AN2521 datasheet(18/33 Pages) STMICROELECTRONICS

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Functional check

AN2521

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correct operation when the short is removed. In Figure 24 and Figure 25 both sequences

are captured during 230 Vac operation but they do not change significantly over the input

mains range.

3.4

Overvoltage and open loop protection

The EVAL6668-75W board implements two different open loop protections: one for the PFC

and another for the flyback stage.

The PFC controller L6563 is equipped with an OVP, monitoring the current flowing through

the compensation network and entering the error amplifier (pin COMP, #2). When this

current reaches about 18 µA the output voltage of the multiplier is forced to decrease, thus

reducing the energy drawn from the mains. If the current exceeds 20 µA, the OVP is

triggered (dynamic OVP), and the external power transistor is switched off until the current

falls below approximately 5 µA. However, if the overvoltage persists (e.g. if the load is

completely disconnected), the error amplifier will eventually saturate low, triggering an

internal comparator (static OVP) which will keep the external power switch turned off until

the output voltage returns to a point near the regulated value.

The OVP function described above is capable of handling "normal" overvoltage conditions,

i.e. those resulting from an abrupt load/line change or occurring at start-up. It cannot handle

the overvoltage generated, for instance, when the upper resistor of the output divider fails

open. The voltage loop can no longer read the information on the output voltage and will

force the PFC pre-regulator to work at maximum ON time, causing the output voltage to rise

uncontrollably.

A pin on the L6563 (PFC_OK, #7) has been provided for additional monitoring of the output

voltage with a separate resistor divider (R3, R5, R11 high, R19 low, see Figure 1.and 2).

This divider is selected so that the voltage at the pin reaches 2.5 V if the output voltage

Figure 24.

EVAL6668-75 W short circuit at no-

load & 230 Vac-50 Hz

Figure 25.

EVAL6668-75 W short circuit

removal at no-load & 230 Vac-50 Hz

CH1: Q2 drain voltage

CH2: SOFT START voltage - pin #11

CH3: output voltage

CH4: VCC

CH1: Q2 drain voltage

CH2: SOFT START voltage - pin #11

CH3: output voltage

CH4: VCC

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