If the transaction log is growing too large then you should increase the frequency of log backups. (The Full database backup does not actually truncate the transaction log at all).

SQL Server 2005，2000，そして7.0において，TRUNCATEはログに記録される操作である。 TRUNCATE操作の実行に関連するトランザクションを含むログのバックアップは非常に機能的で，TRUNCATEを行うトランザクションを含まないログのバックアップと同じくらい簡単に，データベースの復元に使うことができる。

In SQL Server 2005 and later, you can create a transaction log backup while a full or differential backup is occurring, but the log backup will not truncate the log due to the fact that the entire ...

If the transaction log and database are in the same device you can't do a transaction log dump. In this case you need do a Truncate Log on Checkpoint.

[!INCLUDE SQL Server] This topic discusses possible responses to a full transaction log and suggests how to avoid it in the future. The appropriate response to a full transaction log depends partly on ...

So I'm trying to figure out when I should be backing my transaction logs relative to my full/diffs. It seems like right after would be when I'd want to have the checkpoint happen, or does it ...