AN2242

4 Functional Checking

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against lost of channel tuning or video disturbances while the end user is watching TV or

recording programs.

4.4

Short-Circuit Tests

An important functionality of any power supply is the capability to survive in case of load short

circuit and to avoid any consequent failure. Additionally, the power supply must be compliant

with safety rules, which require that, in case of fault, no component will melt or burn-out. The

SMPS protection is another issue of power supplies. Sometimes it is easy to find circuits with

good protection capability against shorts of the load but which are not able to survive in case of

a very hard short like that of an output electrolytic capacitor or of a rectifier or transformer

saturation. Besides, in case of a shorted rectifier the equivalent circuit changes and the energy

is delivered even during the on time, as in forward mode. In case of a short, the voltage at pin

operation, so avoiding the destruction of the components at primary side. The controller

remains in off-state until the voltage across the Vcc pin decreases below the UVLO threshold. It

will then try to restart without success until the secondary short is removed. This provides a low

frequency hic-cup working mode, preventing the power supply from being destroyed.

The board has been tested over the entire input voltage mains range. Two critical circuit

parameters, the Vds and the output current have been checked during short circuit tests. In all

output current has a value lower or close to the nominal one, therefore preventing the

component damage. As indicated by the waveforms in Figure 18. and Figure 19., once an

output is shorted, the circuit begins to work in hic-cup mode, keeping the mean value of the

current at levels sustainable by the component rating. Because the working time and the dead

time are imposed by the charging and discharging time of the auxiliary capacitor C19, thanks to

the L6668 current source and related circuitry, the on-off periods are independent from the

input mains voltage, contrary to controller using a passive start-up circuitry where the on time

CH3: +12 Vout

CH4: +1.8 Vout