provide thermal resistance junction-to-ambient for a standard footprint with some specific amount (such as 1 to 2 square inches) of fill, so if you go for direct connection instead of the reliefs, you could utilize this number from the datasheet.Reliefs of course make it easier to solder if you really won't need the cooling capacity.

author: DaJMasta
author_rank: Super Contributor
timestamp: August 22, 2018, 05:49:07 pm
content: As sort of a side thing, since you have so much space, at least keep your parts away from your connectors. It's easy to run into mechanical clearance issues, and while I don't think these are particularly likely to be an issue, it's nice to have the space to probe/rework/add in stuff especially early in development, and especially the topmost opamp seems both closer to the connector than need be and close enough to make it difficult to debug or measure in circuit when the connector is in use.Not a big thing, but something. Otherwise, smaller size would make sense unless you're fitting into a specific size (could also save some money depending on pcb manufacturer), and the via stitching is nice - my rule of thumb is something around the perimeter where possible, around signal traces (especially sensitive analog or high speed digital), and then a few thrown in to connect some of the areas of filled copper. Makes more of a difference when parts are more tightly packed so you can get the fill to cover more space with adding some extras, but nice to make sure your ground plane is generally low impedance to wherever it goes.

author: Phoenix
author_rank: Frequent Contributor
timestamp: August 23, 2018, 07:38:18 am
content: Hi cbcThere is a major problem with your buck circuit. The IRS21867S gate drive IC requires both high and low side switches for the bootstrap supply. The bootstrap capacitor is usually charged up by turning on the lower MOSFET for a charging path VDD -> D -> C602 ->GND and then holds enough charge to keep Q601 on until the next switching cycle.You only have a high side MOSFET so the bootstrap capacitor C602 will never charge up.

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 24, 2018, 11:47:25 am
content: Make the pcb smaller and add another pcb on the project, I usually add footprint adapters just for example.cost you 0Envoyé de mon iPad en utilisant Tapatalk

author: cbc02009
author_rank: Regular Contributor
timestamp: August 26, 2018, 11:23:03 am
content: Quote from: Phoenix on August 23, 2018, 07:38:18 amHello,Can you explain this a little bit more? I'm using the LTC4449 for a gate driver, not the IRS21867S, and I have both high side and low side mosfets, a pair of BSC010NE2LS. In breadboard prototyping, I used the through-hole FDP8880 instead, and the circuit worked fine.

author: cbc02009
author_rank: Regular Contributor
timestamp: August 26, 2018, 11:24:15 am
content: Quote from: perieanuo on August 24, 2018, 11:47:25 amThat's a brilliant idea! thank you!

author: cbc02009
author_rank: Regular Contributor
timestamp: August 26, 2018, 11:30:27 am
content: Quote from: Siwastaja on August 22, 2018, 04:38:20 pmWe haven't done anything relating to thermal analysis in any of the classes I've taken yet, So I'm not sure what how to go about that. The mosfets I chose are rated for ~100A, of which I'm only using about 5 at the most, So I don't expect them to get very warm.Quote from: DaJMasta on August 22, 2018, 05:49:07 pmProbe points are a great idea, thank you!

author: Julien3146
author_rank: Contributor
timestamp: August 26, 2018, 01:02:25 pm
content: Hi! I'm not a pro designer either, but I just want to share my experience with rated current on datasheets.I once used a 20A MOSFET to drive a 1A load, overkill right ?Well, the frequent thermal shutdown proved me wrong. The max rated current doesn't matter,what really matters is junction to ambiant and/or case temperature provided in the datasheet, in °C/W.I maybe say some utter garbage here, as noone ever told me how to use these values, but from what I understand :Imagine you've got a 60°C/W junction to ambiant temperature value.You're driving 5A with a 0.5ohm Rdson MOSFET, it'll dissipate 12.5W, and there you are with your more than deadly junction temperature increase.Please guys, if i'm totally mistaken, please correct me, that's how I imagine these numbers work, I've never took the time to research it, shame on me.Cheers,Julien

author: cbc02009
author_rank: Regular Contributor
timestamp: August 26, 2018, 02:45:21 pm
content: Quote from: Julien3146 on August 26, 2018, 01:02:25 pmThat is correct as far as I understand it, thank you. The RDSon of the MOSFET I am using is 1 mOhm, which, means the total power on the mosfets should never be more than 5-10mW. the junction temperature increase is 50K/W (not sure why kelvin instead of C it's basically the same thing), which should give a max temperature increase of about .5 degrees.

author: Phoenix
author_rank: Frequent Contributor
timestamp: August 27, 2018, 06:12:31 am
content: Quote from: cbc02009 on August 26, 2018, 11:23:03 amI downloaded your zip file for the "MPPT Controller ext MPU.zip". On second look the PDF schematic in there is for an entirely different board!?

author: cbc02009
author_rank: Regular Contributor
timestamp: August 29, 2018, 03:10:44 pm
content: Quote from: Phoenix on August 27, 2018, 06:12:31 amOh jeez, I'm really sorry about that. That is from a much older version of the board. My sincerest apologies.