MAX712 IC NIMH charging PCB not fast charging - Page 1
Forum section: electronics / projects

author: clark
author_rank: Newbie
timestamp: November 15, 2024, 02:49:27 pm
content: Hi all,This is my first post on here so bear with me!I have been designing a PCB for my speaker to replace the old style 15VDC barrel jack with USB-C. This PCB also provides load sharing by powering the speaker directly and disconnecting and charging the 8s NIMH battery when USB-C power is connected. My USB-C circuit seems to be working exactly as intended, it negotiates 15 VDC from USB-C PD, lighting the 'PWR' LED.However when I originally built the PCB, only the PWR LED came on not the CHARGE LED. I isolated this issue to the THI pin being connected to the V+ pin on the wrong side of the LED. Whilst working this out and poking around with my DMM, I accidentally shorted some pins on the IC and the magic smoke came out. So I built up another on the PCB, with a small jumper wire connecting V+ and THI directly, and cutting the old THI pcb trace. With this change both LEDs came on and the IC appears to be getting correct 5V (4.75) shunted power.The circuit does also then charge the battery. However the MAX712 IC (DATASHEET:https://www.mouser.co.uk/datasheet/2/609/MAX712_MAX713-3470451.pdf) only ever enters trickle charge mode, and also never cuts off the charge after the designed 264 minute timeout.Measurements with a DMM show 0.122A of current across Rsense (R11), which is very close to the designed 0.116A for trickle charge, and in tolerance given my crappy DMM. Measurements also show Temp pin as 0.51V, TLO pin as 0.1V, and THI pin as 4.75V correctly disabling temperature cutoffs on the MAX712 IC, meaning that should not stop the IC entering fast charge mode. The only other reason I could think of was if the voltage being supplied was not high enough for the IC to enter fast charge (even though that should only cause an issue with ending fast charge not entering) So I tried with the USB-C negotiating 20VDC, and still no fast charge.The MAX712 IC also never ends the trickle charge, despite being setup with correct BATT- voltages on PGM2 and PGM3, and in 8S configuration PGM1 BATT-, PGM0 floating (3.4V).I have gone over the datasheet numerous times, and cant find an error in my setup of the MAX712 IC, and am apprehensive about saying its a bad IC given the first built PCB didnt work either (even if i did get the £0.7 AliExpress ICs vs the £8 Mouser ones).Its also my first PCB design in a while so wondering if ive missed something there and thats causing fast charge and timer not to work...Any help will be hugely appreciatedI have attached photos of the Schematic (red boxes show post PCB build changes mentioned earlier), the PCB (including all 4 layers), and built PCB trickle charging an 8S NIMH pack!

author: shabaz
author_rank: Frequent Contributor
timestamp: November 15, 2024, 04:59:09 pm
content: Hi,A detailed, sharply-focused photo of the MAX712 IC will be worth examining, just in case it is a fake part. There's a very high chance the IC is fake, since you mention it was purchased from AliExpress. Sometimes, they are easily identifiable if they have a surface that looks sanded-down, or if the part markings look slightly different from the usual manufacturer markings.

author: clark
author_rank: Newbie
timestamp: November 15, 2024, 09:03:49 pm
content: This is as close a photo as I can seem to get!Doesn’t seem to be sanded off anywhere or anything, the other IC I got from the same supplier that also didn’t work looked slightly different, with a dot denoting pin 1 instead of the line.Surely it would be very bad luck for it to be two bad ICs, both with only the fast charge and timer function not working, but doing trickle charge?Does the rest of my design look okay, no obvious blunders?

author: shabaz
author_rank: Frequent Contributor
timestamp: November 15, 2024, 10:25:31 pm
content: Hard to tell, but I believe it's a fake part (they often have a very thin font that appears to have been engraved in, rather than printed on).The attached photo shows a couple of genuine Maxim parts (old on the left, new on the right). To me the font appears a little thicker on these two parts.Generally it's highly likely to receive fake ICs from AliExpress, I'm afraid those are likely at least part of the cause, if not all. It's been a very long time since I used the MAX712, and would not be able to immediately spot an issue elsewhere in the circuit.

author: tooki
author_rank: Super Contributor
timestamp: November 15, 2024, 11:29:05 pm
content: The way you broke up the schematic into tiny blocks makes it really, really hard to actually understand the circuit you have drawn. In particular, 1) this style of schematic completely hides the flow of signals and power (which by convention flow overall from left to right), and 2) breaking out key parts of the MAX712 circuit into tiny blocks is just a bad idea; it’s smarter to stay close to what the datasheet shows, since then you can easily compare and make sure you got it right.Also, it’s weird that you have “power delivery” components in the charger schematic area; those things don’t have anything to do with PD.With that out of the way: Your temperature disabling is incomplete. The datasheet says that to completely disable temperature monitoring, you must:1. Omit the thermistors and associated resistors and caps.2. Tie THI to V+3. Tie TLO to BATT-4. Tie TEMP to BATT- via a 22k resistor5. Tie TEMP to REF via a 68k resistorYou seem to have left out #3 in the schematic. Is #2 now done properly?The schematic’s description “THI AFTER D3” is meaningless since there is no “before” or “after” in a schematic, and your post just says you moved it, but it’s not clear from where to where. (Is the schematic corrected or not?)Your understanding of the timeout may be incorrect. You say that it “only enters trickle charge mode and also never cuts off after the 264min cutoff.” That suggests that you think it should end trickle charging after the 264mins. But this chip has only two modes: fast charge, and trickle. (See the first note under the table on page 9 of the datasheet.) There is no “off” mode. The cutoff timer forces a transition from fast charge to trickle charge, where it remains until the charger is restarted, or the battery is removed and reconnected.15V is not enough for your battery pack. The datasheet says on page 9 that the input voltage must be at least: (# of cells x maximum voltage per cell) + 1.5V. On the next page they give as a rule of thumb to use 1.9V per cell.So 8 x 1.9V + 1.5V = 16.7VSo you’ll need to use PD at 20V and adjust the value of R7, and you may want to implement the added circuit shown in figure 5 on datasheet page 9, and described on page 10.You are missing two components from the typical application circuit: 10nF and 150 ohms, each from DC IN (VBUS) to DRV. You also added a cap, C4, between V+ and GND. Was that intended to be between VBUS and GND, like the datasheet shows? (You have that elsewhere in your schematic, but I assume that is intended to be separate.I’m not sure if inserting Q3 between the MAX712 and C6 causes any problems, since the cap is now invisible to the chip.What is the purpose of R10?The USB-C standard unequivocally requires the connector shield and ground to be directly connected without any resistor or cap. Seehttps://www.eevblog.com/forum/projects/why-usb-c-gnd-is-being-connected-to-the-shield-of-the-cable-after-connecting/By the way, the MAX712/713 datasheet repeatedly refers to “linear mode” in ways that don’t make much sense. I discovered this years ago when building a circuit with it. Turns out that older versions of the datasheet also described a switching mode. That was later haphazardly removed from the datasheet (which is mentioned in the version history), leaving remnants of text that make no sense, since linear mode is now the only mode described. I’ve attached the old version below for posterity.

author: tooki
author_rank: Super Contributor
timestamp: November 16, 2024, 12:04:51 am
content: Two more things:D2 and D3 will always show the same indication. You’ve labeled D3 as “charge” and D2 as “power”, but D3 is just the power LED for the MAX712, so they will always light in unison.Remember, the MAX712 has only two states: fast charge and trickle charge. If it’s powered up and not in fast charge mode, then it’s in trickle charge mode.Second, the PCB layout. You’ve got BATT- and BATT+ running through dinky thin traces, and in the case of BATT-, going through a single little via, and in the case of BATT+, going to Q3 through an even thinner trace. Yes, for 1A it’s probably ok, but it’s suboptimal, and given that you went for 4 layers, totally unnecessary since you’ve got the room for fatter traces. I would probably have made the top layer fill GND (look at the sheer number of components that need to connect to GND, as opposed to VBUS that you made it instead). For all the power traces that have to jump layers, use bigger and/or multiple vias.

author: clark
author_rank: Newbie
timestamp: November 16, 2024, 11:00:09 am
content: Hi Tooki, thanks for the great replies!You were completely right, I had missed the two components on the DRV pin and left TLO floating!I fixed those issues, as well as the one Cap being in the wrong place, and grounding the USB C shield, and bingo it now enters fast charge!!!I left it with 15VDC PD for these tests as the higher voltage shouldn’t effect it entering fast charge, and it didn’t!Weirdly I now have a small plume of smoke from the PCB when it is powered. The plume doesn’t seem to come from a specific component, and the whole board itself heats up quite hot.Could this be just to the small trace size? They were spec’d for the current with kicads trace size calculator, and I have added secondary current carrying wire for this test on the Bat Q3 traces that were too small in error…

author: tooki
author_rank: Super Contributor
timestamp: November 16, 2024, 01:16:28 pm
content: Glad to hear that it’s now working. Despite this, be a dear and answer the couple of questions I asked in my reply. Remember that forum threads also help future users who search for solutions to a specific problem.As for the heating:It’s been over 4 years since I built my MAX713 circuit, and I vaguely remember that some part would get hot. If not the transistor, then probably a resistor. But I don’t remember which, and I can’t find that board to look at it… (it was built on protoboard, not a custom PCB).It’s unlikely the whole board is generating heat uniformly. Much more likely that one or maybe two components are producing heat, and the big copper pours are spreading the heat around. (Which is actually a good thing, since it’s basically free heatsinking.) I assume the plume is from flux residues left on the board?Do you have access to a thermal camera? See if you can use someone’s. Otherwise, a trick is to spray isopropyl alcohol all over the board and see which components dry off faster than the rest. (I’ve also seen videos where people dust rosin powder over the board and see where it melts.)When you did your trace width calculations, did you remember to account for internal layers typically using thinner copper than outer ones?Regarding input voltage: yes, it’ll enter fast charge, but it may not be able to correctly detect end of charge, and would just keep charging until the timer runs out.

author: clark
author_rank: Newbie
timestamp: November 16, 2024, 08:09:45 pm
content: Sure, here's answers to anything I think I've missed:1. THI is now done correctly, and directly connected to V+. The schematic is wrong and shows it connected to V+ via D3.2. C4 should be connected between VBUS and GND, not V+ and GND as in the schematic3. R10 is a pulldown resistor, it prevents the leakage current of D6 from turning off Q3 when no VBUS is present4. You are correct D3 and D4 light in unison, that was designed this way on purpose, but either could be omitted.I think the smoke plume was from some old flux under an IC or something, as it is no longer happening. I also tried the IPA trick, and it seems to be Q1 (https://www.vishay.com/docs/69075/si3456ddv.pdf) which is getting very hot. Other components are getting warm as expected with ~1A current through them, but Q1 is too hot to touch! Seems strange as RDs(on) is 0.04 Ohms, and P =I^2R, so for max charge current ~1A, should be 0.04W power dissipation. With a max junction-to-ambient of 74degrees/W, it should only have a temp rise of 2.96 degrees C over ambient?!Yes trace calcs were done taking into account inner layer thickness (no traces are ran on internal layers they are both whole ground planes), but if i remember correctly I think I was liberal on the temp rise I allowed for the current capacity, perhaps as high as 30deg. So you are right that wider traces would be more optimal on this board.I agree with needing 20V PD, and this PCB is already designed so a 100k pull up can be added to negotiate 20VDC, but I need to order a new TVS diode first as the current one in the design is only rated for 15 VDC. But once I have this I will also configure it to 20VDC (I will likely design a V2 of th whole PCB with all the changes discussed implemented)

author: tooki
author_rank: Super Contributor
timestamp: November 17, 2024, 04:01:48 am
content: Quote from: clark on November 16, 2024, 08:09:45 pmWell, the actual RDSon of Q1 is clearly much higher than 0.04 ohms, since it’s getting so hot. Try measuring the actual gate voltage it’s seeing. If it is 4V or higher, then you should be getting under 70mOhms, which should still produce negligible heat. (In the MOSFET datasheet, second graph on page 4, that RDSon is significantly higher at the higher junction temperature — and right now you do have an elevated temperature.) Note also that the maximum junction-to-ambient is not 74K/W, but 120 — see footnote C, and the fact that the 74 is in a line that states that applies to time under 5s. But you’re using a steady state, so the higher resistance applies. (Still doesn’t explain the heating you’re seeing, of course.) But yeah, either the gate voltage is actually much lower than expected, or perhaps you got a bad or counterfeit MOSFET that has a much higher RDSon?

author: clark
author_rank: Newbie
timestamp: November 17, 2024, 01:21:26 pm
content: Okay strange one, Vgs is about 0.01V?!!U1 drives Q1, and when I check U1 voltages I get V+ pin = 14.95V (So Vusb is correctly negotiated), But for VbusG i get 13.12V and for Vbus so get about 13.12V. I tested my other board, and it gets VbusG 18.9V and Vbus 14.95V giving a Vgs of 4.25V, and as such Q1 does not get hot.I though this may be a U1 issue as it is outputting the wrong voltages on Vbus and VbusG. So i swapped the U1 chips on the two boards, but the same issue. The board that does fastcharge has 13.12V on Vbus and VbusG, and the other board has 18.9V and 14.95V respectively. So it cannot be a bad U1.I then swapped Q1 with the working Q1, and still it didnt work (always possible i damaged it removing it).So I then fully removed Q1, and wired Vusb to VBUS directly, and the whole circuit works as intended.