AN2521 datasheet(20/33 Pages) STMICROELECTRONICS

20 / 33 page

EVAL6668-75W: thermal map

AN2521

20/33

The flyback stage is also protected against open loop conditions that lead to loss of control

of the output voltage. A divider connected to the auxiliary winding of the transformer is also

connected to the L6668 pin #7 (DIS) and, in case of excessively high output voltage

resulting from loop failure, provides for the triggering of the internal comparator connected to

that pin. In this case operation of the L6563 will cease because the L6668 will stop the PFC

stage operation via the PFC_STOP pin. The VCC powering both the ICs will be maintained

by the HV start-up generator of the L6668. To restart the operation, it will be necessary to

unplug and re-plug the mains, to unlatch the L6668.

EVAL6668-75W: thermal map

4.1

Thermal protection

The EVAL6668-75W is also equipped with thermal protection of the flyback's power

MOSFET (Q5). Its temperature is sensed using the NTC thermistor R58 connected to the

L6668 pin #7 (DIS). If the temperature of the heat sink rises above the maximum allowed

level (80 - 85 °C), the threshold of the internal comparator will be exceeded and the L6668

latched as in the case of open loop. To restart the operation of the circuit, it will be

necessary to unplug and re-plug the mains.

4.2

Thermal map

In order to check the reliability of the design, thermal mapping has been performed using an

infrared camera. In Figure 27 and 28, the thermal measurements on the key components at

nominal input voltage are shown. The correlation between the measurement points and

components for both thermal maps is indicated in Table 7 below. The ambient temperature

during both measurements was 27 °C. All other components on the board work within the

temperature limits, ensuring reliable long-term operation of the power supply.

Figure 27.

Thermal map at 115 Vac-60 Hz - full load

AN2521 Arkusz danych(PDF) 20 Page - STMicroelectronics