Buck boost DC-DC converters - Page 1
Forum section: electronics / beginners

author: yashrk
author_rank: Frequent Contributor
timestamp: May 02, 2014, 12:18:01 pm
content: HEY GUYS, really a short question and may be a silly questionwell I was designing a Boost circuit which will step up solar cell voltage to 5volts by using MC34063A, My question is that if the voltage goes grater than 5V will it ack like a buck and step down the higher voltage to 5v ?

author: Bored@Work
author_rank: Super Contributor
timestamp: May 02, 2014, 01:05:19 pm
content: The output will go to > 5V then.

author: kizzap
author_rank: Unknown
timestamp: May 02, 2014, 01:21:43 pm
content: Look into a SEPIC style converter. They allow for the input voltage to be below, equal or above the output voltage.

author: yashrk
author_rank: Frequent Contributor
timestamp: May 02, 2014, 07:36:33 pm
content: @kizzap can you share any circuit or IC

author: mariush
author_rank: Super Contributor
timestamp: May 02, 2014, 08:47:39 pm
content: See for example LT1317:http://www.linear.com/product/LT1317Datasheet here:http://cds.linear.com/docs/en/datasheet/13177bf.pdfWorks between 1.5v and 12v, outputs as much as you want. Example of SEPIC configuration is on page 11.Read the datasheet carefully, understand why each component is chosen like that (they explain it in "Application Information" section) and if you have to, adjust some components so that your circuit will work better with the most often voltage your solar panels give you. Also pay attention at the inductor because that matters a lot. It's actually a coupled inductor (two inductors in a case) .. google the recommended inductor name and find a similar one, but with the parameters your circuit needs (ex for 5v output instead of 3.3v like the example circuit is shown)Anyway, Linear Tech chips are expensive, but hey, you wanted a circuit and the datasheet has it, and it has lots of information.You can also go to Digikey and filter the long list here :http://www.digikey.com/product-search/en/integrated-circuits-ics/pmic-voltage-regulators-dc-dc-switching-regulators/2556570by selecting at TYPE only the ones that mention SEPIC, and then you can pick only the ones conventient to you ( cheap, easy to use and solder package, work with low input voltages, has example circuit in datasheet, has information to help you etc etc)

author: benjamin545
author_rank: Contributor
timestamp: May 02, 2014, 09:55:22 pm
content: take a look at the spec sheets for the inverting form of the chip. it can output a voltage from virtually zero to whatever you want. the only problem is that it outputs a negative voltage. so its usually not a problem if all you need is one voltage rail, where you would just swap the output and gnd wires to make a positive voltage. this configuration usually only creates a problem if you need to make both positive and negative voltage rails. if you needed it to keep a positive output for whatever reason you basically have to have a 2 stage inverter that uses 2 inductors, and thats basically what sepic does.

author: yashrk
author_rank: Frequent Contributor
timestamp: May 03, 2014, 12:27:36 pm
content: @benjamin545 but will it keep voltage at -5V if the input voltage goes above 5V?

author: benjamin545
author_rank: Contributor
timestamp: May 03, 2014, 03:10:34 pm
content: yes. its suppose to. im no expert by any means but ive been doing a lot of research on this chip in particular to learn how switch mode supplies work. the thing that stood out to me was how the inverting mode did not have the annoying range limit like boost and buck does. its also called a buck-boost converter as you would find it in wikipedia.

author: yashrk
author_rank: Frequent Contributor
timestamp: May 03, 2014, 05:33:25 pm
content: Well thanx I will try it out then

author: Zero999
author_rank: Super Contributor
timestamp: May 03, 2014, 06:22:23 pm
content: Quote from: yashrk on May 02, 2014, 12:18:01 pmAs mentioned above, the output voltage will go above 5V but you need to see why this will happen.Look at the schematic for a typical boost converter.When VIgoes above VO, D will conduct and there's nothing to stop VOfrom rising.

author: Zero999
author_rank: Super Contributor
timestamp: May 03, 2014, 06:26:50 pm
content: Quote from: yashrk on May 03, 2014, 05:33:25 pmOh so now you're talking about -5V, in this case, the output voltage is always below the input voltage so this can't happen.See how the diode is now connected in the opposite direction to the boost converter.You confused everyone by talking about a buck-boost converter, then mentioning 5V output, rather than -5V.

author: yashrk
author_rank: Frequent Contributor
timestamp: May 04, 2014, 02:42:38 pm
content: Ohk let me put it this way I am working with solar cell which will give me around 2-3V in morning but with good amperage because there are two solar plates so I want to use it by boosting it to 5V. But solar cell at their peak they will go above 5v (around 12V max) so at that time I need the circuit to act as buck and reduce voltage to 5V.

author: Zero999
author_rank: Super Contributor
timestamp: May 04, 2014, 07:53:48 pm
content: That's not enough information.Solar panel wattage?Desired output current?It's possible that at lower voltages, there simply isn't enough power available to do what you want. A solar cell acts like a constant current source in parallel with a zener diode. The forward voltage of the zener is depends on the number of cells connected in series and the type of semiconductor used and current is proportional to amount of light shining on the panel.A boost converter needs a higher input current, than output current. At low voltages the output current of the cells will be limited anyway and at high currents, the voltage will be limited by the parasitic zener diode.A buck-boost converter refers to two different topologies: inverting, the output voltage is the opposite polarity to the input (see previous schematic) and a converter which can either work in buck or boost mode, depending on whether step-up or step down is required.http://www.radio-electronics.com/info/power-management/switching-mode-power-supply/dc-dc-buck-boost-regulator-converter-basics.phpIf the polarity of the output voltage is unimportant, just use the inverting topology is it uses fewer components.As far as using solar cells is concerned, you'll achieve maximum efficiency by charging batteries.