Boosting the current of a switching regulator, using a transistor?

rwgast_logicdesign

Ok ive been trying to figure out exactly how I intend to build the DC to DC power system for my robot. From the beginning Ive planned on using switching regulation with filters, the regulators Ive picked are LM2574AN, by recommendation of Dr-Acula. These things are very nice and very simple to set up, im getting a quiescent current of about 2 micro amps using them, thats awesome! I must say im very happy with the chip over all. The only issue I have is there only 500mA output, Id really like to have one of my 5v rails be capable of about an amp 2 would be better, but probably not necessary. I did some reading and apparently switching regulators have the same issue as LDO's as far not being able to connect there outputs in parallel.

Im using the adjustable version of the LM2574AN, this is the simple schematic from the data sheet to get 5v regulation, this doesnt include the filter:

Anyways I was rooting around through bags of junk/surplus and found a few TO-220's labeled P32A im betting these are tip32a power transistors http://www.onsemi.com/pub_link/Collateral/TIP31A-D.PDF I was wondering if maybe there was a way I could set the regulator high enough to compensate for the transistors voltage drop, then run the output from the regulator to the transistor(s base I would think) and the p32a would be able to boost my current and pulse on and off with the regulator. Im sorry im transistor stupid I wouldnt even know how to bias the thing to output an amp or two, or if this would take way more power than just using a 1amp LDO instead.

Dr_Acula

How about the LM2576? Can go to 3A. http://www.futurlec.com/ICLinearRegulators.shtml and http://www.futurlec.com/Linear/LM2576T-3.shtml $1.75 from futurlec.

There are 3V, 5V and adjustable ones. You will need a higher current diode (as per the spec sheet) and also the inductor is 100uH and will need to be rated for as many amps as you need.

rwgast_logicdesign

Hmmm, I will consider that for future projects, or maybe an upgrade later but for now I just want to get this done, so im hoping I can somehow pull this off using a transistor if not I will just be using two rails. I will definitely be using the 76 in the future though, wish id known about it a few months back.

Dr_Acula

I don't think you can boost the regulator you have very easily. You are going to need a new diode with a higher current rating, and a new inductor, and by the time you do that you may as well get a new regulator as well.

Dr_Acula

I found a circuit using a fet http://www.eleccircuit.com/boost-up-current-to-8-amps-from-a-lm2575/

Not sure about transistors. And your TIP32 is a PNP. Lots of circuits out there using a standard 7805 and a power NPN transistor.

Then there are other ways of looking at things. eg for motors and servos often you don't need regulated voltages. eg for servos, the standard supply is 4 nicads. The only regulation you need is for the controlling electronics. You can do the same thing with motors. In the olden days I remember pulling apart radio controlled cars and they often had a 9V battery for the receiver electronics and four large batteries for the motor.

So maybe you don't need a power regulator after all?

rwgast_logicdesign

No I definitely need a 3.3 and 5v rail, 3.3 for the prop and 5v for a custom AVR/Arduino serial motor controller ive built, not to mention the massive amount of sensors . But I was having some issues with my motor control running over 500mA when I pluged the encoders in, im not sure what was going on now that i rewired everything the part of the circuit that needs regulation is only drawing 70mA so 500mA will be fine I guess I was kind of hoping to use one 5v rail for everything that needs 5v in the bot, I think I may be able to pull that off now. having a 1amp rail would still make me feel better about things but if there is no reasonable way to boost current with a transistor I can just wait till later and upgrade the DC to DC supply when I outgrow 500mA.

Dr_Acula

If you are running one thing, like a motor, then obviously you need a supply capable of supplying that thing. But if you have lots of boards, lots of sensors etc, then would it be possibly just to have a number of supplies too?

It may be better anyway for noise reasons to have separate motor and control regulators?

I started buying one or two of those regulators. I now buy them in batches of 10 or so!

rwgast_logicdesign

Oh yes I will have different supplies! I have built a 20nimH pack it is directly driving an h-bridge/motors. Then I plan to use one of these switchers for the arduino/h-bride logic/and encoders. Im kind of stuck wondering whether I should use that same regulator to power an a 3.3 LDO to power the prop, or just give the prop its own dedicated 3.3v switching rail. Im also planning on a third rail for sensors etc. The reason im thinking of having the prop on the same rail as the AVR is maybe its a good idea too keep all the digital stuff micros/sensors etc on a rail, then make a dedicated rail for analog sensors.

RobotWorkshop

If you have time to wait you may just want to buy one of these modules. It is probably cheaper than buying the parts alone:

www.ebay.com/itm/1pcs-DC-DC-Buck-Converter-Step-Down-Module-LM2596-Power-Supply-Output-1-23V-30V-/251066005460

If you need to step up the voltage you could look at this:

www.ebay.com/itm/261096788682

I just used one of the Step up modules on a project and it worked out well.

rwgast_logicdesign

Wow that is cheap!! I acually sampled my chips and inductors, then found some 2amp fast switching diodes on scrap boards. So my set up was basically free! Those ebay boards are probably what ill do in the future when i dont need the regulator on the board.

Do you know if these ebay modules include a filtering circuit? I can only see one inductor, I know this is also using an lm54xx, so id think it would also require a second inductor for the filter. My biggest fear is causing noise issue on my ADCs. I guess it would still be cheaper to just filter this module than to build one with a filted.

its weird no one chimed in on the transistor thing. I would think a transistor switching on and off in sync with the regultor and amplifying the current would be a common scenario. Seems like a transistors perfect for this job!

RDL2004

Hmm.. So an LM2596 chip alone costs like $2 from a distributor (but only in quantities over 500) and they're selling this board for $1.53 with free shipping?

Mike Cook

They sure are handy and cheaper by the 10 pack!: http://www.ebay.com/itm/170892107588

You can't build them for that price, at least in small qty.

RobotWorkshop

rwgast_logicdesign wrote: »

Do you know if these ebay modules include a filtering circuit? I can only see one inductor, I know this is also using an lm54xx, so id think it would also require a second inductor for the filter. My biggest fear is causing noise issue on my ADCs. I guess it would still be cheaper to just filter this module than to build one with a filted!

If you look closely at the picture it shows there are two large surface mount capacitors (silver cans) on board so they have some filtering. Exactly how well they filter you would need to check on a scope to be sure. You could always add additional caps if needed,

rwgast_logicdesign

I see the bypass caps but im sure theres probably a noise filter consisting of another cap and inductor, like th lm2574. Ill take a look at the data sheet in a bit. These boards are so cheap becuase there skimping which is ok most of the time but will cause problems with some things like hi speed adc's. im not sure the cap value but on the output of the schematic i posted you add another bypass cap and 20uh inductor which will drastically filter out the switching noise.

Bits

Is this solved?
Have you considered using a different regulator all together or do you want to just add a circuit?