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Old 5th March 2006, 03:22 AM   #1
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Default TL431 Question

I am using a TL431 for analog control of a 2-switch forward SMPS.
This supply works, and puts out 15V at up to 10 Amps +.

(This is a PC supply on which I removed the --impossible to modify-- analog control circuit and replaced with a 431, then rewound the trafo secondary for 15V.)

The 431 blows once every 20 times or so I turn the SMPS on.

The current into the REF pin is less then 1 mA, and the Cathode current is limited to safe levels by 1K resistors.
Both the REF and the cathode of the 431 are fed from the output filter capacitor node, after the inductor.

But the peak secondary voltage is 60V with a duty cycle of 25%.

I am wondering if, in the first moment during turn-on, the 431 may briefly see the full 60 V on the secondary, until the feedback loop has had a chance to start working??

If that is the case,
it means that the 431 could not be used with peak secondary voltages higher than 36V, without some kind of zener level shifter.

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Old 5th March 2006, 04:49 AM   #2
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A schematic would be helpful to give some comments.
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Old 5th March 2006, 10:47 AM   #3
Eva is offline Eva  Spain
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As you have mentioned, the most probable cause for the TL431 blowing is transient output overvoltage when the SMPS is turned on. This is a serious problem because it may not only blow TL431 references but also output capacitors and whatever load you have connected to the output at power-on.

You have to check output behaviour with oscilloscope (digital storage mode is preferred for slow measurements) and improve your circuit until no overvoltage ocurrs both at turn-on and during the worst case load transient (full load switched abruplty to no load). If the output filter is too peaky, either increasing output capacitance or inductor resistance will help to damp it, altough feedback should be able to correct the effects of peaking to a great extent.
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