This paper discusses general approaches and results of real-time hardware-in-the-loop (HIL) testing for power electronics controllers. Many different types of power electronic controllers can be tested by connecting them to a real-time digital simulator (RTDS) for closed-loop HIL testing. In this paper, two HIL digital controller tests are presented as application examples of the low-level signal interface in the closed-loop tests of power electronic controllers. In the HIL tests, the power system and the power electronics hardware are modeled in the RTDS. The required control functions of the power electronics hardware are not included in the RTDS. Instead, the control algorithms are coded using the native C code and downloaded to the dedicated digital signal processor (DSP)/microcontrollers. The two experimental applications illustrate the effectiveness of the HIL controller testing. Results of the HIL tests and hardware validations are presented to illustrate the real-time HIL testing method for power electronics controllers.