If you are writing to a person by name, then you must use 'Yours sincerely'.
For example,
You start with 'Dear Mr Wong', you will finish with 'Yours sincerely'
You start with 'Dear Miss Chung', you will finish with 'Yours sincerely'
You start with 'Dear 'Dr Robinson', you will finish with 'Yours sincerely'
Whereas, if you write to an unknown person (You don't know the name of the person you are writing to), then you must use 'Yours faithfully'.
For example,
You start with 'Dear Sir'', you will finish with 'Yours faithfully'
You start with 'Dear Madam' you will finish with 'Yours faithfully'
You start with 'Dear Manager' you will finish with 'Yours faithfully'