So how does TL494 feedback and compensation work? - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: May 03, 2016, 08:22:58 pm
content: I have dug through documentation from TI and Fairchild but I cannot find tips on designing the feedback loop for TL494. Help!Attached is the WIP secondary converter for M103v2 modular lab power supply.Specs (M103v2 secondary channel):* Voltage: 5.3V (sensed at the USB output port)* Max current: 3A (polyfuse enforced)* Topology: Forward converterQuestions:1) How to construct the voltage feedback loop?2) How to deal with the unused error amp (since this converter don't need to mind about current)3) Should I want to implement current sensing, how should I add it to the feedback loop? (I already have a working current sensing circuit based on OP2177, and it is trivial to add the required optocoupler should I need it)4) Frequency compensation?

author: Phoenix
author_rank: Frequent Contributor
timestamp: May 04, 2016, 12:47:31 am
content: I can't give you much guidance on designing the error amps but...Which app notes did you use to get your current design? I think your tl494 output driver configuration is incorrect (C1-E1, C2-E2).These are not complementary so you can't just use one to pull down and the other to pull up. They both operate alternating - for use in a push-pull configuration. For your circuit both E should be tied to Gnd and both C should be driving the IRF840, and you will need an additional pull up resistor on the gate. Alternatively they could drive some PNP/NPN totem pole to drive the IRF840.

author: Harvs
author_rank: Super Contributor
timestamp: May 04, 2016, 02:35:05 am
content: I've used this part in a couple of commercial designs so I can't give you the schematics for them, but there's quite a few old designs out there using them. You can look around at old SMPS schematics and you'll find either the 494, or another switch mode controller that uses similar topology.But essentially there really isn't too much to these chips. Unlike a lot of modern converters that do everything for you, they are quite simple building blocks to help you implement your topology. Because of this they are very flexible, but highly un-integrated compared with modern devices.And there in lies the problem, you need to understand how you want your feedback scheme to work and use what parts of the device can help you, this includes feedback compensation. The only reason I've used this device in a current commercial design was we had some hair brained requirements we had to meet that meant totally left field feedback and compensation schemes.So your choices are really to study up on feedback and compensation schemes for your converter topology, or pick a more modern IC that has done the hard work for you and follow their design guides.A tip for your schematic, re-draw the transformer symbol so you've actually go the windings on it and try to lay out the components in a manner which makes their functional purpose clear. Reading that schematic is really hard work at the moment. I think you've got a few issues (such as the mosfet output drive as Phoenix said), but it's difficult to understand the complete intent at the moment.

author: cat87
author_rank: Regular Contributor
timestamp: May 04, 2016, 12:48:21 pm
content: To answer those questions, you must first decide what you want from your supply. For example, if you need to make a feedback loop, you must first decide on how fast you want you supply to act to changes. There are type I feedback networks, which are the slowest, type II, which are a bit faster and type III. Now, what I mean by "slow" or "fast" is the number of switching cycles it takes for your power supply to correct the output value to the preset one, or how long it takes a change to propagate through your loop.I think the best way for you to figure out all of these is to read up on the subject and decide how you want to build the supply. Therefore, I recommend this book: George C. Chryssis High Frequency Switching Power Supplies Theory And Design. It has all the information you're looking for, including practical exampled of how to play with the TL494.Also, it will tell you about how to properly design the feedback loop.And if that won;t be enough, you could search for some Unitrode seminars. These are actually application notes that teach you how to do practical design of SMPSs. They have a specific paper on feedback loop design. If you can't find that specific paper, you can PM me and help you in that respect.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2016, 02:50:54 pm
content: ...If I contribute to this thread too, do I at least get donations from it?

author: diyaudio
author_rank: Frequent Contributor
timestamp: May 04, 2016, 07:56:07 pm
content: Quote from: technix on May 03, 2016, 08:22:58 pmEVERYTHING!!!! you asking for can be found here from the app note. I think you just lazy today.http://www.ti.com/lit/an/slva001e/slva001e.pdfFrequency compensation. (autonomous subject)The web is practically DUMPED with information on how to select poles and zeros and system response tweaking.