How to create unique Alphanumeric auto number for custom entity?

Hi,

As per my requirement, I want to generate alphanumeric unique auto number for my custom entity.

When I click on save it should be generate and every time I update the record it should not increase but it must be locked.The format for my auto number as follows;

Ex: TSN-00001 ......TSN-99999

Please help me to get the accurate result.

Thanks.