Switcher By-Pass
Switcher By-Pass
(OP)
Greetings. Here's a problem I've been working on for a few days. I have a nice little switcher that can convert anything between about 14V and 75V to 12VDC to run some lousy fans and a processor that has sub-regulation from the 12V. This means the 12V can be anywhere from say 11V to 14V and nothing would care in the least i.e. lousy regulation is fine.
The catch.. There's always a catch. I want this product to be able to run from 12V as well as 15,24,36,48V, etc.
Considered solutions. Run a buck/boost instead of a buck. Unfortunately that results in a switcher twice as big and 7 dollars instead of the above one which is only about 4 dollars.
Do you think something like the little network below the switcher with the two transistors will get the job done by essentially bypassing the switcher whenever the voltage is below 14V.
The catch.. There's always a catch. I want this product to be able to run from 12V as well as 15,24,36,48V, etc.
Considered solutions. Run a buck/boost instead of a buck. Unfortunately that results in a switcher twice as big and 7 dollars instead of the above one which is only about 4 dollars.
Do you think something like the little network below the switcher with the two transistors will get the job done by essentially bypassing the switcher whenever the voltage is below 14V.
Keith Cress
kcress - http://www.flaminsystems.com





RE: Switcher By-Pass
If you can use a PNP for Q6 and change the control part accordingly, I think the chances are better. The Uce can then be as low as a few hundred millivolts.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Switcher By-Pass
Dan - Owner
http://www.Hi-TecDesigns.com
RE: Switcher By-Pass
RE: Switcher By-Pass
Thanks for pointing out that detail Skogs. Got me thinking the whole bypass trip point might be too flaky so I've re-tooled it using a comparator with a single chip milliamp switcher to run the comparator and set the reference. Doing this also solves another problem which is thermally controlling one of the fans (for free! Spare comparator.). This change adds about $1.50 to the cost which is better than $3+.
That would probably work too but cost me days of mucking about and prototyping.
Hi Opera. The chip is the LM5010.
It doesn't actually say it won't work. It doesn't do its own UVLO until down about 5.25V I wish it would just turn on its switch and sit there feeding-thru whatever is available but I think it will interject x us of off period causing some sort of negative issue. The plan is to add this other circuitry then crank a board and try it. If it does work adequately I'll not be stuffing the Bypass.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Switcher By-Pass
Apologies in advance if you're way past that point.
RE: Switcher By-Pass
Workbench was invented by National and I'm glad TI hasn't screwed it up yet - like everything else they suck up - but I digress. That's exactly where this design came from. There is one single design that encompasses the 12V in 12V out regulated requirement. That one results in the $7++ cost.
Their BOM estimates are off by 50%. Add 50% to them and they're pretty good.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Switcher By-Pass
The GUI that I was looking at allowed setting Vin to a range as wide as 8v - 75v (not just "12V in"). Given that the issue is the Vin range below 12v, does this minor discrepancy in the statement of Vin requirements lead anywhere useful?
Note: I'm assuming that there are more than one reference design architecture, depending on how you set the requirements fields. That's the impression given to me by the webpage, but I've not gone in deeper to confirm.
Last resort - I've never met an Application Engineer (at the component OEM, such as TI) that I didn't like. They've always been perfectly helpful. Does the modern TI still offer such phone-in technical support? If one of their reference designs is supposed to work down to 8V input, and it doesn't, then this is where they would usually be willing to help review things.
Observation: The Vf voltage drop on D5 is pure waste. Is D5 a low voltage drop Schottky diode? Such a diode might gain you about one-half volt in the difference.
RE: Switcher By-Pass
RE: Switcher By-Pass
I'm sorry if I wasn't clear that this product needs to run on anything between 12V and 50V so 8V is completely useless to me. The 12V is the whole problem since I need 12V out the other side. The one single design in workbench that can provide that functionality is the LM5018 which costs considerably more and requires a double fist-full of surrounding parts to work. If lower voltages like 7 or 8V needed to be supported it would be a shoe-in but just to allow by-passless operation it's overkill.
"Pure waste" unless you provide a warranty in which case it's pure gold.
And yes, it is an (expensive) Schottky to ease your mind. LOL
Keith Cress
kcress - http://www.flaminsystems.com
RE: Switcher By-Pass
Thanks iop. A relay would definitely cost more than a 30 cent FET. So I've opted for the FET. This think is only needing about 500mA.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Switcher By-Pass
RE: Switcher By-Pass
I didn't write " 8V "; I wrote "...a range as wide as 8v - 75v...". That 8v-to-75v range more than encompasses "anything between 12V and 50V". Am I misunderstanding something? It seems clear enough.
Here's the reference schematic diagram that was offered up on the LM5010 page. The diagram indicates that the circuit should accept any Vin from 8 volts all the way up to 75 volts. Their offered information is consistent.
If I were you, I'd want to know why the circuit as you've designed and built isn't working down to about 8 volts. It's an interesting discrepancy, because if it leads to a solution (such that your regulator worked over the range of Vin), then you wouldn't need the bypass. Right?
PS: I'm trying to be helpful. If this discrepancy isn't helpful for some reason that I'm missing (miscommunication?), then apologies in advance.
RE: Switcher By-Pass
RE: Switcher By-Pass
Might be wrong but as this is a step [i]down[/] regulator then the output is by default lower than the input. If you want 12V out you have to feed it something higher than 12V. The converter doesn't have boost capability, it's a fairly straightforward buck converter.
RE: Switcher By-Pass
TTFN

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RE: Switcher By-Pass
RE: Switcher By-Pass
RE: Switcher By-Pass
$3.80 shipped
Same website has quite a few similar products.
RE: Switcher By-Pass
Well that seems to make some sense. I'll let you know as I just sent that design out.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Switcher By-Pass
Man that place as the oddest cr@p. Tends to take a month to receive your orders from there too.
No doubt that regulator has the same issues. It has icky aluminum caps too. :)
Keith Cress
kcress - http://www.flaminsystems.com