Note that there's the ISO standard for weeks which uses slightly different abbreviations:

    SELECT to_char(now(), 'IYYY-"W"IW');

The difference is when the first week of the year starts. Compare yours to the ISO 8601 format for January 1 this year:

    $ SELECT to_char('2017-01-01'::date, 'IYYY-"W"IW');
    > 2016-W52

    $ SELECT to_char('2017-01-01'::date, 'YYYY-"W"WW');
    > 2017-W01