Multi output Fly-back SMPS

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Hello,

 

I design Multi output Fly-back SMPS input +110v ac and output +5 -5. +5V dc of this circuit has feedback and output capacitor is 1000uF. This positive part works well but when I get current from +5V dc the negative part go beyond to -15V and when I get current from -5V this voltage drop to -1V. I use dummy 50ohm load that get current from positive part while I get current from -5 negative part it helps that the voltage decrease to -2.5 voltage in comparison to-1v mentioned voltage. HELP to fix it.

 

 

Thanks for your help.

Attachment(s): 

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That IC seems to be unknown to Google. Please provide a reference.

 

Multiple output switcher are notoriously difficult to design. I have never been successful to make one that provides a wide range of currents to both outputs (high current on one, low current on the other). 

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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The IC is UC3843, Unfotunately I cannot even get 500mA from -5v part. How to correct it?

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Please show the complete circuit. You attachment seems to be only part of the circuit.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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More inductance in T2 (500mA on the negative rail is significant other than for audio power amps)

 

A jelly bean JFET op amp will require significant negative headroom though it's simple to set the center of the common-mode range away from 0v.

Some analog doesn't need much below 0v (-1V may be enough)

Some analog have at least adequate negative rail PSRR (minimal negative rail regulation)

 

"Dare to be naïve." - Buckminster Fuller

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Do yourself a big favor, never draw any connection as a 4 way intersection:

It is very easy to overlook on copied or faded drawings & can cause major blunders if missed during PCB layout or overlooked by a technician.

You will find this advice worth $$$ before you know it.    

 

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Mulitple output switchers are a problem because, normally, only one output provides the feedback. This requires tight coupling between the output with feedback and the other outputs (and this is hard to do). This is  why only one is relatively well regulated. What usually happens is that load changes on the regulated output often results in large changes in the unregulated outputs even though the regulated output may stay relatively steady. Also, load changes in any of the  unregulated outputs will cause large changes in other unregulated outputs and sometimes unexpected changes in the regulated one.

 

Further, when there is  only one controller, you face a major quandary. The switcher only has one PWM signal to control. If the load changes on one of the outputs, then that requires a change in the duty cycle. How do you keep that from altering the voltage on the other outputs, since all have to use the same PWM? The proper solution is a switcher per output. That is why Analog Devices and TI both (and others) have multiple switchers in a single package. Well, there is another solution: add a secondary regulator on each output.

 

For this reason, we really need more of the schematic, because the feedback mechanism is critical. 

 

It is, to use the common idom, "a can of worms".

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

Last Edited: Mon. Feb 17, 2020 - 04:56 AM
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I design Multi output Fly-back SMPS

Then where is your tapped transformer?  A flyback uses energy stored in the core, like a big bucket of energy, ready to be transferred to its final destination.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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I now think that that ls cunningly hidden in the T2 symbol, which gives no hint as to its circuit function except that symbol prefix "T" is often used for transformer.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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I designed a five output flyback switcher, +5, 5 isolated, 5 isolated, +15 and -15 volts. The custom transformer secondaries were wound “quint-filar” (is that even a word?). Feedback was taken from the +5 output. Regulation was very good because of the “intimacy” of the secondary windings.

Tom Pappano
Tulsa, Oklahoma

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Tom is exactly correct. If the windings for separate outputs are very tightly coupled, it is possible to achieve good performance for certain circuit topologies. But, few commercial transformers are built that way. I think what is described are windings that are first twisted together, like a rope, THEN  wound on the core.

 

From the circuit diagram that was offered, I cannot even tell how the various output voltages are created. It the very least, we need to know where the output side transformer windings are connected. Then, where the feedback is generated.

 

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Energy is stored in two places in a SMPS, in the filter caps as a static charge and in the coil as a magnetic charge, using 100uh inductors in multi-Amp(s) power sources seems unwise to me, but my experience with SMPS is limited.  

Jim

 

 

 

 

 

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Energy is stored in two places in a SMPS

A flyback is somewhat "special"...like your favorite cousin

 

in the flyback topology, energy is stored in the magnetic field of the transformer during the first half of the switching cycle and then released to the secondary winding(s) connected to the load in the second half of the cycle. Flyback transformers feature a gapped-core construction, which allows high energy storage without saturating the core. This energy storage aspect distinguishes flybacks from other topologies such as forward-mode where energy transfers immediately from primary to secondary. Flyback transformers are also known as coupled inductors, because they have a gapped core construction and store energy in the core.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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avrcandies wrote:

Do yourself a big favor, never draw any connection as a 4 way intersection:

It is very easy to overlook on copied or faded drawings & can cause major blunders if missed during PCB layout or overlooked by a technician.

You will find this advice worth $$$ before you know it.    

 

 

That is good advice. I shall try to remember it.

 

Four legs good, two legs bad, three legs stable.

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That is good advice. I shall try to remember it.

 If you have a tech wire up a proto & they miss one like that (easy to do from drips of doughnut icing), you will often find some very strange shenanigans occurring!!

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Mon. Feb 17, 2020 - 10:54 PM