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Old 23rd November 2012, 09:36 PM   #11
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Certainly not in series. If one is 'off' it will block the other.
I don't think you'll get two of these to regulate one switcher, maybe you will, but my bet is they'll fight each other.
Does it matter if the grounds are all separate? The aux windings produce what they do not b/c they have a common secondary ground, but b/c of what current goes thru the windings. Post regulate the secondary not used for sensing.
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Old 24th November 2012, 06:24 PM   #12
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If you had matched optos, you could use only a resistor on the LED side (no TL431), parallel the photo{transistor|diode} side, and run it normally, using the controller's error amplifier. By varying the load resistor below (or above, as the case may be) the photo{transistor|diode}, you can control output voltage.

But matched optos don't exist. There are fixed-ratio photodiode types, where one LED shines on two photodiodes, one local and one isolated. The gain and ratio must be calibrated once, but the long-term stability and linearity are within a few percent. These are used for analog coupling applications, where you use an error amplifier to set the LED current, using the same-side photodiode for feedback. The isolated-side photodiode carries a proportional current. There is no precision guarantee of the gain or ratio between photodiodes on chip, nor between chips, so each coupler must be calibrated.

A standard LED-phototransistor device has an initial gain variation of about 4:1 or so, with nonlinearity of 2:1 or 3:1 over the full range of currents (~uA to 20mA), and an overall guaranteed range of about 20:1 over temperature, manufacture and aging. Obviously this is no good for trying to match a ratio of voltage outputs, or coupling any kind of signal beyond gross on-off (digital) signals, or coupling a signal within an overall feedback loop (such as TL431 on a single output).

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