SMPS issues  - Page 1
Forum section: electronics / projects

author: John_Hofmeyr
author_rank: Contributor
timestamp: October 01, 2017, 10:34:55 pm
content: Hello everyone!So lately I have been working on a 150W SMPS. It uses the texas instruments LM5175 Buck/boost controller. I used their reference design as a template for my own and I have attached screen caps of both the reference design and my design.My SMPS specs:Its supposed to take in 22 - 26v DC and output 0 - 35v DC at 10A (the 10A will only be up to 15v, and at 35v I will Only be able to draw 4A)I want adjustable voltage and an adjustable current limit.The problems I'm HavingI have built the circuit on a PCB and it "works" and doesnt blow up, but I have a weird issue where it will only output up to 7.2v DC no matter what I set my feedback pot to. The other oddity is that the lower I put the output voltage (3 - 0v) the LM5175 suddenly pulls way more current and it gets VERY hot. At 7.2v It pulls almost no current at all and it only gets slightly warm. The other issue is that I'm not quite sure how to add the adjustable current limit to my design (yes I know I'm a noob at this). My best guess is to add something like a 0.05ohm current sense resistor on the output and then add a potentiometer in line with it to the ISENSE (+) pin on the LM5175 and to connect the ISENSE (-) pin to the other end of the resistor. Here is an ascii image to give you an idea:________----(output)----|________|---------(to load)---| 0.05ohm || ---- || | _ | ------(to ISENSE -)500k - > | |pot |------(to ISENSE +) ---So to sum it all up:I can't go higher than 7v with a 24v inputAnything below 3v makes the LM5175 get very hotI don't know how to add an adjustable current limit to my design.If you have any suggestions please let me know! Thanks

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 02, 2017, 08:22:08 am
content: The first problem you have is not reading the chip spec, operating range is quoted as"Wide VIN Range: 3.5 V to 42 V, 60 V Maximum"So operation at 3V is untested and not recommended and by the sounds of it destructive!

author: John_Hofmeyr
author_rank: Contributor
timestamp: October 02, 2017, 12:56:31 pm
content: The 3v I stated is on the output, not the input.

author: fourtytwo42
author_rank: Super Contributor
timestamp: October 02, 2017, 07:26:54 pm
content: OK I am sorry for that I misread your question. Which of the two schematics you enclosed in the OP are you implementing, if it is the 2nd its totally unreadable. The current sense is not intended to have such a high impedance in series with it. Can you post a picture of your physical implementation as a chip such as this is sensitive to poor grounding, decoupling and long lead lengths. It is quite likely getting hot either due to partial failure, parasitic oscillation or a mixture of the two.Lastly what are you trying to build ? this chip and it's H bridge is not something I would choose, if you want 0-35V out why not use a higher input voltage together with a simple buck regulator ?

author: John_Hofmeyr
author_rank: Contributor
timestamp: October 02, 2017, 10:07:33 pm
content: In terms of what I'm trying to build, its a Synchronous 4-Switch Buck-Boost converter. The reason I'm doing this is because I'm using a toroid transformer that is capable of 24v at 10A or 48v at 5A and I'm not sure I will be able to achieve a 10A output at 15V if I have the transformer set to output 48v at 5A (maybe I’m wrong, if that's the case then yes, I'll just use a buck converter), also this controller is said to be 97% efficient so I thought it would be a good choice. The other reason for building my SMPS this way is because someone told me that it is very difficult to step down such a large voltage (48v down to 0V or close to 0v) especially if I want to be able to output 10A. Also I'm using the second schematic if it is unreadable (sorry, I’m new to eagle and I’m really bad at it), it's pretty much a carbon copy of the first one with just a few more capacitors to filter the output and a 12v regulator. The only other difference is that I have replaced the 357k resistor on the output to a 500k pot for the feedback to the FB pin. If you need to you can just use the first one, they are identical. Also when you say physical implementation do you mean a picture of the PCB?Thanks

author: John_Hofmeyr
author_rank: Contributor
timestamp: October 03, 2017, 02:24:34 am
content: Question:Due to the complexity of this circuit do you think it would make sense to switch to the LT8711. It looks to be WAY easier to use and it only requires 2 transistors not 4 which will make the whole design less complex. Also as you can see in the image I attached they give you a sample circuit and all the necessary formulas to make your own. I think it would be a lot easier to just use it instead, and at the end of the day all I want is an efficient, stable and well built lab supply and this seems like it can do it with ease. What do you think?