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TOP 264EG- Flyback Power Supply Transformer Issue

Posted by: semcee on

Hello Sir,

I have made 2 design which i have put this file below.But unfortunately, although their output turns are same, the input turns are different.Which one I must choose?And why there is difference between them?And also I wonder, how it is possible in the 5V output voltage with 4  Turns and 15V output voltage with 7T.Because of the 15/5 = 3 shouldn't it be 7*3 = 21 T.

Thank you for your support in advance

Best Regards

Semih Senyuz 

Files

Attachment 大小
TOPSwitch-JX_PIDesign6.pdf 2.64 MB
TOPSwitch-JX_PIDesign7.pdf 2.63 MB

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Submitted by PI-Wrench on 01/30/2023

The difference between the two designs lies in the value chosen  for VOR, or reflected output voltage. This is expressed as VOR = (Vo + Vd) Np/Ns, where Vo is the supply output voltage, Vd is the forward drop of the output rectifier, and Np and Ns are transformer primary turns and transformer secondary turns, respectively. The value of VOR is an important part of the design, as it dictates things like peak drain voltage, operating duty cycle, primary peak current, secondary peak current, and output diode reverse voltage.

In general, if VOR is increased, you get lower primary peak current, higher duty cycle, higher peak secondary current, and lower inverse voltage stress for the output rectifier..  Decreasing VOR gets you higher peak primary current, lower operating duty cycle, lower peak secondary current, and higher  reverse voltage for the output rectifier.  As for choosing between the two designs, one has a VOR of 135V, which is a little on the high side. The second design has a VOR of 100V, which is a little on the low side. You might want to run a new design with a compromise value for VOR like around 120V.

As for the secondary turns mentioned, the designs have a 15V output stacked on top of a 5V output, so the actual total number of turns for the 15V output is 4T + 7T = 11T

The spreadsheets both show a close match of predicted output voltage to the desired output voltage.

There are two floating outputs for each design, one with 5V output, and one with 12V output. For better cross regulation on these 2 outputs, I would suggest installing preload resistors on both those outputs. A suggested value for each would be around 10% of max load.  Depending on how variable the load is on the 5V and 15V outputs, you may need a preload resistor on the 5V regulated output as well. The preload resistors will keep the output voltage on the two floating outputs from going  totally  crazy when the load on the regulated outputs is at maximum. A preload might be necessary on the regulated outputs as well if the load there has a chance of going close to no-load, as in that case the floating outputs will collapse without a well-defined minimum load on the regulated outputs.

Submitted by semcee on 02/01/2023

Hello Sir,

Thank you for your response.Actually, it scaries me adding external things that the PIExpert doesn't put.I have attached my schematic which the tool produced.Could you suggest to put preload for this design if so kindly show to me where i should put the preload and which value?And the VOR value is correct becuase I have read that it must be 90-135V.

Thank you for support again

Best Regards

Semih Senyuz

 

Attachment 大小
PI_shared.pdf 549.14 KB
Submitted by PI-Wrench on 02/02/2023

Because this power supply has two floating outputs, cross regulation will be challenging. It is likely unreasonable to expect perfect regulation over all conditions of line and load. 

Since the TL431 regulator/error amplifier is connected only to the 5V output, that output will regulate to suit itself, meaning the regulation might suffer on the other outputs. If the 5V output is unloaded, the voltage on the two floating outputs will collapse, and regulation for the 15V output may also suffer. By the same token, if the 5V output is loaded to max load and the other outputs are unloaded, the output voltage on the other outputs will likely rise beyond regulation limits. It is important for good regulation from a supply  of this nature to have a well- defined minimum load for each output. 

If you expect the load on any of the outputs on this supply to go to zero, it will be necessary to use a resistive preload at that output to keep it from going out of regulation when the 5V load is at maximum. A good starting value is 10% of maximum load. For the 5V floating output, suggested resistive preload is 47 ohms, 1W. For the 12V floating output, suggested resistive preload is 120 ohms, 2W.

If you expect to encounter a situation where the load on the main 5V output can go to zero, it will be necessary to add a preload resistor to that output as well to keep the voltages at the other outputs from collapsing. The required preload may be as much as 20% of full load., so in the case of this supply, it may be necessary to ad a 24 ohm, 2W resistor to the 5V main output.

Expect to spend some time on the bench optimizing the preload values for proper regulation, depending on the characteristics of the actual system that will constitute the load for this supply. If that system presents a well-defined minimum load to all the outputs, it will be much easier to get this power supply to properly regulate.

Don't expect perfect regulation for the two floating outputs. 10-15% regulation  may be the best obtainable for those 2 outputs, depending on the nature of the actual load. If this is unacceptable (and may be for the 5V floating output), it may be necessary to design one or more of the floating outputs for a higher output voltage and use a linear post-regulator.

I will leave it up to you to choose the transformer design for this project as both are within the recommended VOR limits.. 

I will warn you that it is unreasonable to expect perfect regulation for a supply like this if you expect the load on each output to vary from 0-100% load.