Help Me Design my SMPS  - Page 1
Forum section: electronics / projects

author: John_Hofmeyr
author_rank: Contributor
timestamp: May 30, 2017, 02:56:48 am
content: Hi everyone! Lately I have been interested in designing my own 0 - 30V 10A SMPS. I understand the basics of how a SMPS works but this helps very little and I cant seem to find a schematic that fits my design anywhere. I want to keep the power electronics very simple so I don't want to mess with any switching transformers and figuring out how to wind the primary etc. I first was considering using an ATTiny 216 as a controller for the MOSFETS but the programming was just too much and I got completely lost. So I turned to using an all-in-one controller IC. I settled on the TL594 becuase all of the other ones that I would love to use are SMD and I want to use as little SMD parts as possible.In my design I want to use an arduino to take the input of a potentiometer and then activate either a Boost or Buck converter. Im doing this so that I can get the absolute highest efficiency out of both the boost and buck converters instead of combining them. It might not be the best way of doing it but it's the way I'm interested in doing it.My real frustration starts with the TL594. I have spent countless hours trying to get a simple boost converter to work using it and it just never works. Im constantly blowing up MOSFETS and I can't figure out how to controll the output voltage. I'm also confused on how to use the error amps and feedback on the TL594 in the boost converter so that the pulse width changes based on the current drawn. I'm also having a tough time figuring out the components I should use. I have looked at the Data sheet for the TL594 but it hasn't helped me much (maybe because I dont understand it that well).Anyway I'm just curious if this is even a half decent design, what I'm doing wrong, how I would go about designing this power supply, and what IC I could use to replae the TL594 because it's really annoying me.Thanks

author: mdszy
author_rank: Unknown
timestamp: May 30, 2017, 02:58:50 am
content: So you want to design a 300W power supply... with no experience whatsoever, while just looking for already-done schematics that will fit the bill? I'd suggest you tone back your expectations a bit and start simpler. Dealing with that much power isn't easy, and it seems like you think it will be.

author: John_Hofmeyr
author_rank: Contributor
timestamp: May 30, 2017, 03:15:09 am
content: That’s my final goal, to make a 300W power supply. In the meantime I do want to work with MUCH lower power designs to first learn what I need to. Also I'm looking for schematics so that I have examples to work off of to make my own power supply. I just can’t find any that help me design the one I have in mind.

author: Someone
author_rank: Super Contributor
timestamp: May 30, 2017, 05:38:10 am
content: Quote from: John_Hofmeyr on May 30, 2017, 03:15:09 amThe problem is a simple (single) boost or buck stage doesn't scale up to larger power very well if at all. Also trying to make buck convertors work across a wide output range is difficult, so you're adding in multiple difficultrequirements. If its a bench supply there is a wealth of information out there already, but why add the complexity of different switching stages if you only need 0-30V? Almost every design will start with a higher rail and buck down from there, because its simpler.

author: strawberry
author_rank: Super Contributor
timestamp: May 30, 2017, 06:14:34 am
content: https://www.onsemi.com/pub/Collateral/TL594-D.PDFin page 10.reason why you are blowing MOSFETS is inproper gate drive.. over 15V or too slow gate pulse rise/fall timeMOSFET cant be used as source/emiter folover in switching aplications , because of drain to source voltage drop aprox 3Vit is better sinhronous BUCK converter to go in this power level. you can use 2xMOSFET and IR2104(deadtime) insted of transistor and diodefeedback is always negativeto change output voltage/current need to change reference voltage or feedback network

author: P90
author_rank: Frequent Contributor
timestamp: May 30, 2017, 06:19:02 am
content: I was interested reading until I got to the arduino part... LOL

author: strawberry
author_rank: Super Contributor
timestamp: May 30, 2017, 12:33:40 pm
content: Quote from: blueskull on May 30, 2017, 06:33:52 amyou mean insulated gate drives with integrated insulation transformer?FPGA is right direction or dedicated arm'ish processorMicrocontroller ADC is too slow to go above something 10W

author: John_Hofmeyr
author_rank: Contributor
timestamp: May 30, 2017, 01:27:26 pm
content: Ok so the TL494/594 is off the table. I still don't want to use a mircocontroller to switch the MOSFETS because programming is definitely not my strong point but if I really have to then I'll just have to learn the programming. Do you have any recomonedation for a through hole IC that is relatively easy to use and actually works well that I can use to switch the MOSFETS. I definitely want to use a controller that lets me use an external switch so that it can handle more current. Also I have decided to use an audio amp transformer for my first low power revision and just use a buck converter to convert the transformers 45v output to a 0-30V output at 1A.

author: John_Hofmeyr
author_rank: Contributor
timestamp: May 30, 2017, 01:37:39 pm
content: @strawberryWhat happens to the MOSFET is that when I'm driving them with the TL594 I use a small transistor to convert the low power signal from the TL594 to a stronger 9v signal for the main power MOSFET (for some reason the output square wave disappears when I connect it to the MOSFET). The boost converter will work for about 30 seconds but I cant controll the output and the voltage instantly jumps to something like 55v. After that the output if the TL594 dies on my ocilliscope and the MOSFET pulls 5A and then blows.

author: strawberry
author_rank: Super Contributor
timestamp: May 30, 2017, 02:44:30 pm
content: MOSFET gate capacitance is reason why driver square signal disapear. you can adjust driver first with 2nF capacitor insted of blowing MOSFET'sIn BOOST converter you should be using higher voltage N-MOSFET than output voltageIn BOOST converter can't be used pwm more than 50% duty cycleWhy voltage jump up, Feedback loop might be pozitive/wrongIntegrated chips usualy come below 30V something about

author: diyaudio
author_rank: Frequent Contributor
timestamp: May 30, 2017, 02:47:27 pm
content: Quote from: John_Hofmeyr on May 30, 2017, 01:27:26 pm

author: debininja
author_rank: Contributor
timestamp: May 30, 2017, 04:07:06 pm
content: I'm actually doing something with the TL494 (less capable brother of the 594) myself, but my topology is different from yours: step-down, flyback, with isolated feedback. It's low powered, 5Watts (5Volt, 1Amp). I wanted to start off small and build up, but I'm interested in how your project will turn out. It's very ambitious, that's for sure.As of right now, I have a cheap, $3 buck-boost PCB from eBay which has a range of 2V to 32V at 2Amps max recommended current. Even drawing 1Amp and 30V (30Watts) from it, using my 12V ATX PSU as the input, the PCB gets quite hot from the switching inductors and ICs, which get to around 95C. And you are wanting 10Amps at 30Volts. Depending on your frequency, maximum PWM width (45% for the 494/594), inductors and switching mosfets/bjts, your circuit will be affected in terms of efficiency, dimensions, and so on.You say you don't want to "mess with any switching transformers and figuring out how to wind the primary etc," but I think that's not a good way of going about when working with SMPS. As someone completely new to building SMPS myself, I often struggle to understand many of the design equations for the different topologies around, but I don't limit myself by saying "oh, I don't want to use calculations."Anyways, I think I get what you mean when you say buck-boost using a TL594, but I don't think a microcontroller is required. It's apparently called a non-inverting 4-switch buck-boost converter. I don't know if this schematic will even work well, but here's what someone came up with:

author: John_Hofmeyr
author_rank: Contributor
timestamp: May 30, 2017, 06:09:16 pm
content: @debinninjaThanks for the reply, really helpful! I am curious though what the difference is between the TL494 and 594. I have both but they seem to be the exact same. Now that I know what I have been doing wrong with the TL494/594 I'll try to use them again but I'm still open to using different IC's. With regards to the output power that I'm aiming to get I have realized that 30v at 10A is WAY too much. I do still want a 10A output capability but I want the total output power to be around 90 - 120 watts instead of 300 to lower the "danger factor". Now I just need to find the right topology. I'm planning on using this power supply as a second supply for projects because my current one only has one output and it can be really annoying to power projects that need different voltages. So should I go with an Isolated or non-isolated converter. I like non-isolated converters because they seem to be much simpler to make but I'm open to trying to make an Isolated converter and making my own transformer if I need to for the amount of power I'm working with. Im also confused with what topology of Isolated or non-isolated converter I should use. Oh and BTW I'm going to be using a buck converter to step down the 45v from an audio transformer down to 0-30v.Thanks.

author: nctnico
author_rank: Super Contributor
timestamp: May 30, 2017, 06:19:08 pm
content: I'd go for a synchronous DC-DC converter controller with integrated MOSFET drivers. The trick to selecting the right MOSFETs is finding ones with the smallest gate capacitance which meet the worst case voltage (short cricuit) and current (SOA graph). Also have an 8V power supply to drive the MOSFETs as this will usually be a good trade-off between RDSon and switching speed. The PCB layout is the most tricky part of these kind of designs so it is a good plan to start with an evaluation board of the DC-DC controller. For now forget about the boost configuration.

author: John_Hofmeyr
author_rank: Contributor
timestamp: May 30, 2017, 07:11:49 pm
content: @nctnicoIs it possible to use an external MOSFET driver along with the TL494/594 or is it best to just buy an all in one chip that just needs the external MOSFETS and other parts. Also I have been using IRFZ48N MOSFETS from an old inverter, are they any good?

author: Zero999
author_rank: Super Contributor
timestamp: May 30, 2017, 07:40:41 pm
content: You've omitted what range of input voltages this needs to work off, from your specification. Please don't say you're planning to power this directly off rectified mains.Start off with 12VDC.Is this intended as a bench power supply?If so, to achieve low noise/ripple it may be better to use a linear regulator on the output of the SMPS, although it will reduce the efficiency.

author: John_Hofmeyr
author_rank: Contributor
timestamp: May 30, 2017, 07:51:59 pm
content: @Hero999Are you crazy?? I would never power it from rectified 120VAC. No for the final supply I'm thinking of using a transformer to step down mains voltage to something like 45V, for now I'm just going to use 30VDC from my current power supply to test my design. I would like the supply to run off of 35 - 45V DC so that my max output voltage of the supply is 30V DC. Also I can’t think of any linear regulator that will be ok with passing 90 - 120 watts through it (9v @10A or 12v @10A) .

author: Zero999
author_rank: Super Contributor
timestamp: May 30, 2017, 08:05:12 pm
content: Quote from: John_Hofmeyr on May 30, 2017, 07:51:59 pmGood. We've had crazier people than that here before, so I had to check. lolQuoteSo you're intending to use a buck converter? That will work fine for the higher any of the voltage range but will become challenging at low voltages. Why not shoot for a voltage half way between the two, about 12V to 15V, then use buck-boost? Despite the extra complexity it'll make achieving the large range of voltages easier.QuoteThe point is to use the switcher, as a pre-regulator, to reduce the voltage to a few volts above the linear regulator's output, so the power dissipation will be no more than 30W.

author: John_Hofmeyr
author_rank: Contributor
timestamp: May 30, 2017, 08:14:58 pm
content: @Hero999I'm kinda confused. What difficulty will the buck converter have with lower voltages, and also could you clarify a bit on the linear regulator, sounds interesting I just don't understand exactly what you mean. Also if I were to use a buck and boost converter would it be a good idea to have two completely separate converters and then use a micro controller to look at the requested voltage and output voltage and then either activate or deactivate the buck or boost converter.thnaks

author: strawberry
author_rank: Super Contributor
timestamp: May 30, 2017, 09:48:54 pm
content: non-isolated coupled inductor SEPIC topology

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 30, 2017, 10:49:03 pm
content: Looks like it is time to go through this thread like a scythe through wheat...Quote from: strawberry on May 30, 2017, 09:48:54 pmBad suggestion. High stress on semiconductors and coupling capacitor. Prone to high circulating current and oscillation/ringing from coupling capacitor resonating with inductor. Very difficult to stabilize due to right half plane zero and a non-linear input/output transfer function.Quote from: Hero999 on May 30, 2017, 08:05:12 pmThe buck-boost (aka non-isolated flyback) also subjects the semiconductors, input and output capacitors to high stresses, and also suffers from the right half plane zero if the inductor current goes continuous. Without a transformer the output polarity is inverted as well.Getting a buck converter to go down to near 0V isn't a problem, either *if* the PWM controller can skip pulses (usually this is intended for maintaining regulation at very light load but it can apply to very high step-down ratios as well).Quote from: Hero999 on May 30, 2017, 08:05:12 pmThis, however, is good advice - the best in this thread, in fact. I would just reiterate that a buck converter would be the best to use as the pre-regulator, not a buck-boost or SEPIC or Cuk or whatever. Note to OP: Linear Technology has an App Note on doing just this with, IIRC, the LT1074. I can't remember the number but it should be easy enough to find with google.Quote from: nctnico on May 30, 2017, 06:19:08 pmI agree in principle, but this is the first SMPS for the OP and so I wouldn't recommend a synchronous topology which is more challenging to get working because of shoot-through and operating in forced continuous conduction mode all the time.Quote from: strawberry on May 30, 2017, 12:33:40 pmI have no words... No one in their right mind is going to use an FPGA or an ARM to control a 0-30V/10A power supply. And the speed of a MCU's ADC is not the limiting factor for how much power can be controlled; if there is sufficient inductance present (regardless of topology) and the loop bandwidth doesn't need to be too high (ie - more than a few kHz) then even a crappy 8b PIC or AVR can perform acceptably quickly in multi-kW converters (I know, I've designed and built them). Just don't use an Arduino fercryingoutloud...Quote from: strawberry on May 30, 2017, 02:44:30 pmYes, yes, NO.Duty cycle can definitely go above 50% in a boost converter. The transfer function does get a bit twitchy as duty cycle exceeds 80%, and 100% duty is verboten, but pretty much every PFC stage on the planet needs to go to at least 95% duty near the zero crossings of the mains to achieve distortion requirements.Quote from: blueskull on May 30, 2017, 06:33:52 amI mostly agree with this, except to reiterate once again that this will be the OP's first SMPS and so he probably shouldn't mess with MHz class switchers which is what an integrated inductor implies. Nor should he be messing with a hybrid digital and analog controller. A simple circuit using well-established controller ICs like, e.g., the SG3525A, would be much more appropriate and deliver excellent performance.Quote from: Someone on May 30, 2017, 05:38:10 amI agree with most of what you wrote except* that single stage buck and boost converters scale just fine up to nearly any power level. It is true that it is difficult to cover a very wide range of input/output voltage conversion ratio with a single stage unless pulse-skipping is employed, but that is where Hero999's suggestion of using a switcher as a pre-regulator for a linear regulator comes into fore.* - forgot to insert this word in original post

author: John_Hofmeyr
author_rank: Contributor
timestamp: May 31, 2017, 12:35:04 am
content: Whoa! So much info, thanks! Just to make sure I understand what's been said:So if the non-isolated coupled inductor SEPIC topology and non-isolated flyback converter impose stresses on the components, then what is a better topology that I should use? Also, I'm still not quite sure how to use the linear regulator. If I'm not mistaken, you are saying that the output of the switching regulator is set a few volts higher than the desirred output, and then finally filtered by a linear regulator to get a smoothe output at the required voltage.

author: debininja
author_rank: Contributor
timestamp: May 31, 2017, 05:38:47 am
content: QuoteExactly. The power lost as heat, following that method, becomes: (VoltageIn - VoltageOut)*CurrentOut. Say you had a 10V fixed output and you wanted 1V out to power some widget. If you used a linear regulator by itself, it would have to drop 9V. You multiply that by the current, say 1Amp. So that poor linear regulator has to dump 9Watts as heat. You would need a beefy heatsink.Compare that to if you have an adjustable SMPS, you can set the voltage at 1V plus a little more than the maximum voltage drop (usually 2.5V at worst, but there are linear regulators that are "low dropout", which have 0.5V or so of drop). So, you set the SMPS to 4V on the output, and the linear regulator drops 3V, at 1A for example. The power loss as heat is now only 3Watts. You can put on a tiny heatsink and it'll work just fine. Seehere for the calculations.

author: strawberry
author_rank: Super Contributor
timestamp: May 31, 2017, 07:55:24 am
content: @MagicSmoker50% is max boost ratio ( x 2 )at max output powerdouble stress but you can use one MOSFET insted of twoslow 8bit microcontroller will give delay in feedback loop (it resoult in output voltage overshoots..instability) to process all (digital voltage adjust, monitoring, false modes..USB) instructions and hard to implement current protection..

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 31, 2017, 10:35:28 am
content: Quote from: John_Hofmeyr on May 31, 2017, 12:35:04 amWhat I said is that the SEPIC and flyback impose higher stresses on various components compared to the buck, and that they are also much more difficult to compensate/stabilize while maintaining good transient response.Otherwise, what I am saying is to use a linear regulator as the master regulator which not only supplies the output power but which also provides a reference - offset by 2-3V - for a buck converter "pre-regulator." E.g., if the linear regulator is set to 5V and in operating in Constant Voltage (CV) mode then the buck pre-regulator would be set to deliver 5V + the offset (e.g. - 7-8V) to the input of the linear regulator. Read the App Note from Linear Technology for the LT1074 that covers this (among other ideas):www.linear.com/docs/4155EDIT - the LT app note is for a 1.2V to 28V @ 5A max design which doesn't quite meet your requirements, but it also has hints on how to expand the output voltage range down to 0V (and you should be able to increase the output beyond 28V as well). You will learn a lot just from getting a simple but known-good design like this working, and with that knowledge you will have a much better chance getting a scaled-up version that delivers 10A or more to work.Also, the key to understanding switching power supplies is a good grasp of the magnetic components. A very useful site that will generate waveforms and spit out a table of reasonable magnetic component designs for the most popular SMPS topologies is here:http://schmidt-walter-schaltnetzteile.de/smps_e/smps_e.htmlI can't recommend this site highly enough for the beginning (or even an old pro) switcher designer. It's very handy and fairly accurate.HINT - You always design the buck inductor for the maximum input - output differential at maximum load current.

author: VEGETA
author_rank: Super Contributor
timestamp: May 31, 2017, 10:45:01 am
content: I think a simple solution is to have a buck pre-regulator followed by a linear post regulator. Something like LT3081 which offers 1.5A or LT3083 for 3A. Switching pre-regulator should be relatively easy as there are many designs in the forum which has one, including my own which has SEPIC pre-regulator with a N-channel MOSFET post regulator.

author: debininja
author_rank: Contributor
timestamp: May 31, 2017, 07:14:41 pm
content: Quote from: MagicSmoker on May 31, 2017, 10:35:28 amPERFECT. That will help me out nicely since I'm in the works of making my first SMPS. A little 5V 1A flyback. It's just about done. I need to redo the transformer windings.

author: John_Hofmeyr
author_rank: Contributor
timestamp: May 31, 2017, 10:07:08 pm
content: Ok, so I took a look at this website (http://www.smps.us/topologies.html) to see what different topologies there are and I'm thinking of either a half bridge isolated buck converter or just a plain old buck converter. I'm leaning towards the isolated half bridge because according to the graph at the bottom of the website a half bridge converter will be able to handle the 90-120W of power that I want this supply to be rated at. Which one would you guys recomend.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2017, 10:27:58 pm
content: Quote from: MagicSmoker on May 30, 2017, 10:49:03 pmI was referring to the type of buck-boost converter made with four switches, two transistors and two diodes. The output voltage is the same polarity as the input and it's no more stressful to the semiconductors than the standard boost and buck configurations. For a 0 to 30V output design, an input voltage of just under half the maximum would work nicely. The main disadvantages are increased complexity and switching losses.https://en.wikipedia.org/wiki/Buck%E2%80%93boost_converter#Principles_of_operation_of_the_4-switch_topology_by_vannak_pov

author: John_Hofmeyr
author_rank: Contributor
timestamp: June 05, 2017, 01:17:36 am
content: Ok so I have been doing some reaserch and this is the B.O.M I have come up with. If I'm missing anything (mostlikely) please let me know.Topology: Buck/Boost converterBOM:Switch controller - SG3525AMOSFETS - IRFZ48NShotky Diode - FFSP10120A100uH Inductor rated at 15ASome random huge full bridge rectifier from a pc power supply120vAC to 20VAC transformerATtiny85 for temp regulation10K thermistor for temperatureVoltage and current readoutLT1084 for final regulation50k Potentiometers for adjustment2X 5600uF 56V capacitor for filteringHuge-ass heatsinkLM7805 for ATtiny