AN2864 Datasheet, PDF (22/29 Pages) STMicroelectronics – VIPER17L demonstration board with wide-range

Testing the board
Figure 20. Operation with output shorted (V_IN = 115 V_AC)
Ch1: V_DD
Ch2: V_FB
Ch4: I_DRAIN
AN2864
3.6
Secondary winding short-circuit protection
The VIPER17L is equipped with an adjustable first level of primary overcurrent limitation that
switches off the power MOSFET if this level is exceeded. This limitation acts cycle-by-cycle
and its main purpose is to limit the maximum deliverable output power. A second level of
primary non-adjustable overcurrent protection is also present, fixed at 600 mA (typical
value). If the drain current exceeds this 2^nd OCP (second overcurrent protection) threshold,
the device enters into a warning state. At the next cycle, the MOSFET is switched on and if
the second level of overcurrent protection is exceeded again, the device assumes that a
secondary winding short-circuit or a hard saturation of the transformer has occurred, so the
MOSFET is no longer allowed to switch on and the device stops operating. In order to
enable the MOSFET to switch on again, the V_DD voltage must be recycled. That is, V_DD
must go down to V_{DD_RESTART}, then rise up to V_{DD_ON}. When the VIPER17L is switched on
again (V_DD equals V_{DD_ON}), the MOSFET switching can restart. If the cause of the 2^nd
overcurrent protection activation is not removed, the device switches into auto-restart mode.
This protection was tested on the STEVAL-ISA058V1demonstration board. The secondary
winding of the transformer was shorted in different operating conditions. Figure 21 and
Figure 22 show the behavior of the system during the tests.
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Doc ID 15282 Rev 1