floating regulator questions - Page 1
Forum section: electronics / general

author: uranium235
author_rank: Contributor
timestamp: June 29, 2010, 05:20:22 am
content: is there any difference in a typical regulator or it is just the same?? and how doed it work?

author: DJPhil
author_rank: Frequent Contributor
timestamp: June 29, 2010, 09:54:37 am
content: Quote from: uranium235 on June 29, 2010, 05:20:22 amI think I know what you're asking about, but I'm not sure so if none of this makes sense simply disregard. A floatingsupplyis somewhat of a separate problem if I remember correctly.From National's LM317 page:QuoteFloating 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) hasan 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.

author: saturation
author_rank: Super Contributor
timestamp: June 29, 2010, 10:20:18 am
content: 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.

author: Unknown
author_rank: Unknown
timestamp: June 29, 2010, 11:00:56 am
content: 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.

author: uranium235
author_rank: Contributor
timestamp: June 29, 2010, 11:29:31 am
content: 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 LM723http://www.datasheetcatalog.org/datasheet2/5/0w87150igkpyrpq0eu5rxzgh6c3y.pdf

author: Zero999
author_rank: Super Contributor
timestamp: June 29, 2010, 12:03:43 pm
content: Quote from: alm on June 29, 2010, 11:00:56 amIf 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.

author: saturation
author_rank: Super Contributor
timestamp: June 29, 2010, 12:08:17 pm
content: 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=falseQuote from: alm on June 29, 2010, 11:00:56 am