Using a LM2679 for a power supply, question about ratings - Electronic Circuits Projects Diagrams Free

FusionITR · 18th April 2006, 09:55 AM

I'm going to be using a LM2679 regulator for a switching power supply that outputs 5V as 5A, but the question I have is for the input for the regulator. If I use a transformer and bridge retifier as the DC input for the regulator, what does the transformer need to be rated at? Do I have the option to use any transformer as long as I have a minimum of 25VA of power? What about the bridge rectifier after the transformer, does the voltage/current rating need to match the transformer?

FusionITR · 18th April 2006, 03:33 PM

Anybody?

FusionITR · 18th April 2006, 09:50 PM

Looking for a bit of help here.

docel · 18th April 2006, 10:03 PM

The basic rule of thumb: get a transformer atleast twice the rectified/regulated output current. The diodes rating MUST exceed the transformer current ratings several times ( atleast 4), for better safety and reliability, although you can getaway with 1/2 ratings fro a bridge rectifier.

FusionITR · 18th April 2006, 10:10 PM

Quote:

Originally Posted by docel

The basic rule of thumb: get a transformer atleast twice the rectified/regulated output current. The diodes rating MUST exceed the transformer current ratings several times.

Do you mean if I want 5V/5A output I should get a transformer that can handle 10A? I thought it was about output power versus input power, such as for 25W of output power I would need 25VA of input power (such as 12.5AC at 2.5A would support this, even though the current rating for the transformer is lower than the regulated output current).

audioguru · 19th April 2006, 02:10 AM

For 25W of output power the transformer must be rated for 25VA plus more current to heat the filter capacitor and rectifiers.
If you have a full-wave bridge rectifier then it will have a 1.5V or more voltage drop. The rectifier's loss is difficult to calculate due to the pulsing but is roughly 1.5V x 5A= 7.5W.

mmelikaa · 19th February 2008, 03:24 PM

I have a problem with LM2679 adj ,I used the software in national semiconductor and i connect the exact same quantities,but my circuit doesn't work,I just have around 0.6 of output current and 3 amperes in input!
please help me,I really need it.

mneary · 19th February 2008, 10:25 PM

A drawing or photo of your work would help us to help you.

Hero999 · 22nd February 2008, 11:03 PM

Quote:

Originally Posted by docel

The basic rule of thumb: get a transformer atleast twice the rectified/regulated output current. The diodes rating MUST exceed the transformer current ratings several times ( atleast 4), for better safety and reliability, although you can getaway with 1/2 ratings fro a bridge rectifier.

Where did you hear that?

It's not true, a rectifier only needs to be rated for half the current since it only conducts for half the time. Most rectifiers can also handle some pretty heavy surges and shouldn't be damaged charging the reservoir capacitor.

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18th April 2006, 09:55 AM	(permalink)
FusionITR	Using a LM2679 for a power supply, question about ratings I'm going to be using a LM2679 regulator for a switching power supply that outputs 5V as 5A, but the question I have is for the input for the regulator. If I use a transformer and bridge retifier as the DC input for the regulator, what does the transformer need to be rated at? Do I have the option to use any transformer as long as I have a minimum of 25VA of power? What about the bridge rectifier after the transformer, does the voltage/current rating need to match the transformer?

18th April 2006, 09:50 PM	(permalink)
FusionITR	Looking for a bit of help here.

18th April 2006, 10:03 PM	(permalink)
docel	The basic rule of thumb: get a transformer atleast twice the rectified/regulated output current. The diodes rating MUST exceed the transformer current ratings several times ( atleast 4), for better safety and reliability, although you can getaway with 1/2 ratings fro a bridge rectifier.

19th April 2006, 02:10 AM	(permalink)
audioguru	For 25W of output power the transformer must be rated for 25VA plus more current to heat the filter capacitor and rectifiers. If you have a full-wave bridge rectifier then it will have a 1.5V or more voltage drop. The rectifier's loss is difficult to calculate due to the pulsing but is roughly 1.5V x 5A= 7.5W. __________________ Uncle $crooge

19th February 2008, 03:24 PM	(permalink)
mmelikaa	I have a problem with LM2679 adj ,I used the software in national semiconductor and i connect the exact same quantities,but my circuit doesn't work,I just have around 0.6 of output current and 3 amperes in input! please help me,I really need it.

19th February 2008, 10:25 PM	(permalink)
mneary	A drawing or photo of your work would help us to help you.