Voltage feedback in Buck-Boost converter - Page 1
Forum section: electronics / beginners

author: Unknown
author_rank: Unknown
timestamp: May 29, 2012, 03:26:15 pm
content: Hallo everyone!I'm facing big problem with my buck-boost converter and I hope You can help meMain requirement: Vin 20-400 V (sic!), fixed output voltage -80 V (becouse of topology it's opposite to the ground), Pout = 25 W.It's works, but I have problems with feedback loop. Let me shortly describe the circuit and the problem:It's typpical buck-boost topology. The PWM 61,5 kHz is generated by Atmega32 (I've got evaluation board, in final model I'm going to use smaller one) and ampliefed by typpical mosfet driver. 0,47R+33n is a snubber. What is a big problem in Buck-Boost converters is inevitability to connect PWM's generator ground to transistor's source, so 2 "grounds" are in the circuit. In my model I use external extra socket-supply (not sure if it's proper name in English, it's kind of cell phone charger) to supply Atmega and Mosfet Driver.Problem: At the moment I use a potentiometer at evaluation board to set PWM duty cycle to keep output voltage = -80 V for various input voltage. I want to apply some feedback, sense resisotrs (see 2 resistors, i.e. 800k + 39k at output) to automaticly sense Vout and set duty cycle, but: Atmega's ground is different than supply voltage. So my -5 V on sense output resistor is measured with respect to "big" ground. With respect to "small ground" (Atmega's ground) is around |Vin|+|Vout|, so 100 ... 480 V ! What is more it changes with respect to PWM cycle.Now is the question:what is the method to apply feedback and sense resistors to such type of converter?

author: Neilm
author_rank: Super Contributor
timestamp: May 29, 2012, 06:41:57 pm
content: Why don't you have the micro connected to ground and drive the FET with a proper driver?.In answer to your question, you are entering the realm of digital control theory. To under stand that you need to know about Z transforms, PID algorithms and reasonably high speed DSP. For example a good digital control loop will have a high speed low resolution ADC so it will respond quickly to transients.If regulation is not too much of a concern you could fudge it with a slow ADC but the regulation would be terrible.Neil

author: Unknown
author_rank: Unknown
timestamp: May 29, 2012, 07:06:07 pm
content: Dear Neilm,at the moment I really don't care if A/D is slow or not, because the problem is how to "feed" this A/D converter the voltage measured by sense resistor placed on the output. Voltage on this resistor is below |5| V (with respect to ground), but with respect to A/D converter's ground it is +100...480 V, in addition changes in PWM pulse... In other words: how to avoid this huge difference between output's ground and Atmega ground?What is unusual in this project is very wide range of input voltages. Right now20-400 VDC. In the future also24-380 VAC + PFC. I don't think if any driver or in general Integrated Circuit will meet such requirements

author: Bored@Work
author_rank: Super Contributor
timestamp: May 29, 2012, 07:34:05 pm
content: You try to solve a minor problem, while you want to ignore your major problem. And that major problem is using a dead slow microcontroller in the control loop.Regarding your minor problem, did you even spend a minute to study existing designs? Have you never seen an opto coupler or any other kind of isolation barrier in an SMPS design? People put them there for a reason. But since you have to ask such questions, do you think you are qualified to work with 400 V DC?

author: BravoV
author_rank: Super Contributor
timestamp: May 29, 2012, 07:40:03 pm
content: Where is the control loop for regulation ? I can not spot it.

author: Kremmen
author_rank: Super Contributor
timestamp: May 29, 2012, 07:42:37 pm
content: Please stop while you are ahead and above all, alive.I am not saying this to be rude but if you ask questions on this level it shows that you need to study the subject a lot more. Off line SMPS circuits can be tricky bastards and definitely unsafe at such voltages. What you are attempting is not a project for a novice and everything in your question spells novice.Still, to answer your question - isolate the drive using a pulse transformer or level shifting fet driver. This is the done thing.

author: Neilm
author_rank: Super Contributor
timestamp: May 30, 2012, 08:44:08 pm
content: Quote from: dziegiel on May 29, 2012, 07:06:07 pmPersonally, I would investigate the use of an isolated fly-back topology with an opto-isolator to feedback the output voltage using a referance to switch the opto.Re ADCs, to get decent regulation ADCs in the order of >1MHz are not uncommon.I would suggest doing this using some off the shelf chip that will control this.Neil