One method to implement a bi-directional isolated DC/DC converter is to use two full-bridge converters interconnected with a transformer. Both full-bridge converters are controlled to produce a 50% duty cycle square wave voltage at their terminals. If the square wave voltages are directly in-phase then no real power is transferred through the transformer. If the phase angle between the two square waveforms is changed then real power is transferred through the transformer. The direction of the power flow depends on the sign of the phase angle. Whereas the magnitude of the power transfer is dependent on both the phase angle and the ratio of the output to input (u2:u1) DC voltages.

By using this applet it is possible to observe the relationship between the power flow and the phase angle, as well as the current waveforms. If the phase angle is zero and the output to input DC voltage ratio is no zero, it is interesting to see that current still flows between the two isolated sides, however on average no real power is transferred.

• Change the visualisation of the transformer to an inductor by clicking on the transformer.
• Adjust the phase angle between the input and output bridges by dragging the red dot on the uS square-wave voltage.
• Change the u2:u1 voltage ratio by dragging the red dot on either the input voltage, output voltage, or both, of the converter.