Flyback switching power supply designed by UC3843

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ROSELJR

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I have a UC3843 flyback switching power supply. The transformer design is very strange. It has 6 turns of primary coil, 81 turns of secondary coil, DC12V input, and 350V output. According to the calculation, doesn't the secondary coil need at least 175 turns? Why also get 350V voltage output?
But the calculation formula is not related to frequency?
The timing resistor is 750 and the capacitor is 743. What is the switching frequency?
Thanks in advance.
 

Bo Deadly

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I know nothing of SMPS design. The off-the-shelf ones are good enough and cheap enough that I don't think I ever will be. But for some strange reason I do like to muse about such things (actually it's not strange, I'm procrastinating) so ...

From glancing at the datasheet, you are using the formulas in section 9.2.2.2 yes?

It seems to me the turns ratio depends more on the voltage ratings of parts and not entirely on the desired output voltage. The chip is going to moduate the output to try to produce right output voltage regardless of turns ratio.

As for switching frequency, again, the frequency doesn't matter that much because it's the pulse width is what controls the ultimate output voltage / current. There are all sorts of factors that go into selecting an operating frequency.
 

JohnRoberts

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I have never designed a flyback switcher but recall fixing one in the high voltage PS section of a very early heathkit computer monitor. It had a blown darlington transistor that was very long lead time so I had to cobble together a discrete darlington using two devices.

My understanding about flyback PS is that the operating principle is not unlike the Kettering auto ignition coil. One winding builds up the magnetic field in the core, then when that charging current is interrupted the magnetic field collapses creating the high voltage spike across the secondary. I don't think there is a simple turns ratio to find.

Good luck.

JR
 

ruffrecords

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Are you sure the primary is not centre tapped so the ratio is really 3 to 81. I have seen quite a few designs with 3+3:90 windings.

Cheers

Ian
 

Newmarket

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My understanding about flyback PS is that the operating principle is not unlike the Kettering auto ignition coil. One winding builds up the magnetic field in the core, then when that charging current is interrupted the magnetic field collapses creating the high voltage spike across the secondary. I don't think there is a simple turns ratio to find.

My understanding about flyback PS is that the operating principle is not unlike the Kettering auto ignition coil. One winding builds up the magnetic field in the core, then when that charging current is interrupted the magnetic field collapses creating the high voltage spike across the secondary. I don't think there is a simple turns ratio to find.

Basically yes. Similar principle to ignition or 'Autotransformer'.
Output Voltage is function of both turns ratio and Duty Cycle .
This link is pretty helpful for starters.

A Guide to Flyback Transformers | Coilcraft
 
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