Circular definitions always define a term using the very same term. If a premise is relevant, it must be true. By definition, relevance is independent of the truth of a premise. The truth of a premise is considered part of the Acceptability condition. Inconsistent premises can not be true at the same time.
Inconsistent premises are, by definition, contradictory.
This means that if one is true, the other is false. So, they cannot both be true at the same time. A Straw Man argument is bad because its premises are false. A Straw Man argument fails to meet the Relevance condition. Straw Man arguments 'put words into someone's mouth,' thus changing the subject and failing the R condition.