Author Topic: floating regulator questions  (Read 9060 times)

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Offline uranium235

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floating regulator questions
« on: June 29, 2010, 05:20:22 am »
 is there any difference in a typical regulator or it is just the same?? and how doed it work? ???

Offline DJPhil

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Re: floating regulator questions
« Reply #1 on: June 29, 2010, 09:54:37 am »
is there any difference in a typical regulator or it is just the same?? and how doed it work? ???

I think I know what you're asking about, but I'm not sure so if none of this makes sense simply disregard. A floating supply is somewhat of a separate problem if I remember correctly.

From National's LM317 page:
Quote
Since the regulator is “floating” and sees only the input-to-output differential voltage, supplies of several hundred volts can be regulated as long as the maximum input to output differential is not exceeded, i.e., avoid short-circuiting the output.

Floating in this sense simply means that the regulator's ground pin is not at the same voltage as the load current return path (usually the rest of the circuit's ground). Take as an example a 50VDC unregulated supply with a few volts of ripple and regulating it to 40VDC. You can use a standard LM317 (Max input of 37VDC) if you 'float' the ground up by putting a (lets say) 30V zener diode in series to the circuit ground. This tricks the ground pin into thinking ground is 30V higher, so now the supply looks like 20VDC with a few volts of ripple going down to 10VDC. The only problem you have now is ensuring that you don't short the supply and show the regulator enough voltage too cook it. :)

NEC (which was bought out by someone else) has an excellent short guide on three terminal regulators, and there's a couple of examples on page 18 of high input/high output voltage regulation. There's a lot of information in this guide on various protection and high current designs as well, and it might be a good place to start.

Hope that helps.
 

Offline saturation

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Re: floating regulator questions
« Reply #2 on: June 29, 2010, 10:20:18 am »
Floating just means the regulator circuits work without a true ground connection.  It is fooled into working above the true ground of the circuit, so it can maintain the specified safe difference between its unregulated Vin and its regulated Vout.

Its a technique to regulate > 40 V.  Most regulators are designed to work under ~ 40V.
 
The advantages are you increase the maximum voltage output that typical regulator designs can regulate.

The disadvantages include higher external parts count compared to if you used its non-floating configuration.  









« Last Edit: June 29, 2010, 12:00:20 pm by saturation »
Best Wishes,

 Saturation
 

alm

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Re: floating regulator questions
« Reply #3 on: June 29, 2010, 11:00:56 am »
How do they handle a short circuit on the output? That is the classic issue with using something like an LM317 on >40V (or LM317HV on >60V). The regulator will have almost the full input voltage between input and output in that case.
 

Offline uranium235

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Re: floating regulator questions
« Reply #4 on: June 29, 2010, 11:29:31 am »
how about LM723 >40v as floating regulator with pass transistors, it is possible to adjust the voltage and limit the current? and if added with a short circuit protected, the circuit protection must or can be floating or not?

here's the datasheet of LM723

http://www.datasheetcatalog.org/datasheet2/5/0w87150igkpyrpq0eu5rxzgh6c3y.pdf

Offline Zero999

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Re: floating regulator questions
« Reply #5 on: June 29, 2010, 12:03:43 pm »
How do they handle a short circuit on the output? That is the classic issue with using something like an LM317 on >40V (or LM317HV on >60V). The regulator will have almost the full input voltage between input and output in that case.

If the input-output voltage is greater than the maximum rating (40V for the LM317) the short circuit protection is lost. This means it's risky putting large capacitors on the output which can cause the IC to current limit and blow. One way round this is to connect a 39V zener diode between the input and output terminals and use a poly switch to provide short circuit protection.
 

Offline saturation

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Re: floating regulator questions
« Reply #6 on: June 29, 2010, 12:08:17 pm »
Practically all such designs require protection diodes to allow safe discharge of the excess currents, and preferably zener diodes to limit the maximum voltage across the regulator and its adjust terminal. The zener is chosen for both power dissipation and the maximum voltage allowed between Vin and Vout.

http://books.google.com/books?id=dIMW5eoujncC&lpg=PA11&ots=_2oBK08wzR&dq=floating%20linear%20regulator&pg=PA11#v=onepage&q=floating%20linear%20regulator&f=false

How do they handle a short circuit on the output? That is the classic issue with using something like an LM317 on >40V (or LM317HV on >60V). The regulator will have almost the full input voltage between input and output in that case.
Best Wishes,

 Saturation
 


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