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Date: 03/31/03 at 2:39 PM
Posted by: Russ Kincaid
E-mail: russlk@netzero.com

In Reply to: SMPS feedback posted by Joe Cocker on 03/31/03 at 4:00 AM:

1. I don't know what regulated voltage you are talking about, but believe you are confusing the reference voltage (internal to the TL431)of 2.5 volts with it's output. The output (cathode symbol) of the TL431 can vary from 2.5 volts up to a max of 37 volts, depending on the ratio of the feedback resistors R32 & R34. The TL431 is being used as an amplifier in this case. The diodes, D14 & D15 provide a DC supply to the anode of the opto coupler LED (U3). The TL431 sinks current to turn on the LED and complete the feedback path.

2. That is a good question. I suspect it is an error.

3. NO. The feedback can have an AC component which is damped by the compensation. If those resistors are removed, the system could oscillate.

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