DC-DC boost circuit for AA batteries - Page 1
Forum section: electronics / projects

author: foxr
author_rank: Newbie
timestamp: January 12, 2021, 06:31:37 pm
content: Hi! I'm currently working on a project with lots of op-amps and need to design a PSU for it. I want the project to be powered by 4 AA batteries and to output +-6v at about 100/200ma. I don't have much experience working with batteries and DC-DC converters so I need some help. I'm not sure how I should connect the batteries (eg: 2 in series and 2 in paralel, 4 in series, 4 in paralel) to get the most performance out of it and don't know how to design a boost circuit that will work with the batteries from their max voltage(1.5V) to 1V. I'm not sure how I should create the positive rail if anyone has any suggestions I'd greatly apreciate. For the negative rail I think I'll just use a MC3406 to invert the voltage and create a -6v rail. BTW the positive voltage doesn't have to be exactly 6V and the negative rail doesn't have to be exactly the negative of the positive rail.Oh and BTW this is my first post on the EEVBLOG so sry if I did something wrong.

author: Dennis Frie
author_rank: Contributor
timestamp: January 12, 2021, 07:29:49 pm
content: First, the batteries: You likely want to put them in series, to get as high a voltage as possible. 4 * 1.5V = roughly 6 V.If you can live with that, including the voltage-drop when your batteries gets discharged (expect them to drop down to something like 4-5V at least) you can use that directly as your positive supply.As far as converters goes, I can recommend to take a look here for a general overview of converter typologies.https://www.ti.com/lit/ml/sluw001g/sluw001g.pdf?ts=1610459584353&ref_url=https%253A%252F%252Fwww.google.com%252FIf you want a more stable positive supply, using a boost-converter is fairly easy. TI webench (or similar) can be a good place to start, to get an idea about the controllers available and get a quick "reference design".The negative supply is always the "problem". If you can live with virtual ground, it might be easiest to simply boost the 6V to 12V and use 6V as your virtual ground.

author: CaptDon
author_rank: Super Contributor
timestamp: January 14, 2021, 02:38:19 pm
content: One downside to boost converters running on batteries. Since the load can be assumed to be constant, as the battery voltage goes down the current required will go up and batteries really don't like to push even more current as their output voltage drops. You need really good long life high current demand batteries. We had a Marantz Minidisc recorder used by our news team and it was full of SMPS voltage converters. They ate batteries like throwing them in a fire!!! We had to go to digital media like SD recorders for reliability and being able to be 'on scene' recording for an hour or more without battery death.

author: Zero999
author_rank: Super Contributor
timestamp: January 14, 2021, 07:39:05 pm
content: Quote from: CaptDon on January 14, 2021, 02:38:19 pmThat applies to any switched mode power supply.QuoteThat's bad design, rather than the fault of a switched mode power supply. If the same device used linear regulators, the battery life would be worse, because they're less efficient.Suppose we have a device which requires 5V, at 100mA, running off six alkaline cells.With a linear regulator the input current is equal the output current, so it's 100mA, whether the batteries are fresh, or old.Now consider a switched mode power supply, say with an efficiency of 90%. The output power is 5✕100mA = 500mW and input power 500/0.9 = 556mW. With fresh batteries, 1.5✕6 = 9V, the current draw will only be P/V = 0.556/9 = 62mA, much lower then the linear and with dead batteries, say 5.5V, the current will be 0.556/5.5 = 101mA, virtually the same as the linear regulator. In reality the efficiency of the switched mode regulator will vary, depending on the current. Generally simple buck and boost regulators are more efficient, when the input voltage is closer to the output voltage, so efficiency will improve, as the battery discharges.