Testing the board

AN2929

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Doc ID 15333 Rev 2

The duty cycle of power delivery is very low (around 1.55%) so the average power

throughput is also very low (see the figure below).

Figure 22.

Operation with output shorted

2.6

Secondary winding short-circuit protection

The VIPER27LN is provided 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 overcurrent protection is also present and in this case it is not adjustable, it is

protection) threshold, the device enters a warning state. The next cycle the MOSFET is

switched on, and if again the second level of overcurrent protection is exceeded, the device

assumes that a secondary winding short-circuit or a hard saturation of the transformer is

is not removed, the device goes in auto-restart mode.

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

was shorted, in different operating conditions. The following figures show the behavior of the

system during these tests.

Ch1: V

DD

Ch2: V

FB

Ch4: I

DR AIN

V