I am designing a gate drive for a high-side P-channel mosfet. The high rail is at 30V, logic is at 12V, and some sort of level shifter is needed to avoid exceeding the maximum Vgs of +/- 20V (See: Block Diagram).
Here are the options I am considering with my comments attached to each:
Option A: Simple voltage divider, but turn-on is slowed by the relatively large resistor (R_on) needed to limit current through the transistor. This design is mentioned, but then pretty much dismissed in most of the application notes I have seen. Why?
Option B: Faster turn-on, but turn off is slowed by the need to have R_off large enough not to overstress the zener.
Option C: Based on a Vishay application note (AN804). Turn-on is slower by the relatively large R_on. Biggest advantage is that the gate drive is relative to V+, not to ground.
Option D: Used R_on to bypass some of the stress on the zener in Option B. Main disadvantage is that gate voltage is related to ground, so if V+ were reduced a lot, there might be insufficient gate voltage. Fast turn-off, but greater ringing.
I am leaning toward Option C or D, as each has only one additional part (zener) compared to a simple voltage divider (Option A). I am swayed by the prestige of Vishay toward Option C, but dont like the slow turn-on and dont understand why Option A seems to be so neglected.
Please share freely your comments and experience related to the four options. I apologize in advance for the length of this question and the large number of attachments. I have screen shots of an oscilloscope trace for each, but left out Option B because of the limit on attachments. It is available, if ayone wants to see it. Thanks. John
Edit: I increased the resolution on the Composite schematic.
Here are the options I am considering with my comments attached to each:
Option A: Simple voltage divider, but turn-on is slowed by the relatively large resistor (R_on) needed to limit current through the transistor. This design is mentioned, but then pretty much dismissed in most of the application notes I have seen. Why?
Option B: Faster turn-on, but turn off is slowed by the need to have R_off large enough not to overstress the zener.
Option C: Based on a Vishay application note (AN804). Turn-on is slower by the relatively large R_on. Biggest advantage is that the gate drive is relative to V+, not to ground.
Option D: Used R_on to bypass some of the stress on the zener in Option B. Main disadvantage is that gate voltage is related to ground, so if V+ were reduced a lot, there might be insufficient gate voltage. Fast turn-off, but greater ringing.
I am leaning toward Option C or D, as each has only one additional part (zener) compared to a simple voltage divider (Option A). I am swayed by the prestige of Vishay toward Option C, but dont like the slow turn-on and dont understand why Option A seems to be so neglected.
Please share freely your comments and experience related to the four options. I apologize in advance for the length of this question and the large number of attachments. I have screen shots of an oscilloscope trace for each, but left out Option B because of the limit on attachments. It is available, if ayone wants to see it. Thanks. John
Edit: I increased the resolution on the Composite schematic.