What controller chip to use for smps? - Page 1
Forum section: electronics / beginners

author: Embed
author_rank: Newbie
timestamp: March 08, 2017, 12:49:11 am
content: I built a linear power supply about 20 years ago and has served me well. I needed to use it for a project recently and the circuit didnt work. Come to find out the filtering capacitors are leaking all over the place. Instead of building another linear power supply I figured I would try my hand at a switch mode based system. The theory I have a good grasp of so the first thing I needed to do was order a controller chip. I want 1.2- 20 volts variable out with a audio or digital pot . I want 5 amps out . The 5 amps out is the problem. After doing dozens of searches on mouser and digikey and the web all I see only go up to 3 amps. I searched so long last night I was about to pull what hair I have left out and gave up. The closest one I could find was a Ti LM2678T-ADJ/NOPB. It's $6.00 for 1 though ! As I'm probably going to blow up a few of these figuring them out I don't need to pay that much for 1 chip. I figure I could put a mosfet in circuit to up the amperage but what chip or chips are easy to implement or use with a mosfet? I thought about just using a pic microcontroller and pwm an out put to a mosfet then to the inductor but I have to worry about resets, timeouts etc etc. If that chip ever stalls stuff starts blowing not unless I do a pretty hefty supply path for the voltage until the chip restarts. Any ideas on a good all around cheap chip I can blow a few up until I figure out how exactly I want the circuit? Thanks in advance !

author: singapol
author_rank: Frequent Contributor
timestamp: March 08, 2017, 02:21:38 am
content: Honestly it's cheaper to buy a smps than build one yourself and it looks more nice.Save you all the headaches.

author: Embed
author_rank: Newbie
timestamp: March 08, 2017, 02:50:52 am
content: I can't learn anything like that. Besides, I like blowing stuff up. Thanks though .

author: mariush
author_rank: Super Contributor
timestamp: March 08, 2017, 03:00:10 am
content: Plenty of switching regulators that can do 5a or more4.18$ Diodes Incorporated AP1501A-K5G-13 :http://www.digikey.com/product-detail/en/diodes-incorporated/AP1501A-K5G-13/AP1501A-K5G-13CT-ND/4497349Description IC REG BUCK ADJ 5A TO263-5L Buck Switching Regulator IC Positive Adjustable 1.23V 1 Output 5A TO-263-6, D²Pak (5 Leads + Tab), TO-263BAVoltage - Input (Min) 4.5VVoltage - Input (Max) 40VVoltage - Output (Min/Fixed) 1.23VVoltage - Output (Max) 37VCurrent - Output 5AFrequency - Switching 150kHz3$ : Sanken SI-8008HD-TL :http://www.digikey.com/product-detail/en/sanken/SI-8008HD-TL/SI-8008HD-TLDKR-ND/4175675IC REG BUCK ADJ 5.5A TO263-5 Buck Switching Regulator IC Positive Adjustable 0.8V 1 Output 5.5A TO-263-6, D²Pak (5 Leads + Tab), TO-263BANumber of Outputs 1Voltage - Input (Min) 4.5VVoltage - Input (Max) 40VVoltage - Output (Min/Fixed) 0.8VVoltage - Output (Max) 24VCurrent - Output 5.5AFrequency - Switching 150kHz4.3$ Texas Instruments TPS54540DDARhttp://www.digikey.com/product-detail/en/texas-instruments/TPS54540DDAR/296-39422-6-ND/5143913Description IC REG BUCK SPLIT RAIL 8SOPWRPAD Buck, Split Rail Switching Regulator IC Positive Adjustable 0.8V 1 Output 5A 8-PowerSOIC (0.154", 3.90mm Width)Voltage - Input (Min) 4.5VVoltage - Input (Max) 42VVoltage - Output (Min/Fixed) 0.8VVoltage - Output (Max) 41.1VCurrent - Output 5AFrequency - Switching 100kHz ~ 2.5MHz1.1$ : Richtek USA Inc. RT8298ZSP :http://www.digikey.com/product-detail/en/richtek-usa-inc/RT8298ZSP/1028-1157-1-ND/3078160Description IC REG BUCK ADJ 6A 8SOP Buck Switching Regulator IC Positive Adjustable 0.8V 1 Output 6A 8-SOIC (0.154", 3.90mm Width) Exposed PadVoltage - Input (Min) 4.5VVoltage - Input (Max) 24VVoltage - Output (Min/Fixed) 0.8VVoltage - Output (Max) 15VCurrent - Output 6AFrequency - Switching 600kHz1.66$ : Richtek USA Inc. RT2805AGSP :http://www.digikey.com/product-detail/en/richtek-usa-inc/RT2805AGSP/RT2805AGSPCT-ND/5640520IC REG BUCK ADJ 5A Buck Switching Regulator IC Positive Adjustable 1.222V 1 Output 5A 8-SOIC (0.154", 3.90mm Width) Exposed PadVoltage - Input (Min) 5.5VVoltage - Input (Max) 36VVoltage - Output (Min/Fixed) 1.222VVoltage - Output (Max) 26VCurrent - Output 5AFrequency - Switching 500kHz

author: Embed
author_rank: Newbie
timestamp: March 08, 2017, 03:08:09 am
content: I should have mentioned I would like through hole for prototyping . The final version I dont mind if its surface mount but working with surface mount while prototyping would be a lot more work. I really prefer mouser over digikey because their website is a lot easier to navigate and they have the user projects space . but I did search digikey and came up with nothing. Do you mind telling how you searched? And thanks !

author: mariush
author_rank: Super Contributor
timestamp: March 08, 2017, 03:57:21 am
content: Started fromhttp://www.digikey.com/product-search/en, pressed Ctrl+F (find) and entered "switching regulators" to scroll down to PMIC - Voltage Regulators - DC DC Switching Regulators (25734 items)So I went to DC-DC switching regulators, then selected current 5a and above, 1 outputs, function (all with step-down listed), then selected input voltage 22v and above (used Ctrl and clicks to select all the valid ranges, it's not very "friendly" digikey" here) .. basically filter down the results starting with easiest filters to apply, and after this you can also select maximum output voltage to 20v or whatever you want (you can see i didn't do that as I listed the 15v max one in the previous comment).Afterwards, I entered quantity 10 above the list of results where it says "enter quantity" and pressed enter, so that afterwards when I sort by price I actually get the cheapest parts in quantity 10 or more, instead of having the first few pages full of "2500pcs minimum order" results.But for this purpose, you may want to sort by switching frequency as well, because for example a switching regulator running at 500-600 kHz could require a 0.5$ inductor to work while a 150kHz switching regulator could require a 3$ inductor to work .. lower switching frequencies means bigger inductors with bigger peak currents are required... also a 150kHz may require electrolytic or polymer capacitors while a 500-600kHz could probably work with just a 100-220uF polymer capacitor that would be much cheaper and reliable.Downside of those high frequency regulators is you can't really work with them on breadboards and they'd barely work on prototyping boards - you can solder wires on their leads or use adapters (soic to dip , stuff like that) but these regulators really like thick ground traces, short traces between inductor and the pins on the IC the inductor is connected to.. layout is very important.Even with 150kHz switching regulators you would see much better performace on a pcb instead of a prototyping board (you can solder those dpak sanken or ap1501 i linked above for example on a proto board and play with them) but they won't work as well as on a proper pcb.So you may not want to get the cheapest regulator as you have to take in account the other components such regulators require.You also have the DC-DC controllers section on digikey, where there's loads of controllers but with those the current limit depends on what mosfets or power transistors you use with them, so it's harder to filter.Also.. you should be aware that some switching regulators have minimum and maximum duty cycles ... you may not be able to do 24v in, 2v out at 100mA even though the switching regulator advertised that, as that could be too low duty cycle .. see links below ... it would depend on the switching regulator / controllerSeeDuty-cycle is one key to buck converters' output current capability :http://www.eetimes.com/document.asp?doc_id=1225917TUTORIAL 2031 DC-DC Converter Tutorial :https://www.maximintegrated.com/en/app-notes/index.mvp/id/2031Linear Tech appnote 140 : Basic Concepts of Linear Regulator and Switching Mode Power Supplies :http://cds.linear.com/docs/en/application-note/AN140fa.pdfbtw... Linear has awesome regulators but they're usually expensive.See LT1370 for examplehttp://www.digikey.com/product-detail/en/linear-technology/LT1370IT7-PBF/LT1370IT7-PBF-ND/962877or LT1374 :http://www.digikey.com/product-detail/en/linear-technology/LT1374CT7-PBF/LT1374CT7-PBF-ND/889614or the low frequency LT1170http://www.digikey.com/product-detail/en/linear-technology/LT1170CT-PBF/LT1170CT-PBF-ND/890607

author: Embed
author_rank: Newbie
timestamp: March 08, 2017, 04:53:20 am
content: Thanks mariush ! Yeah thats one reason I've been looking at Ti's webbench so I can change the frequency around to see if I can use some of the inductors I have on hand and change the size as needed. Has anyone been able to do a switch on input reliably ? Say I'm changing voltage on the output I would have a sense circuit to change over to different tap on the input transformer ? The frequency on the input transformer would stay the same but I would be able to put components in the circuit better suited for a specific range instead of trying to make the same components work over a wider range.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 08, 2017, 02:25:50 pm
content: You may be missing some things from your theory,Do you mean an offline PS, mains input, 1.2-20V output?Or just a DC-DC, where the source is handled somehow (perhaps an iron core transformer and rectifier)?There are options for both, but the first is considerably more difficult -- it covers alotmore domains of electrical theory, to build a successful design.You do have a greater chance of success, with less theory, with the DC-DC converter.As for "controller" chips, understand: aregulatorcontains integrated switches, and is only good up to so much voltage and current, at its switch. There are integrated regulators for converters and isolated supplies, but they are limited in power. Usually up to 100W (mains powered) or ~5A (converter).In contrast, acontrollerhas gate-drive outputs: you add your own switch, and support components. It can be scaled up to any voltage or current you desire, though there are some practicalities, such as: control and protection methods, that become desirable at higher power levels; where to source auxiliary power from; how much adjustable range is desired; and how feedback, isolation and regulation is handled.For example, at a <100W power level, for an offline converter, you could choose the venerable UC3842 peak current mode controller. It's a controller, so you have to supply it with auxiliary power, and add your own switch, current sense (shunt) resistor, timing and feedback components, etc. The usual approach is: switching into an isolation transformer (actually a tightly coupled inductor, as it stores energy), with a TL431 performing load-side regulation (the controller is simply wired as a follower, so it follows the TL431's command proportionally). This works great for fixed supplies, but if you want more than about a 40% adjustable range (i.e., 12-20V say), the aux supply to the UC3842 disappears (it's self powered from an extra winding on the transformer), and it browns out and goes into a clicking auto-restart mode. You can solve this by adding a dedicated supply (which can be built from a regulator chip and parts, or a drop-in module), but then you can only adjust down to a few volts before the TL431 itself runs out of headroom to operate in. The flyback topology is also a poor choice to use for wide output voltage ranges: the lower the voltage, the longer the discharge time is, which changes the controller's response. It's not designed for that regime (continuous current mode, CCM), so it's better to avoid going too far that way.There are plenty of other options. If flyback isn't suitable, then you might consider forward topology. The classic controller here is the TL494, but it has a number of quirks that makes it tedious and slow, and operate poorly. (To make it work optimally, you need more external parts.) The traditional design would have an auxiliary supply (separate chip or module), which puts a steady, say, +12V on the secondary (load) side. This supplies the TL494, which is on the load side. Two transformers are used: one couples the TL494 outputs (via gate drive IC) to the switching MOSFETs, the other is the main power transformer. Because the gate drive signal has to pass through a transformer, it's limited on how big the transistors can be, and how fast they can switch. This is suitable up to about a kilowatt or so, before you want something more elaborate (like isolated driver circuitry). Since the TL494 is powered separately, it can regulate the output voltage over any range, and the forward converter topology is suitable for wide range outputs like this.Tim

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 08, 2017, 02:51:39 pm
content: Assuming you will be using a standard 50/60Hz step-down transformer + bridge rectifier + filter capacitors for the unregulated supply then I would recommend the LT1074 (available at Digikey), and also look up design manual AN-44 which has lots of details on using this device.I'm recommending this because one of my first successful smps projects was using this IC as a preregulator for an LM338K-based linear regulator. I built that circuit 20+ years ago and I still use it as one of my bench power supplies alongside some very expensive Kepco and Glassmann rack mounted jobs. It is effectively bulletproof yet has relatively high efficiency - the LT1074 maintains about 2V of drop across the LM338 - with much lower noise and better transient response than a straight switching regulator.

author: mariush
author_rank: Super Contributor
timestamp: March 08, 2017, 04:58:18 pm
content: Say I'm changing voltage on the output I would have a sense circuit to change over to different tap on the input transformer ? The frequency on the input transformer would stay the same but I would be able to put components in the circuit better suited for a specific range instead of trying to make the same components work over a wider range.Changing taps on the input transformer is regularly done by adjustable linear power supplies.Here's some schematics to show you how they do it