AN3095 datasheet(7/55 Pages) STMICROELECTRONICS

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AN3095

System description

Doc ID 16555 Rev 3

7/55

The output sensing and relays board was realized to interface the power system and the

grid. This task is accomplished with the implementation of a proper control algorithm which

requires both grid-current and grid-voltage sensing. For this reason, the board is equipped

with current and voltage Hall effect sensors. Two relays, controlled by an I/O of the

microcontroller, are also placed on the same PCB to interrupt/connect phase and neutral of

the system to phase and neutral of the grid. Moreover, this board is provided with two-way

connectors for electrical wiring of the LCL filter to the main power board.

The multi-output power supply board implements two independent offline flyback

converters, with wide input voltage range, based on VIPER technology, to generate the

following output voltages:

●

+5 V to supply DC-DC converter gate drivers

●

+5 V to supply DC-AC converter gate drivers

●

+5 V to supply the microcontroller

●

+/-15 V for LEM sensors supply

●

24 V for relays supply

The main advantage of an offline solution is the availability of a power supply for circuits

dedicated to communication and data transfer even at night or in the case of weak PV field

energy production. The price to pay for such an advantage is higher power consumption

during standby mode of the main power unit.

The specifications in Table 1 for the PV system are used as inputs for the design of the

boards mentioned above. All parameters are assumed to be equal to their nominal value if

not otherwise stated.

Table 1.

System specifications

Specification

Value

DC-DC input voltage

200 V - 400 V

DC-DC output voltage

450 V

DC-AC output voltage

230 Vac

Nominal output power

3 kW

DC-AC switching frequency

17 kHz

DC-DC switching frequency

35 kHz

Transformer turns ratio

1.2

Grid voltage

230 Vrms +/- 20 %

Grid frequency

50 Hz

Power factor above 10 % rated power

>0.9

THD@ full load

<5 %

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