AN3290 datasheet(23/34 Pages) STMICROELECTRONICS

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AN3290

Protection features

Doc ID 18038 Rev 1

23/34

5.3

Secondary winding short-circuit and transformer saturation

protection

The ALTAIR04-900 is equipped with a hiccup mode overcurrent protection level.

If the voltage on the SOURCE pin exceeds the VCSdis level (1 V typ.), it means the primary

drain current exceeds the value 1/RSENSE (where RSENSE is set according to the total output

power requirement), the device enters a warning state. If, at the next switching cycle, the

hiccup mode level is exceeded again, the device assumes that a secondary winding short-

circuit or a hard saturation of the transformer has occurred, so the device stops operating

and the MOSFET is no longer allowed to switch on.

In order to enable the MOSFET to switch on again, the VCC voltage must be recycled. That

is, VCC must go down to VCCrestart, then rise up to VCCon and the MOSFET switching can

restart.

Of course, if the cause of the hiccup mode overcurrent protection activation is not removed,

the device again enters auto-restart mode.

Thanks to the extremely low fault quiescent current value, the restart repetition rate is

extremely low, resulting in very safe and reliable protection.

This protection was tested on the demonstration board. The secondary winding of the

transformer was shorted in different operating conditions. Figures 25 to 28 show the

behavior of the system during fault.

Figure 25.

Hiccup mode OCP: protection tripping at VIN=115 VAC and full load

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