eem2am
Joined: 22 Jun 2008
Posts: 753
Helped: 13
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 01 Oct 2008 19:50 ncp1337 |
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Hello,
I am designing a valley switching flyback with NCP1337 IC.
I am using opto feedback for isolation as in the datasheet.
However, the opto transistor is connected back to the feedback (pin 2) pin via a resistor. Unfortunately, the documentation does not give how the value of this resistor is arrived at.
I would be very grateful if any reader could shed any light on the finding of this resistor value.
The only explanation given of this resistor is as follows:........
on page 9 of the following app note:.....
http://www.onsemi.com/pub_link/Collateral/NCP1337CRTTVEVB_MANUAL.PDF
there is a resistor (R31) into the FB pin of NCP1337 (pin 2). On page 8 of the same document this resistor is said to be 18K.
In all of the available documentation, there is nowhere where it is shown how the value of this resistor is worked out.
The datasheet for NCP1337 is here:......
http://www.onsemi.com/pub_link/Collateral/NCP1337-D.PDF
In the above datasheet, the feedback pin is explained at the bottom of page 11.
The following app note:....
http://www.onsemi.com/pub_link/Collateral/AND8266-D.PDF
gives more explanation of the feedback pin on bottom page 3 & top page 4
....Unfortunately it is not possible to find how the feedback pin resistor is calculated. Any help or thoughts much appreciated.
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VVV
Joined: 26 Nov 2004
Posts: 1584
Helped: 290
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02 Oct 2008 17:50 smps feedback isolation |
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As you can see from the datasheet the feeback pin uses the current, rather than voltage, for regulation. The range of valid current is 40ľA to 220ľA. Below 40ľA the circuit thinks the opto is open, above 220ľA it detects a no-load condition. Further, at 500ľA it can no longer maintain the voltage constant.
Anyway, if you are going to use the circuit with an optocoupler, the resistor value is of little importance, because the opto output is a current, so it will drive the feedback pin correctly, irrespective of the value of that series resistor.
However, some limits have to be imposed on the value of the resistor for correct operation. Thus, the maximum current is about 500ľA. You should make sure that if the opto is fully on (or shorted) the current into the feedback pin is not much higher than 500ľA. So the feedback resistor will be Rfbmin=(Vcc-Vfb)/500ľA=9V/500ľA=18kΩ. That is probably why you see the 18k resistor.
I don't think this value is critical, as there seems to be internal protection if the current exceeds 500ľA. (Of course, keep it reasonable, don't let it go into the mA range by choosing a resistor much lower than 18kΩ.)
The maximum value for the feeback resistor should ensure that the maximum current of 220ľA can still be reached when the opto is fully on. Rfbmax=(Vcc-Vfb)/220ľA=40kΩ.
As you can see, the feedback resistor can vary widely. Any value between 18kΩ and 40kΩ will work.
But your safest bet is to go with the minimum value of 18kΩ, which will ensure that the 220ľA can be reached and the circuit will operate properly, while at the same time limiting the current to a safe value of about 500ľA.
Otherwise, because Vcc and Vfb have tolerances, as do resistors, you may find for example that you cannot reach 220ľA. This should be reached under no-load conditions. But this never happens because you selected too large a resistor. So your output will go out of regulation because the circuit never sees 220ľA and instead of switching to burst operation it keeps pumping power into a non-existent load, forcing the output voltage to increase, exceeding the limit. Effectively your feeback loop is open at this point.
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