Battery power adjustable power supply, advice. - Page 1
Forum section: electronics / projects

author: David_
author_rank: Contributor
timestamp: November 06, 2017, 10:55:37 am
content: Hello.I want to design a power supply, that is supposed to make it easier for me to prototype and test out circuit ideas in general.As such I want it to be safe as in not an offline supply, and instead a self contained device though with batteries that are charged externally, ether I'll go with 2*20700A(a couple of mm larger than 18650) or 2*26650. Probably 2*26650 as they are made with over 4000mA/h.So the first question is, should I connect the batteries in parallel and make a DC-DC converter for an input of 4,2V - 2,75V, or should I connect them in series in order to make a DC-DC converter with an input range of 8,4V - 5,5V?Secondly, what kind of topology would you recommend?I want to design something like an flyback or a forward converter with a isolating transformer, the following is not really the reason for why I want to do it that way, but if I later on would go for a larger Lipo battery or something I still want completely isolated rails even if I would derive multiple rails from one battery. I want to be able to produce something like 4-6 units of this design so that I have 4-6 readily available power supplies that can be connected in different configurations.Is it possible to design something like this that can operate in parallel with each other to enable a higher current to be drawn?The output voltage range will as high as 15V and as low as 3,3V, I don't need it to go down to 0V but as I plan to use a linear regulator as a post-regulator, and control that LDO with a DAC connected to the feedback voltage divider through a third feedback divider resistor, the output voltage will go down to some fraction of a a volt.But I am really unsure about how to make the pre-regulating isolated DC-DC converter to be controlled by the LDO output voltage, if there where no isolation then I would know how to use a PNP transistor to maintain around 1,7V above the LDO's output voltage(assuming that the pre-regulating converter have a feedback pin), but how to do the same with an isolated converter.Do you know how?The current would be allowed to be something like 1A, maybe 2A,Regards

author: ebclr
author_rank: Super Contributor
timestamp: November 06, 2017, 11:08:09 am
content: http://www.instructables.com/id/Digital-Battery-Operated-Powersupply/

author: Ian.M
author_rank: Super Contributor
timestamp: November 06, 2017, 11:30:22 am
content: Why do you think a battery powered supply is safer than a mains powered supply using a transformer with an appropriate isolation rating? If anything goes wrong you risk a Lithium battery fire or even an explosion. The exception when it would be safer would be if you need a supply that can be floated at several KV with respect to ground. Other reasons to use a battery supply and a linear regulator *WITHOUT* a switching pre-regulator would be if you require extremely low output noise.Now that's out of the way, and assuming there are other valid reasons for a battery supply, its generally preferable to start from a higher voltage and use a buck converter because I2R losses will be lower. If you have to boost, at least start with the highest input voltage possible, so a series battery configuration is preferable, assuming you are willing to budget for an effective balance charger.Tracking switching preregulators before a linear regulator can be more trouble than they are worth as many linear regulators have insufficient PSRR at higher frequencies, and radiated EMI from the switcher is also a problem so its difficult to minimise switching frequency noise on the output. Also maintaining the target voltage for the preregulator X volts above the final output typically results in interesting challenges stabilising the interacting control loops, and simply tracking the output set point rather than tracking the actual output tends to require more complex circuitry, especially if you need to reduce the dissipation while in final regulator current limiting mode.SMPSU feedback, for power supplies that use a primary side controller chip and actually have secondary side feedback is typically via an optocoupler driven by a TL431 adjustable shunt regulator on the secondary side, used as a comparator. Its Adj input is typically ground referenced and has a 2.5V threshold. The only issue with applying your tracking feedback compared to a non-isolated switching regulator is the extra delay and phaseshift in the TL431 and optocoupler which will make it harder to stabilise.

author: ebclr
author_rank: Super Contributor
timestamp: November 06, 2017, 01:27:52 pm
content: You can use the power supply on a bicycle or on a bus can you do that with a mains powered?

author: plazma
author_rank: Frequent Contributor
timestamp: November 06, 2017, 01:33:16 pm
content: I made one out of a DPS5005 modulehttps://www.eevblog.com/forum/projects/dps5005-portable-mini-lab-psu/msg1286429/

author: ebclr
author_rank: Super Contributor
timestamp: November 06, 2017, 02:10:09 pm
content: For that purpose the DPH series will be better, But the instructable one have better accuracy and less noise

author: Freelander
author_rank: Regular Contributor
timestamp: November 07, 2017, 12:14:56 pm
content: Hi, I doubt the 'instructables' one would have better accuracy than the DPH series, similar maybe, but highly unlikely to be 'better'. As for noise... I would have to see a real comparison at the equivalent voltage and current. As for price for price and value for money, there is simply no comparison or contest. The DPH winds hands down. There is also a brand new DPH5005 buck boost unit arriving shortly . 1ma and 10mV resolution. The Accuracy spec can be seen on their site. This has full comms availability also. It comes with the excellent control software provided on the other comms units and also the ability to control it via anything via Modbus RTU . That will be a nice option. I am planning on using the existing DPH case with the power board rotated and the rear banana plugs and fan driver removed, also mounting the fan outside the case. This should enable a 2200mAH 3 cell LiPo 20C WMD thermo nuclear device to be 'inserted'. Should make a nice unit.The instructables unit though is a really nice variation on the dust gatheringEEV unfinished project and is a real credit to the constructor. It is so good to see projects like this actually being built.