Fast turn-on protection of a cascode switch

1. A power converter comprising:

a cascode circuit including a depletion mode field effect transistor and an enhancement mode field effect transistor electrically coupled in cascode, the cascode circuit configured to provide a cascode node voltage; and

an overcurrent detection circuit configured to detect a period when the cascode node voltage gradually decreases and to indicate an overcurrent fault when the period exceeds a threshold time duration.

2. The power converter of claim 1 , wherein the overcurrent detection circuit comprises a comparator.

3. The power converter of claim 1 , wherein the overcurrent detection circuit comprises an AND gate.

4. The power converter of claim 1 , wherein the overcurrent detection circuit comprises a controller.

5. The power converter of claim 1 , wherein the depletion mode field effect transistor is a gallium nitride (GaN) depletion mode transistor.

6. The power converter of claim 1 , wherein the depletion mode field effect transistor is a silicon carbide (SiC) depletion mode transistor.

7. The power converter of claim 1 , wherein the enhancement mode field effect transistor is a low voltage field effect transistor.

8. A method of detecting overcurrent in a gallium nitride (GaN) cascode circuit comprising:

turning on the GaN cascode circuit with a transient step;

receiving a cascode node voltage;

determining a period when the cascode node voltage gradually decreases; and

indicating an overcurrent fault in response to the period existing for greater than a threshold time duration.

9. The method of claim 8 , wherein turning on the GaN cascode circuit comprises:

providing a gate signal to a low voltage field effect transistor.

10. The method of claim 9 , wherein receiving the cascode node voltage comprises:

receiving a drain voltage of the low voltage field effect transistor.

11. The method of claim 10 , wherein determining the period when the cascode node voltage gradually decreases comprises:

comparing the drain voltage of the low voltage field effect transistor with a threshold voltage.

12. The method of claim 11 , wherein indicating the overcurrent fault in response to the period existing for greater than the threshold time duration comprises:

determining the threshold time duration using a controller.

13. A method of rapidly detecting overcurrent in a power converter comprising:

turning on a cascode switch with a transient step;

receiving a cascode node voltage;

determining a period when the cascode node voltage gradually decreases; and

indicating an overcurrent fault in response to the period existing for greater than a threshold time duration.

14. The method of claim 13 , wherein the cascode switch comprises a depletion mode field effect transistor and an enhancement mode field effect transistor electrically coupled in cascode.

15. The method of claim 14 , wherein turning on the cascode switch comprises:

providing a gate signal to the enhancement mode field effect transistor.

16. The method of claim 15 , wherein receiving the cascode node voltage comprises:

receiving a drain voltage of the enhancement mode field effect transistor.

17. The method of claim 16 , wherein determining the period when the cascode node voltage gradually decreases comprises:

comparing the enhancement mode field effect transistor with a threshold voltage.

18. The method of claim 17 , wherein the threshold voltage is between one volt and ten volts.

19. The method of claim 17 , wherein indicating the overcurrent fault in response to the period existing for greater than the threshold time duration comprises:

determining the threshold time duration using a controller.

20. The method of claim 19 , wherein the threshold time duration is between one-hundred nanoseconds and three-hundred nanoseconds.