TL494 loop compensation - Page 1
Forum section: electronics / projects

author: PetAd
author_rank: Newbie
timestamp: August 29, 2015, 06:19:14 pm
content: I want to redo the loop compensation for a TL494 converter in an old ATX supply to make a benchtop PSU. But I need some answers, because I couldn't find the solution on the net.What is the gain of the PWM modulator inside a TL494? I searched the internet for this and I found 2 solution: 1/VRAMP or Vin/VRAMP, where VRAMP is the peak to peak voltage of the sawtooth waveform, Vin is the supply level of the comparator in Volts.So in my case the VRAMP is 3V (found it on the datasheet of the TL494). But I don't know that the PWM comparator supplied by the built in 5V regulator or by the VCC of the chip itself.Another question is, the transfer function of a simple non resonant half-bridge, like in the old ATX PSUs, is simply D*n?Where D is the duty cycle of the pwm signal, in this case 0-~0.47, and n is the winding ratio of the main transfomer.How should I calculate the D if it can vary depending on the current requirement and if I want to change the reference signal?Or just simply calculate the min. and the max. values of the D, and make the control loop unconditionally stable in this region?

author: PChi
author_rank: Frequent Contributor
timestamp: August 29, 2015, 07:01:26 pm
content: Section 3.3.3 Pulse-Width Modulation of the Designing Switching Voltage Regulators With the TL494 athttp://www.ti.com/lit/an/slva001e/slva001e.pdfdescribes the PWM generation. So the duty cycle goes from 0 to 100 % (almost) for a 3 V change in the error amplifier output.I am guessing that you are correct that with a forward converter the transfer function is V input x duty cycle x turns ratio, when it is operating with the inductor in continuous conduction.I would have thought that the output filter characteristics wont be known so calculating stability isn't possible. Is the best approach to just resort to trial and error?

author: PetAd
author_rank: Newbie
timestamp: August 29, 2015, 08:19:50 pm
content: V input in my case is the rectified and buffered mains voltage?The supply will also remain in CCM, I placed a current sink on the output. The output filter will be calculated also.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 29, 2015, 10:18:52 pm
content: Transfer function of the transformer is straightforward, yes, but the filter adds another two poles. The gain of which varies with supply voltage. Ideally, you'd have a current sensor on the filter choke, and close the loop on that (so the loop is first order and the compensator can be type 1 or 2 with a pole at nearly the same frequency). Unfortunately, TL494 is not made for current mode operation, so it takes extra stuff to do this.Doing it voltage mode is even worse, because you have no control over inductor current (startup transients or shorts? goodbye inverter transistors!) and you have two poles in the loop (so the loop can only be excessively slow, even for a type 3 compensator).Tim

author: PetAd
author_rank: Newbie
timestamp: August 30, 2015, 08:15:37 am
content: I've used slow start at the deadtime pin against startup transients, and used the second error amplifier for current limiting overcurrent protection., and it works really good. I've used a current shunt monitor with high-side current sensing to measure the current.QuoteThen I need to calculate the min. and the max. value of the mains voltage also?I'm still confused about the half-bridge and the PWM modulator transfer function. So the half-bridge tf is Vin*D*n?The PWM tf is Vin/VRAMP or 1/VRAMP, and the Vin is the 5V inside the TL494 or the main supply voltage of the IC or the mains voltage after rectifying and buffering?I increased the switching frequency to make the transient response better, thus I need to change the LC filter and the loop compensation, but I need the system transfer function.

author: PChi
author_rank: Frequent Contributor
timestamp: August 30, 2015, 08:39:39 am
content: I think that the transfer function of the TL494 PWM modulator is (Duty Cycle, D)/(Error Amplifier Output, Voltage at the FEEDBACK pin). Which is - 0.97 / (3.5 - 0.5) approximately.The transfer function of a buck regulator / forward converter with no transformer in continuous conduction mode is (I think) (Output voltage)/(Input voltage) = D. This is assuming no losses so isn't very realistic.There is a fair amount of information on the web such ashttp://www.ti.com/lit/an/slva301/slva301.pdf, special test equipment and books written about SMPS stability.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 30, 2015, 04:02:40 pm
content: Quote from: PetAd on August 30, 2015, 08:15:37 amI've used it this way, too, but it's only ever a cosmetic hack-job. Just because you've limited startup transients by moving cautiously, doesn't mean they can't still happen, in the same way, under other conditions (besides output shorts, line dropout could do the same, assuming the aux supply happens to stay live for longer than DC bus).I don't think I've seen a current feedback method that's agreeable. All the ones I've tried hiss and scream -- typically you're using a current transformer and peak detecting diodes to sense and control primary current. But this responds to switching spikes and rapid changes just as easily as it responds to average output current, making the loop unstable and chaotic. It's a horrible mess, in control terms.This could be easily solved by adding e.g. a hall effect sensor to the main output inductor (assuming a single dominant output), but those are expensive (relatively speaking).The error amp isn't laid out correctly, anyway, so you need a second op-amp to actually finish the job (i.e., inner current loop, outer voltage loop; both fighting each other at the same hierarchical level again makes bad dynamics). This is okay if you need isolation between the load side and the TL494 (use an optoisolator, and TL431 for the voltage error amp), but that's not usually the case.So I suppose, the sad truth is, it's a relic from an earlier time, when voltage mode circuits were considered merely sufficient. There are much better ways, and much better chips to use now, but by sheer production history, we're (sometimes, almost) forced to use these extremely cheap, hacky parts...(On a related note, there ARE better (cheap and historical!) chips than MC34063, so we never need to consider that one, at least.)QuoteNonono, not VCC, Vin of the inverter, the DC bus. Regardless of what the TL494 is doing, or thinks it's doing, the output voltage from the inverter must necessarily be proportional to its supply. (This is the basic reasonPWM multiplicationworks.)The consequence of this is to design the loop so it's stable under the worst condition (highest voltage = highest gain?), then to double-check that it's still good at the least condition (lower gain, but lower pole, too). (It won't necessarily be stable inbetween those points either, but as long as we're doing simple loops with just a few dominant poles, it should be reasonably well behaved, and checking the edge cases should be sufficient. A little extra overhead is necessary to account for load variation, anyway.)Tim

author: PetAd
author_rank: Newbie
timestamp: September 05, 2015, 01:50:27 pm
content: Thanks for your reply!I know that the TL494 is obsolete chip, but I want to completely understand a simple voltage control loop, and design the loop compensation by myself. Placing zeroes and poles for compensation is easier, because there are a lot of information on the web.QuoteSorry I'm just using current sensing on the secondary side and after the LC filter, I don't want to make the loop current mode, thus I don't want to sense the primary current. It can work without it, of course it's not the best solution, but my goal is to study the half-bridge voltage mode control loop with TL494.But first I need the transfer function of the PWM modulator and the half-bridge converter. How about this? GPWM = 0.45/3 GHB = Vin*n*D, where 0.45 is the maximum duty cycle, 3 = 3.5V-0.5V, Vin is the rectified mains voltage.There is a peak current protection in the Designing Switching Voltage Regulators With the TL494 document at page 19, can it solve the overcurrent related problems?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 05, 2015, 06:55:15 pm
content: Quote from: PetAd on September 05, 2015, 01:50:27 pmYeah... but it's a terrible hack, still not current mode, and should be integrated to begin with. Consider UCC3808 for example.Tim

author: PetAd
author_rank: Newbie
timestamp: September 06, 2015, 01:02:55 pm
content: Terrible hack if fine for me, I just want to learn with it. There is only one thing I need, the transfer function of the modulator and the half-bridge, that's what I didn't found on the web, yet.