Bulk induced dynamic R_ON in GaN-on-Si HEMTs is a serious performance limiting instability which remains a problem even in some commercially available power switching devices. Its origin is now reasonably well understood, however until now it has not been possible to simulate it using a realistic epitaxial stack. For the first time we successfully simulate the controlled suppression of bulk dynamic R_ON by adding a specific model for leakage along threading dislocations. This was undertaken using a commercially available standard TCAD simulator, allowing realistic device optimization in an advanced GaN HEMT design flow.