MOS - delay for time off

Discussion in 'General Electronics Chat' started by adinellu, Jul 28, 2015.

  1. adinellu

    Thread Starter New Member

    Jul 27, 2015
    5
    0
    Hello,

    I have 4 square signals generated with a MSP, so with the logic level 0-3.3V. I need this signals with the logic levels 0-15V. I made a circuit like in the image attached. I used for the NMOS : NDS331N from Fairchild Semiconductor (1.3A, 20V) and for the PMOS : NTR4502PT1G from On Semiconductor (-1.95A, -30V). The switch ON is good, with no delay, but the switch OFF is taking ~25us. I tried to change the 2 pull-down but the switch OFF is always 25-30us. My signals have to be precise at less than us, because I have some signals with a time ON of 2us. See the attached photos for an example of my problem. Do you have any idea from where the delay appears from?

    Thanks,
    Adina
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,762
    1,099
    It's due to the gate capacitance charge time via R2. R2 could be reduced, but ideally should be replaced by an active pull-up device.

    Edit: You'd get a faster response by using BJTs instead of MOSFETs.
     
    Last edited: Jul 28, 2015
  3. adinellu

    Thread Starter New Member

    Jul 27, 2015
    5
    0
    Thank you, I tried with a 400ohm and the delay was reduced to ~1us. I will look into the solution with the BJTs.

    Thanks again
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,973
    3,219
    Some BJT's have a significant turn-off (storage) delay but for those designed for switching, such as the 2N2369, this delay is quite small.

    What is the load you are driving?
    Why did you use such an odd value for the resistors?
    Why did you use two MOSFETs?
    What is the part number for the MOSFET?
     
  5. adinellu

    Thread Starter New Member

    Jul 27, 2015
    5
    0
    I wanted to use the standard value for the resistors, 10k, but we didn't have it, so I choose the next closest value I found in the lab. I didn't realised the value will have such a big impact on my circuit. I am not really at ease with the analog electronics. I am doing this signals to test an FPGA program I developped. The load will have 100mA max.
     
  6. crutschow

    Expert

    Mar 14, 2008
    12,973
    3,219
    What transistors do you have available to you?
    Can you readily order parts?
     
  7. adinellu

    Thread Starter New Member

    Jul 27, 2015
    5
    0
    Yes, I can order parts, I don't have any transistor, the ones I used where ordered for this application.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,515
    1,246
    Just to be clear, let me restate the project. You have an input logic signal, 0V to +3.3V, and you want an output signal of 0V to +15V that can source and sink 100 mA, and has rise and fall times of less than 1 us. Is that correct?

    If yes, a while ago I posted a voltage shifting driver circuit, and I think a variation of it will work for you.

    ak
     
  9. crutschow

    Expert

    Mar 14, 2008
    12,973
    3,219
    Below is the LTspice simulation of a circuit using 3 MOSFETs in a push-pull output to get fast rise and fall times.
    I didn't have the models for your exact devices, but used ones that have similar characteristics, so your results with the actual parts should be reasonably close to the simulation.
    The delay/rise-time/fall-time from input to output is less than 0.2μs.
    R2 is to limit the shoot-through current during the switching transient.

    Driver.GIF
     
  10. adinellu

    Thread Starter New Member

    Jul 27, 2015
    5
    0
    AnalogKid yes, that is my project. I looked into your history but I found nothing about a voltage shifting driver circuit altough I found some pretty interesting posts.
    Crutschow, thanks for your circuit, I suppose the 3rd MOSFET is the active pull-up device Alec_t was talking about?
     
  11. Bordodynov

    Active Member

    May 20, 2015
    637
    188
    Good Driver 3.3v --> 15V Driver3V3to15V.png
     
    AnalogKid likes this.
  12. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,515
    1,246
    Bordodynov - what is the output waveform when the output load is 150 ohms to ground?

    ak
     
  13. Bordodynov

    Active Member

    May 20, 2015
    637
    188
    Rload=150 Ohm-->VHight=14.6V.
    Cload=100p --> Tdelay~5nS
    Cload=1000p --> Tdelay~7-8nS
     
  14. Bordodynov

    Active Member

    May 20, 2015
    637
    188
    Low cost Driver 3.3v --> 15V

    driver3V15V.png
     
Loading...