linear algebra – Under what condition can a symmetric matrix becomes PSD

I’ve just learned the concept of PSD in class, and I’ve got a little confusion.

If A is a symmetric matrix represented by [a b ; b c], what conditions should be satisfied to becomes a PSD?

What I’m thinking is that all principle minors should be greater or equal to 0, thus a>=0, c>=0, and ac-b^2>=0. Is these all we need? How about b?

Thank you!