portable PSU - Page 1
Forum section: electronics / projects

author: Boschi
author_rank: Regular Contributor
timestamp: October 28, 2016, 08:33:20 pm
content: few days ago i started feeling the need of a decent PSU (not the 12V -> lm317 arrangement that i was using before), and the portable PSU of dave really inspired me a lot.because im an arduino fanboy (well, to say the truth is the only micro i've ever used) i want to use it as the brain of the PSU, but i really want to keep the things as simple as possible, so instead of using discrete dac/epot i want to use two PWMs dac on arduino. the standard ide library allow you to use the 16bit timer as a 8bit one, but with some bitbanging i want to use the 16 bit timer to count up to 2^12, just to have a frequency of about 4khz.with those PWM dac i want to set current and voltage (0-15V and 0-1A), but the problems come with the voltage regulator.i want to use a hibryd model, with a MC34063 to boost the voltage and a linear regulator to get better regulation, but to achieve 0v the usual lm317 wont work, so i was thinking to create my own discrete linear regulator.i started out with this:as pass transistor i want to use a IRLU9343PBF,wich is a PMOS, and as op amp a classic LM358.how can i avoid a self-oscillating circuit and what problem could it have this arrangement?thanks for any answer, and sorry about my bad english

author: prasimix
author_rank: Unknown
timestamp: October 29, 2016, 07:30:58 am
content: Hi, different people have different understanding of "decent". I started my project two years ago with "decency" in my mind and for the short time I also played with mentioned MC34063. I was attracted with its price, and that is mentioned on many places but what I got can be better described with terms "messy" and "banal". Today you can choose between hundreds or even thousands of controllers that can be use boost converter. And the procedure to obtain them is I believe the same everywhere: just go to the web shop of your favorite supplier and make an order.Question about stability based on the generic block schematic that you presented cannot we answered. If you are going to make an programmable PSU you can check some designs that I collected on thispage. Also if you spend some time on this forum you could learn a lot from some members (follow e.g.Kleinstein).

author: Kleinstein
author_rank: Super Contributor
timestamp: October 29, 2016, 03:59:08 pm
content: As described the project has three relatively independent parts:1) the DACs / filters for control2) the switched mode pre-regulator3) the linear mode regulatorI would first consider one part at a time, though they will depend on each other later.For the switched more regulator it is difficult to go higher and lower than the input voltage. So the usual way is to go down only. The MC34063 is rather old, and more a low cost option for lower currents (e.g. < 500 mA so that no external power switch in needed) if efficiency is not important.It depends of the type of switched mode stage use whether it is better to use an N-Channel or P-Channel MOSFET / BJT for the linear stage. To make the stage work at low voltages, one would need to use the battery output or an extra auxiliary supply. A possible combination that can work without an extra auxiliary supply would be something like a negative side buck converter (e.g. using an N-MOSFET) and a positive side LDO (e.g. with an P-MOSFET) for linear regulation. So neither side of the battery will be connected directly to the outputs.To avoid oscillations one should look at the loop gain for the regulation loop. The simple way is to have a single slow part and loop gain going below 1, when the second slowest stage comes into play. Today one can do this stage in a simulation first. For the output stage there are mainly two classes:1) current driving stages like in LDOs. These need a well chosen output capacitor and a relatively well adjusted loop compensation.2)follower type output stages with a low output impedance. These can get away with no or little capacitance at the output, but than need a standing current. Loop compensation might be a little easier - if the output stage is well behaved, but current regulation / limiting can be slow or more difficult. To get close to the full output voltage one would need an extra, higher driving voltage and to get a standing current / 2 quadrant drive a negatve auxiliary supply would help.

author: Boschi
author_rank: Regular Contributor
timestamp: October 29, 2016, 08:33:03 pm
content: tanks for the answers!Quote from: prasimix on October 29, 2016, 07:30:58 amyou are right, the things i want in this design are just CC mode and not too much rms noise. im not really in depth in analog stuff, so i'll use this PSU to power up mostly digital stuff and probably some analog, nothing fancy, so i think that aiming for something like 5mV pk-pk of noise would be enought for me. you are free to correct me, i really dont know how much noise could impact on a circuit.about the switch regulator do you have any suggestion? and its better to go with a low frequency so the pssr would be higher or go with a higher frequency (and less ripple) and lower pssr?Quote from: Kleinstein on October 29, 2016, 03:59:08 pmim not sure if i get it right. are you talking about step-up/down an sepic converter?i was thinking to use a step up converter, and when the output is 0 volt it work just the linear regulator.about the MC34063 i realized that i have to change it. same question, its better with low frequency or high frequency?about the rest, before saying anything else i should probably readthis

author: prasimix
author_rank: Unknown
timestamp: October 29, 2016, 11:21:51 pm
content: Quote from: Boschi on October 29, 2016, 08:33:03 pm5 mVpp noise figure is very low, unless you are not talking about small current (e.g. below 100 mA). If you check reference designs of many manufacturers you'll see that figure is regularly over 10 mVpp a goes easily to couple of hundreds mVpp for few amps. Even some solutions with linear post-regulator claims "massive" ~10 mVpp (LTCDC2132Demo board). Better figure is rms and something below 10 mV is easily achievable. Measuring output ripple is quite challenging and depends of the measuring point and method (single ended, differential), output mode (CV or CC) and output capacitance (and total impedance connected to the PSU output) end result could vary to the great extent. Also despite the fact that "standard" scope bandwidth for such measurement is 20 MHz, some manufacturers have no problem to list results with bandwidth as low as 1 MHz.It will be much easier to continue discussion if you present at least minimum info, like output voltage and current range, max. power, etc.Yet, even without such info I can say that making "serial" hybrid when linear post-regulator follows smps pre-regulator is pretty challenging, you can expect pre-regulator switching all around and PCB layout is of paramount importance. I didn't use 4-layer PCB yet trying to keep overall cost down, but that is something that manufactures recommends by default. Another default is usage of SMT with min. numbers (or none) of THT parts.I heard just recently about "parallel" hybrid topology that is used for powering of very sensitive circuits (see attachment document):That is something reserved for expert league, where I'm not belong, but if someone has some spice model, schematic, etc. I'd like to build one and compare it with what is already done.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 30, 2016, 08:44:17 am
content: That hybrid circuit seems to be used for constant current drive, like powering a large electromagnet that needs a low ripple current. So the high power needs a switched mode regulation for the main part. Instead of doing series regulation as one would do for a voltage source they use the parallel mode as they need a current source.Using a step up converter only for the higher voltages is possible, the lower voltages would than use linear only regulation and be less efficient, but could have low ripple. So it depends on the main use. Starting with 2 Li cells (6-8 V) could be a good option for this configuration.The permissible output current depends the voltage - so likely less current at 15 V and more at 5 V - possibly again less at 1 V.I don't think CC mode only makes much sense - this would be a kind of LED driver. A simple supply is more like CV mode with a limited accuracy current limiting for protection.For the frequency of the switched mode converter, modern converters tend to use higher frequency. This allows smaller inductors and filtering caps. For a converter with external MOSFET this makes the layout more tricky. Usually the converter chip sets the suitable frequency range. I would not go extra high or extra low.

author: Boschi
author_rank: Regular Contributor
timestamp: October 30, 2016, 05:16:46 pm
content: @prasimix: abou the noise you you are right, probably 10mv rms are really enough, and to say the truth im not even sure if i can measure it properly with just a digital scope.for the voltage and amps they are 0-15v and 0-1A (i said them in the first post)@Kleinstein: i'd prefer to use just a lithium cell, in this way i can charge the whole thing with usb, and in my junkbox i just found some tp4056 board.i'll probably work mostly at 5V, and sometimes at 9 or 12V, 15V are just in the case i have to recover some dead batteries.about the CC, you are right, i get confusedwhat range of frequency is better? something like 0.1-0.5Mhz?