Wide input voltage power supply design challenge for marine electronics - Page 1
Forum section: electronics / projects

author: TVHeadedRobots
author_rank: Contributor
timestamp: June 08, 2015, 02:47:07 pm
content: Hi,Here is the challenge; I am designing a product that uses a SIM900 cell module and in testing I've observed pulse currents for ~1.2ms durations at ~2.5A during TX. I also observed unexpected behaviour from brownout conditions during these events with an inadequate power supply. The rest of the system's power requirements are minimal. The issue is that the system requires a wide input voltage range of around 10V - 36V to accommodate 12V & 24V systems. I was also hoping to use a module to reduce design and testing work. I managed to find a 1.5A module (OKI-78SR-3.3/1.5-W36-C) that fits the voltage range and is relatively inexpensive, but that doesn't give me the 2A suggested by the datasheet that I should have. The other issue is that the system needs to be battery powered in the event of a main power failure. Backup battery power isn't required for more than 24hrs. I was hoping to use 6x lithium primary AA cells, but now this pulse current is scaring me. Obviously I need to test this further, but I'm curious to get some input on possible solutions to these requirements.The simplest option I was considering was adding bulk capacitance at the SIM900. The 1s avg current draw from the system during operation is at most 120mA. My concern is that; 1, bulk cap is just not a good solution here and 2, that the primary AA cells won't be able to handle the pulse load and keep the DC-DC operating during a main system power failure and SIM900 TX conditions.The not-so-simple solution that I've considered is adding a LiPo and charge controller. In this design the system would be powered from the LiPo at all times. Unfortunately this mean added complexity and cost. Since I still have a wide input voltage range I'm likely going to still need a DC-DC (haven't researched this) in front of the charge controller. This solution also adds some concern due to the use of a LiPo in a potentially harsh environment. Although we can suggest where to install the device and provide warnings on the hazards for LiPos there is still the possibility, since this is a consumer device , of all kinds of shenanigans in salt water (enclosure is IP65.) For this reason I was attempting to keep it simple with primary cells. It also keeps cost down some.Thanks for the input.

author: Ian.M
author_rank: Super Contributor
timestamp: June 08, 2015, 03:36:17 pm
content: Its worse than that. A marine installation is a much higher risk for a load dump transient than a land automotive installation. Vibration from big engines & propellers + pounding in waves + battery switches that most operators use daily = a much higher risk of the battery becoming disconnected with the engine running. If your system's PSU cant survive a 200V half second pulse, you've got problems.Enough bulk capacitance on the sim900 module's supply rail will vastly ease the design of the rest of your PSU. Supercaps rated for pulse discharge at >3A would probably be the way to go. Low Rds_on high side switch after the cap bank so you can powerup/powerdown the module cleanly.Backup is a whole other issue. In an AA form factor, designing for anything other than regular Alkaline cells is stupid as, unless you implement a load test routine, you cant control what the customer puts in the battery holder.

author: TVHeadedRobots
author_rank: Contributor
timestamp: June 08, 2015, 05:31:14 pm
content: Ian.M,Thanks for the reply. Glad to hear that you think that bulk capacitance might work. As far as large voltage pulses go, I have a TVS on the system VIN that is rated to handle 10W peak power for 1s according to the data sheet (if memory serves.) So I did take this scenario into consideration. You are absolutely correct about the ugliness of transients on a boat. Just in my test setup up with a single 12V lead acid cell, dummy load at 1A and marine battery switch I saw transients of >60Vpp. Granted they were for much shorter times that 500ms, but I can imagine much worse in a real system. I'm curious if you think further measures should be taken above the TVS. Thanks for the advice on the switch for the power up/down. I've actually implemented such a switch already, but to do a hard reset of the module if it fails to respond. I'll look at the rdsOn of the MOSFET I'm using.Regarding the AA batteries you are probably right about that. I've tested with standard duracells so far, but had intended to supply energizer lithiums in the box. Perhaps it wasn't necessary to mention that the AAs were lithium, but I did get some good feedback because of it. I think it is suggested that standard AA alkalines shouldn't be expected to provide CC loads of much more than 1A. I think that the ISR increases dramatically after that. I couldn't actually find any real pulse load stats. If anyone has any please share. The duracell MN1500 datasheet shows ISR increasing to between 800 mOhms & 600mOhms @ 2A for 1s (I guess that is a pulse load stat but it isn't specified like it is with the Energizer Lithiums.) This would likely be adequate for the application, but it is probably cutting it a bit close. I guess the other option, would be something like eneloop rechargeables (staying away from LiPos.) I can probably get away with a very simple trickle charge circuit and keep the diode OR that I have in place for swapping between main and backup power sources.

author: Ian.M
author_rank: Super Contributor
timestamp: June 08, 2015, 06:19:22 pm
content: The *nasty* routine load dump on a powerboat is if anyone runs a 12V microwave (or a mains microwave on an inverter). Unless on shorepower, its usual to run the engine due to the very high current consumption, and as soon as the battery voltage drops the alternator puts out between 60A and 100A depending in the microwave. Unfortunately most microwaves pulse the magnetron with a duty cycle of a few seconds rather than implement continuously variable power control. Due to the self-inductance of the field coils on the rotor, the alternator cannot respond quickly to a change in load current and if the battery is too cold its internal resistance can be quite high limiting its capacity to absorb 60A to 100A pulses each time the magnetron cuts off.You can spin up an alternator to a reasonable speed with a high power electric drill + a socket to fit the pulley nut on an adapter in its chuck, Repeat your test with an alternator properly connected to the battery and a pulsed load (4 seconds on, 1 off) that pulls 50A and see what happens!I suspect you've chosen a TVS that clamps at over 36V, and its going to be clamping between 50A and 100A for anything up to half a second so unless you have a current limiter upstream of it it could be seeing peak power of over 3.6KW! I suspect a 10W/1s TVS is likely to die explosively. In most cases, wiring resistance will limit the current and save your butt and/or an upstream fuse or circuit breaker will blow/trip limiting the energy that reaches the TVS diode, but worst case: If they install your gadget on a 1m long heavy wire permanent feed direct from the battery terminals, it would be helpful if it doesn't explode or catch fire! There are circuits that use a HV MOSFET that can disconnect the input if the supply voltage goes too high so your gadget can ride out the load dump pulse off its bulk capacitors without having to handle the load dump current.As long as the SMPSU can keep up with the worst case average transmit current, the supercaps can provide the 2.5A 1ms pulses. The current limit of the SMPSU will control their recharge rate and therefore set the maximum draw from the backup batteries. If you cant keep it under 1A, and you insist on using AA cells for backup, you may need twelve, in two six cell strings to halve the current per cell, each string with its own Schottky diode onto the SMPSU's input power rail so that mismatched cells don't get recharged. Alternatively load test the batteries when they are first inserted and if they cant support the required load current, alert the user.

author: TVHeadedRobots
author_rank: Contributor
timestamp: June 08, 2015, 07:37:46 pm
content: I hadn't considered the microwave scenario that you described. Thanks. You're right on about the diode's specs, and at 3.6kW the diode would certainly explode, likely along with other stuff. I do have an inline 2A fuse connected to the positive input of the device. I'll attempt to replicate the test you suggested, if for no other reason than it should be interesting. I should be able to get my hands on the alternator relatively easily. Unfortunately I don't have an active load that can sink anything close to that kind of power. I'll need to build something. Maybe just a MOSFET switching the load into a bank of big wirewound resistors? I could control the switching easily enough with an MCU and a transistor. I think it should be that simple. Thinking about it a bit more, I doubt that the drill would be able to spin the alternator under a 50A load, we'd be talking about a serious drill to be capable of that. I actually don't have one of those either. Still you've made your point, and I could still test it to see what happens. Let me know if I'm not understanding the test scenario correctly.As far as the batteries are concerned, I have a good enclosure that can hold 6AA, but no more. I've been designing around that. If I had to move up to 12x AA I think I might aswell just rethink the battery decision all together. I like the alert idea!

author: langwadt
author_rank: Super Contributor
timestamp: June 08, 2015, 07:50:55 pm
content: the peak current should not be huge issue, cellphones manage to work on a small lipo and usb modems manage to stay under the 500mA USB maximum with a big capacitor.And with a dc-dc converter from a higher voltage the peak current is smaller

author: TVHeadedRobots
author_rank: Contributor
timestamp: June 08, 2015, 08:13:05 pm
content: Langwadt, I went into this with that very assumption. Unfortunately it doesn't seem to be the case. The SIM900 apparently isn't as efficient as a modern cell phone or modem. Full disclosure, I have 100uf of capacitance on it and it is working so far, but with 10uf it certainly didn't. Granted the datasheet recommends 100uf, but they also supply a reference design using a 3A supply. I thought that it was just a big safety margin. I was wrong. I can tell you that the SIM900 will not work reliably, (using the reference design linear supply) powered over USB. I know this because that's what lead me to testing and observing the 2.5A current pulses during TX. I think we do agree that the capacitance should handle the load transients, but I still wanted to get some other input on the subject. Also, fingers crossed, I was hoping I might run across someone with experience with the SIM900.

author: Dragon88
author_rank: Regular Contributor
timestamp: June 08, 2015, 08:28:54 pm
content: Have you checked out any other (newer) Simcom modules? The 800 and 808 say that power supply current "up to 2A" is required in the data sheet. If peak current truly is lower than the 900, perhaps that can lessen your problem.

author: Ian.M
author_rank: Super Contributor
timestamp: June 08, 2015, 08:30:46 pm
content: If you cant get 50A use a smaller battery and a smaller load current. The Microwave scenario is likely to be with a 220AH or even bigger house battery bank. Use a 65AH battery and a 15A pulsed load and the results should be broadly comparable except the alternator field current will recover far faster so the load dump pulse width will be over 3 times narrower. Switch it with a heavy duty relay, contactor, honking big MOSFET or whatever you've got handy. Get the battery and alternator from a car breaker as you *WANT* them to be old and crappy! Try the effect of chilling the battery to 0 deg C overnight.You can use stainless steel wire immersed in a plastic tub of distilled water for the load. If you use 18AWG 304 stainless you'll need a bit under 4' for 1R. zigzag it to fit and make sure the terminations are immersed. See:http://www.pelicanwire.com/products/resistance-wire/304-stainless-steel/

author: lewis
author_rank: Frequent Contributor
timestamp: June 08, 2015, 08:31:33 pm
content: I did a similar design with the Telit GM862, that needed 3x 470uF 6.3V electrolytic in parallel IIRC for reliable operation. 10uF is bugger all in this application, you need something much juicier. You can get away with lower capacitance (ergo reduced PCB area) if you use os-con caps, but the lower ESR can cause ringing and regulator instability.I've linked this paper many times (it's not mine) since it's very useful for designing to meet transient immunity.http://www.radiocad.co.uk/files/1413/6921/7114/LoadDumpPaper-final.pdf

author: TVHeadedRobots
author_rank: Contributor
timestamp: June 08, 2015, 09:16:32 pm
content: Ian.M, I'll take a look at that resistance wire. I was scratching my head a bit about how to dissipate the heat from that big of a load. Also, I will likely need to scale it down a bit as you suggested. It's also nice round math that way.Lewis, thanks for the info. You're right, 10uF is nothing. Clearly I was being naive. I'll check out the paper you linked in. Thanks again.

author: Ian.M
author_rank: Super Contributor
timestamp: June 08, 2015, 09:27:52 pm
content: It doesn't have to be speciality resistance wire. Unlay some wire rigging or get an offcut of stainless MIG welding wire etc. At a pinch, try mild steel wire, but you wont be able to leave it in the water bath when you aren't using it and will need to wipe it down after use with a tissue and some WD40

author: TVHeadedRobots
author_rank: Contributor
timestamp: June 09, 2015, 01:05:59 pm
content: Ian.M, Gotcha, I'll try to put my hands on some. Thanks.

author: DTJ
author_rank: Super Contributor
timestamp: June 09, 2015, 01:42:37 pm
content: What voltage does your modem unit run at?I worked on a similar project, 4 x 1500uF 4V SMD tantalum caps placed right at the modem connector worked well for supplying the pulse current demands.

author: Ian.M
author_rank: Super Contributor
timestamp: June 09, 2015, 02:07:27 pm
content: I'm surprised you used 4V caps. Running Tantalums near their rated voltage with high pulsed currents is not a good idea as it significantly increases the risk of them failing catastrophically and catching on fire.

author: langwadt
author_rank: Super Contributor
timestamp: June 09, 2015, 02:16:30 pm
content: Quote from: TVHeadedRobots on June 08, 2015, 08:13:05 pm+2A is standard for cellphones, you need much more capacitancehttps://www.fairchildsemi.com/application-notes/AN/AN-8019.pdf

author: Marco
author_rank: Super Contributor
timestamp: June 09, 2015, 02:41:29 pm
content: How about bucking to 5V with around 2.2 mF of capacitance and then LDO for the drop to 3.3V, that should cover 1.2 ms.

author: Ian.M
author_rank: Super Contributor
timestamp: June 09, 2015, 03:01:43 pm
content: That would waste power reducing the backup battery life.As the SIM900 supply voltage range is 3.2 ... 4.8V, why not simply run it at 4.5V nominal off a current limited switching regulator. 3mF of capacitance would hold a 2.5A load for 1.2ms while dropping 1V, still within the permitted input voltage range, but a larger cap would be advisable. It probably still needs a 100uF ceramic as well to keep the ESR down

author: TVHeadedRobots
author_rank: Contributor
timestamp: June 09, 2015, 03:44:44 pm
content: The idea of running the board at a higher voltage is an option, but it would add complexity over just adding caps. I have several 3.3v device on the board, none are very high current, but it would still require a second reg for the 3.3v rail and will add efficiency losses. I'd like to avoid that. I am going to just stick more capacitance in there and continue testing. I'll be doing my best to stay away from tants all together. As Ian.M mentioned, when they fail it's known to be catastrophic and in flames. When I'm forced to use them I make sure I have a large safety margin on the voltage rating.I'll take a look at that datasheet langwadt.Thanks for all the input thus far.

author: Marco
author_rank: Super Contributor
timestamp: June 09, 2015, 05:55:23 pm
content: Quote from: Ian.M on June 09, 2015, 03:01:43 pmAt 4.5V you're still going to be burning more power ... but more importantly max droop during burst is 400 mV, assuming the other components on the board can take higher voltage and 400 mV droop.These are the options as I see them :- Run with >6 mF and a bucked voltage sufficiently high to keep supply above 3.4 with droop, other components have to be able to take the higher voltage and droop.- Buck to higher voltage with less capacitance and use a LDO, you get the best regulation.- Use a higher power switched power supply which reacts fast enough to keep droop below 400 mV like OKI-T/3-W40 or APXW003A0X3-SRZ, you'll get regulation within a couple 100 mV for the first ... the latter is able to do it within 10s of mV but needs external components.

author: TVHeadedRobots
author_rank: Contributor
timestamp: June 10, 2015, 03:58:43 pm
content: Marco, thanks for the breakdown. I with you on this. I'll take a look at the parts you mentioned. Thanks.