Mosfet gate drive_level shifter

Discussion in 'The Projects Forum' started by jpanhalt, Mar 7, 2008.

  1. jpanhalt

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    Jan 18, 2008
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    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 don’t like the slow turn-on and don’t 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.
     
  2. studiot

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    Nov 9, 2007
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    Ask yourself what would happen if the zener failed short circuit.
     
  3. jpanhalt

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    Jan 18, 2008
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    Nothing catastrophic as the highest current through the 2N3904 would be less than 100 mA. In B and D the drive would continue to work, but the mosfet might fail at 30Vgs. (The Fairchild version of the same mosfet is rated at 30 V. That may also be a consideration.) In C, the drive would stop working. But, why is Option A so frowned upon, except for the slow turn-on? How frequently does one expect a zener to fail like that?

    As an aside, the schematic was quite readable in the original png but now, I find it unreadable. I going to try and post a readbale version.

    John
     
  4. rwmoekoe

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    Mar 1, 2007
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    i'll go with the c, and add a "booster" capacitor (in series with a small resistor), parallel to the R_ON_C.
     
  5. jpanhalt

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    Jan 18, 2008
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    You get a gold star for that! :D It sure speeds up the turn on. I found a value of 150R and 470 pF were just about right. Gate charge at 15 V is about 30 nC so I shot for something around a nF. Turn-on time (estimate) is reduced from about 620 nS to 140 nS.

    Still looking for comments on Options A and D. Presumably the same high-pass filter trick would work on Option A as well, right?

    John
     
  6. Audioguru

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    Dec 20, 2007
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    Why re-invent the Mosfet Hi-side driver IC?
     
  7. jpanhalt

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    Jan 18, 2008
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    Is there one you can recommend to drive a P-mosfet at 30V? I looked up several, and they seemed either limited in voltage to less than 30V or were ground referenced, which brings the question back to level shifting.

    At the outset, I thought it would be a fairly simple level shifter. It worked on a prototype board. So, I etched the PCB. Ran into some wild overshoot and ringing, which has been resolved. But in the process came up with the options presented here.

    I guess at this point it is a combination of intellectual curiosity, challenge, and the desire not to re-design completely the already-made PCB. I can fit a few components with solder and bridges, but not a complex IC. Nevertheless, any advice for an IC would be appreciated for the future.

    John
     
  8. n9352527

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    Oct 14, 2005
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    Using the capacitor on option A is not recommended as the gate voltage would, for a brief moment, reach the supply voltage.

    Option A, as you say, is slower than other options however it works and it is simple and cheap. I've used A, B and C before. B worked wonders with gate turn-off circuit and produced clean switching. C is a good and solid design.

    If you don't need the fastest switching, then A is a good option. B coupled with gate turn-off circuit would be my choice for the fastest switching and C fills the rests.
     
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