How robust are boost converter ICs against input voltage variations? - Page 1
Forum section: electronics / projects

author: thecake90
author_rank: Newbie
timestamp: August 07, 2015, 01:05:38 pm
content: So I am currently designing a boost converter but here is the tricky part, the input voltage is gonna change constantly and will keep changing dynamically, we are talking about 500Hz-1000Hz of variation on the order of magnitude of +-20V (of course it will always have a reasonable DC average and never drop below a certain threshold).I have been simulating a standard boost converter circuit using matlab and had obtained reasonable results using just feed-forward control. (See attachments for figures)Anyhow, if I want to avoid the complexity of this control problem, can I just use a normal boost converter controller IC? Like this for examplehttp://www.ti.com/lit/ds/symlink/lm5032.pdfI don't think their control logic was designed to handle such dynamic variations in the input voltage. Does any body know any suitable controllers that I can use? Or any closed loop control mechanisim. I imagine that a standard Type 3 componesator would not be enough in this case.Please helpany input is highly appreciatedEDIT: Just to make things clearer, I would like to have a constant output voltage (60 V in the simulation pictures below). So I am using a boost converter to try to get that constant output even though the input voltage is changing constantly. Is this possible?EDIT2: Of course the switching frequency is gonna be much higher than the frequency of voltage variation of the input.

author: macboy
author_rank: Super Contributor
timestamp: August 07, 2015, 02:05:45 pm
content: 500 to 1000 Hz is not so much just variation in the source, it is noise. And it is high enough in frequency to be relatively easily taken care of. Did you consider an LC filter on the noisy input? This should be obvious. In fact, an LC or CLC filter can be advisable to help block switching noise from your boost converter back into the supply.Modify your simulation to include the LC or CLC filter between your noisy source and the boost converter input. Also add a reasonable series resistance to the source to approximate cable resistance... some milliohms at least. Don't forget to simulate the series resistance of the L; this helps dampen the LC and adds helpful series impedance to the filter.

author: thecake90
author_rank: Newbie
timestamp: August 07, 2015, 03:20:48 pm
content: Quote from: macboy on August 07, 2015, 02:05:45 pmYou are right I can use an LC filter. Just tried it in my simulation, had a cut off frequency lower than my input voltage variation frequency and it worked fineThis solution however is not ideal, as I would need big inductor and cap. I need them to be rated at 30A, so you can imagine that size will be an issue.And the input voltage variation is deliberate, there is an input battery bank and the batteries are being dynamically switched using MOSFETs. So I would not call it "noise", it is my main input voltage.

author: MagicSmoker
author_rank: Super Contributor
timestamp: August 08, 2015, 11:28:42 am
content: The attached pictures from your simulations show the input varying over the range of 10V to 50V. While this is possible to accommodate, it would require a maximum duty cycle of ~85% to deliver 60V when the input is at 10V and the LM5032 can be programmed for a max of 76% (and even that is a bit high to run a boost, as the transformation ratio goes to infinity as the duty cycle goes to 100% [not literally possible, of course]).My suggestion to reduce the variation in input voltage as seen by the boost converter is to put a diode in series with the input and then use a bigger than normal reservoir capacitor to basically provide "hold-up time". The amount of capacitance needed (assuming a constant power drain) is:(2*P*t)----------- = C(Vh² - Vl²)where t = us and C = uFE.g. - for 2000us of hold-up time from 50V down to 20V with a 60W load (e.g. - 60V @ 1A output) you'll need a whopping big 114uF (and your max duty cycle reduces to ~60%).One other caveat with respect to the LM5032 is that it is designed for optocoupler-based voltage feedback which basically means that duty cycleincreaseswith voltage at the feedback pin (ie - COMP1 or COMP2); that is the inverse of how normal feedback inputs operate.Don't forget about the right half plane zero in the boost (and flyback) topologies. You want to set the compensation crossover frequency well below this zero to avoid catastrophic instability. Limiting the input voltage swing (and therefore maximum duty cycle) will help immensely here, as well as going with lower inductance (ie - operating with higher peak to peak ripple current).