4 Functional Checking

AN2242

16/31

To avoid spurious start-up attempts when power supply is plugged in with abnormal, lower

voltage mains, the HV current source has a turn-on threshold so that it is not activated if the

input voltage is lower than the Start voltage (typ. value is 80V).

As visible in Figure 14. and Figure 15. captured at the nominal mains voltages there is not any

overshoot, undershoot, dip or abnormal behaviour during the power supply start-up phase.

The power supply has been checked over the entire input voltage range with same positive

results as in the above figures.

4.3

Power-down

Even at turn off the transition is clean, without any abnormal behaviour like restart or glitches on

both the auxiliary and the output voltages. This is still provided by the HV start-up circuitry that

controls the power down sequence: at converter power-down the system loses regulation, then

Vcc drops and IC activity is stopped as it falls below the UVLO threshold (8.7V typ.). To prevent

restart attempts of the converter and to ensure monotonic output voltage decay at power-down

an internal logic re-enables the HV current source only if the Vcc voltage goes below the

Figure 16. and Figure 17. also the hold-up time at both nominal mains voltages is measured

115 Vac the input Elcap stores enough energy to keep the regulation for 20mS at full load. After

that the converter loses regulation and the output voltages drop. The measured time is enough

to provide the required immunity of the Set-Top box against standard mains dip or short

interruptions tests, required by the standard rules such as the IEC1000 and protecting the unit

CH3: +12 Vout

CH4: +1.8 Vout