AN2844 datasheet(7/37 Pages) STMICROELECTRONICS

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AN2844

Adapter features

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The operational voltage of the converter ranges from 125 to 1250 VDC, which enables the

demonstration board to be used in various technologies, particularly in metering

applications. The output voltage is 5 V and the maximum output power is 15 W.

The board is protected with a 1-A fuse in the primary area. A negative temperature

coefficient (NTC) resistor is inserted in series with the input line to protect the demonstration

board from inrush current. For voltage purposes on the input DC line, the main 450 V

filtering capacitors are connected in series.

Figure 3.

ESBT - internal schematic and symbol

A non-dissipative active startup circuit has been implemented to optimize the converter’s

efficiency. The alternative option of using a pure resistive startup circuit was rejected due to

unacceptable power losses. The active startup circuit has been designed with Q4, Q5 and

related passive parts. The R4, R6, R11, R15, R19 and R25 resistors provide the supply

current to the PWM driver during the start-up phases and have been calculated from the

minimal input supply voltage of the converter and required supply current of the PWM

controller L6565, plus the related supply current required to charge the C15 filtering

capacitor within a reasonable time to maintain an acceptable startup time. The balance

resistors R8, R13, R17 and R23 are used to ensure the same voltage drop across each

input capacitor. R28 and R30 supply the current into the base of the Q4 high-voltage

Darlington transistor. When the rising voltage on the C15 capacitor reaches the start-up

threshold of the L6565 PWM controller, the voltage on the transformer's auxiliary winding

turns on the Q5 transistor, which in turn shorts the base of the Q4 transistor. This means

that the active startup is blocked. The main power dissipation under normal working

conditions of the startup circuit is due to the balance resistors. Refer to AN2454 "Universal

input voltage power supply for ESBT-based breaker and metering applications" for more

information on active startup issues.

The self-supply circuit that provides the supply energy to the controller has been built

around Q1, which acts as a linear voltage regulator. This voltage regulator offers a stable

output supply voltage, which guarantees the performance of the converter’s overall input

voltage range at very low or no loads. The voltage regulator is mandatory in such

applications with a wide input voltage range. The value of the auxiliary voltage is set with the

Zener diode D7 and is approximately 14.5 V. The primary side of the converter incorporates

the L6565 PWM controller that includes all the features required for building a complete

system working in QR mode with a minimal number of external components.

Information relating to the zero voltage switching comes from the transformer’s auxiliary

winding. The auxiliary winding is also used for the controller’s self-supply. To keep a

relatively constant output power across the entire input voltage range, the line voltage is fed

through resistors R9, R14, R18, R24, R29 and R31 into the line voltage feed-forward pin

(VFF) of the L6565.

AM003535

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