How does the AC Fail signal work in a power supply?

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I used to be not too long ago speaking to one in every of our Design Engineers about my weblog and he urged a clarification of the operation of the AC Fail signal can be a good matter.  He said that he was usually requested “at what enter voltage does the AC Fail signal function?”

A power provide’s AC Fail signal is used to supply a warning to the person that the AC enter power has both been misplaced, or is dropping in voltage to a level that the power provide will quickly not be capable of regulate or present power.

Clients utilizing such a signal will then have a brief time frame (sometimes 5 to 10ms) in which to retailer any information or begin an orderly shutdown of their system.

Inside to the power provide, the AC Fail circuit is normally a easy circuit evaluating a reference with the voltage of a primary aspect housekeeping provide.  In the occasion that voltage drops, drive is faraway from an opto-coupler and the person supplied with an AC Fail signal state change.

Earlier than the widespread use of Power Issue Correction (PFC), the AC Fail did certainly function at a set enter voltage.  I bear in mind as a Check Technician decreasing the enter voltage with a variable transformer (variac) to verify the perform.

On these non PFC power provides, the AC enter is peak rectified as proven beneath.  The primary switching converter operates off that unregulated high voltage buss, the worth of which is a direct perform of the AC enter voltage – between 120 and 375VDC.

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On power provides with PFC although, that high voltage buss is regulated utilizing a enhance circuit – to round 360VDC.  Now any change to the AC enter voltage (inside the regular working vary) isn't mirrored in a change in the DC buss; therefore a totally different check technique have to be used.

Use a storage oscilloscope to watch the output voltage, the AC Fail signal and if desired, an isolation transformer to display the AC enter voltage.

Flip off the enter voltage and measure the time between the AC Fail signal going low and the output voltage beginning to drop.  That is the quantity of warning time you'll have.

In contrast to with a non PFC power provide, this warning time is not going to be associated to enter voltage.