Power supply linear? switching? both?  - Page 1
Forum section: electronics / projects

author: soubitos
author_rank: Frequent Contributor
timestamp: February 16, 2018, 11:43:48 pm
content: I need a power supply for some of my projects. I know i can go the easy and perhaps cheaper way of getting a 19V/2-5A laptop "brick" which is a switching power supply and i will be ok...On the other hand, i enjoy the idea of using a 24V/50VA toroid transformer with all the bells and whistles (line filters, fusing, rectification, large capacitors etc) ...I think i go with the first option anyway for the "final product" as it is cheaper and would really make my life a whole lot easier but i am also considering the second option inside a nice case to serve as the basis for a small bench power supply.. I just wonder... do i feed the voltage after rectification to a switching mode voltage regulator to begin with or is it better to use a linear voltage regulator first, drop the voltage a couple volts even then feed it to the switching mode voltage regulator (Or a module likehttp://bit.ly/2C32kSe)What would you do?

author: Paul Price
author_rank: Super Contributor
timestamp: February 17, 2018, 01:05:55 am
content: If'n I was in your shoes I would get more learn'n about power supply circuits before going any further.A nice high-current laptop brick makes an excellent power source to start with and replaces the mains power transformer, rectifiers, filter capacitors needed while it also features overload protection.Just figure out how to stick a LM317 on a large heatsink, find a pot and a box to put it in, and you've got 90% of a good bench power supply that will give you a well-regulated supply with an output from 1.2V to 17V(The highest regulated output voltage at full load using a LM317 would be less than 17V, but worked well at something like <500mA.)More than plenty of info about LM317 power supply design here if you search this forum or just Google it.One of my first bench power supplies I made was a LM317 linear regulator that was fed power from a HP laptop 21V/3.5A brick.I actually used a LM2976 switching regulator to act as a preregulator( it had its own front-panel pot for voltage adj. ) and when I wanted to have a really smooth regulated output I also internally connected the output of this switching supply to the LM317 and would adj. the really quiet regulated output with a second output voltage control. This way I could keep things cool and quiet to work with my projects. So my power supply had two outputs, the switching regulator output and the LM317 regulator output and I could switch back and forth the panel voltage/current meter to set both supplies to fit the custom requirements of the circuits I would be working with.To get this ganged-power supply to work I would first set the LM2976 switcher to about 4V above the LM317 regulator's set output voltage.Most of the time, using only the LM2976 output was plenty good enough to use for most of my experiments, but sometimes I needed two different supply voltages and I would use both the switching and LM317 P/S outputs.The big heatsink I needed was salvaged from a free of cost dead PC ATX power supply, which also provided me with free electrolytic capacitors, the large inductor I needed for the LM2976 switcher, and at the same time many other salvage parts that I could experiment with such as NPN transistors, a LM339 comparator to play with, as well as a nice 12V 4-in cooling fan.

author: jbb
author_rank: Super Contributor
timestamp: February 17, 2018, 09:10:33 am
content: Staring with a laptop brick is a good start. It’s got all the mains stuff done and is quite small.One area where doing a 50 (or 60) Hz transformer + rectifier + linear reg is an advantage is getting low output noise. But you do have to worry about getting the mains stuff right. There are some safety issues to think about (e.g. does it need to be earthed, what happens if a wire comes loose and flaps about, are there sharp edges which could rub through insulation?)Doing at least a basic set of dials is good too.

author: HSPalm
author_rank: Regular Contributor
timestamp: February 17, 2018, 09:44:18 am
content: If you go for a stationary pc psu you have many high current rails at useful voltages and even dual rails. You could use it with the voltages provided or as a base for a more complex benchtop supply. Just beware that these psus can deliver very high short circuit currents, and are much more likely to destry your gadgets if something goes wrong, than for instance with the LM317 solution mentioned above.

author: cellularmitosis
author_rank: Unknown
timestamp: February 17, 2018, 10:09:09 am
content: I’ve been happy with the CSI1802X.https://www.circuitspecialists.com/bench-power-supply-csi1802x.htmlhttps://www.eevblog.com/forum/testgear/picked-up-a-csi1802x-bench-supply-($50-0-18v-0-2a)/msg379364/#msg379364

author: soubitos
author_rank: Frequent Contributor
timestamp: February 17, 2018, 03:56:59 pm
content: Quote from: Paul Price on February 17, 2018, 01:05:55 amThis is why i consider it the most possible solution to power my "end product"Quote from: Paul Price on February 17, 2018, 01:05:55 amI do not need a bench power supply.. the devices to be powered are all already switching at their input or output at some point. I just happen to have a 24v/50va toroid and most of the parts to start from 220vac and bring it down to a rectified output... the question really is, do you take the rectified output (transformer-bridge rectifier-storage caps) directly to a switching voltage regulator or it would help if a linear voltage regulator comes in first? i dont want to add (serious at least) heatsinks and cooling as much as i can avoid it because most probably the case i will use to house all this is going to be ABS plastic with aluminum bottom.Quote from: jbb on February 17, 2018, 09:10:33 amUsing a linear power supply means at 50va of the transformer i will definitely need amble cooling.. that i like to avoid. I think i will go a switching solution which should be ok.. other than that, i think i am able to manage a "proper" buildQuote from: HSPalm on February 17, 2018, 09:44:18 amI need something to power small modules chargers etc, something i can either "make" or have ready made off the shelf.Quote from: cellularmitosis on February 17, 2018, 10:09:09 amI would be happy to have one too as a bench power supply but unfortunately, i don't see how i could get one in Greece, not without spending at least 3-4 times the selling price of it in shipping and taxes... on the other hand, i always say yes to free stuff if anyone wants to send me one LOLWell, thank you all guys... i really appreciate the time you took to post, i think i'll go the laptop psu way all the way... as for the toroid, i might use it for a bench power supply once i find an interesting schematic!!!

author: jbb
author_rank: Super Contributor
timestamp: February 17, 2018, 07:48:39 pm
content: Quote from: soubitos on February 17, 2018, 03:56:59 pmOK, now we're talking.Raw DC => Linear => Switcher doesn't make sense, because a (properly designed) switcher will be able to deal with variations in the DC supply.Raw DC => Switcher is probably the right move.Raw DC => Switcher => Linear. This type uses the switcher to get close, and the linear to finish up (and remove some of the noise) but is a bit fiddly to design.One thing to look out for is that your raw DC rail may be a little higher than expected. We start with 24V AC.- Add 10% for situations where you're in a building with 'naturally' high line voltage.- Add 10% for voltage surges- Multiply by sqrt(2) to turn AC RMS into AC peak- Subtract 0.5V for diode rectifier drop (assuming rectifier is warm and current is low).And we get 40.5V DC max. So, adding a margin of 30%, your switcher must be rated to 52V input (round up as appropriate).It would be reasonable to crudely assume that a buck regulator has a lower duty cycle limit of roughly 1% meaning that you can probably have a controlled output down to 0.5V (actually I would expect 1V due to the way feedback loops are done for most SMPS ICs)Now to look at your maximum reliable output voltage. We'll start with 24V AC- Subtract 10% for situations when you're in a building with 'naturally' low line voltage.- Subtract 10% for voltage sags.- Multiply by sqrt(2)- Subtract 1.0V for diode rectifier drop (assuming rectifier is cold and current is high)And we get 27V. However, there will be some 100Hz ripple because the 50Hz power comes in lumps, and the bulk DC caps are required to smooth it out. If we assume there's a 10% dip, we get 24.6V minimum.To get 24V output, we would expect a duty cycle of 97.3%, (in reality a little higher), so I suggest you look for a switching regulator with a max 99% duty cycle.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 17, 2018, 09:48:28 pm
content: That was really helpful mate.. appreciated...