Help with replacing transistor with faster speed

Discussion in 'The Projects Forum' started by globalone, Jun 18, 2013.

  1. globalone

    Thread Starter New Member

    May 27, 2013
    6
    0
    Hello,

    I am building a fast switch driver circuit . Switching speeds are similar to the bipolar design.

    The circuit provides 35nSec rise and fall times. This circuit will turn the switch on and off within 50nSec.

    My target is to have faster speed for Turn-On/Turn-Off . 35nSec would be okay for the T-On/T-OFF , but I want to reach between 15nSec ~ 30nSec.

    Do you think that BSS84DW (http://www.diodes.com/datasheets/ds30204.pdf) can be replaced with NDC7003P or NDS9948 ?.

    What speed would i reach If I am going to use NDC7003P or NDS9948 ?.
    http://www.fairchildsemi.com/ds/ND/NDC7003P.pdf
    http://www.fairchildsemi.com/ds/ND/NDS9948.pdf

    Are you familiar with (DUAL P-CHANNEL ENHANCEMENT MODE MOSFET). I have a working circuit that accept a logic input (0V/+5V) to bias a switch VC1 & VC2 (0V/-40V).

    I had to use BSS84DW (http://www.diodes.com/datasheets/ds30204.pdf) , but the circuit provide a switching speed on and off within 50nSec and rise/fall 35nSec.


    I have already found a replacement to BSS84DW with NDC7003P (http://www.fairchildsemi.com/ds/ND/NDC7003P.pdf) which should be faster , but I am not sure if it will use the same resistors , capacitors etc..

    I want to know what should I get a Turn-OFF/Turn-ON speed if I am going to use the new PowerTrench MOSFET (NDC7003P) ?.

    Please see the Driver circuit of the spdt switch.
    http://obrazki.elektroda.pl/3646151600_1371494410.png

    I really appreciate your help


    Ghassan
     
  2. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    What influences mainly the time it takes to fully turn on or off the FET are:

    - the ON resistance of the gate driver and therefore current drive capability
    - the internal resistance of the gate driver supply (a low ESR electrolytic capacitor adjacent to the driver pins is recommended
    - the total gate charge of the FET to be switched
    - the length of the wire (inductance) the gate drive signal goes through
    - the gate resistor value

    The lower any of those values the faster the FET can be turned on or off. Use a FET that has adequate voltage / current ratings for your application, not much more , in order to choose one with minimum total gate charge.
     
  3. globalone

    Thread Starter New Member

    May 27, 2013
    6
    0
    Hello again,

    Thank you so much for the quick reply.

    Can you please see the circuit that I have drawn according to the recommendation that i got from the company (the owner of the SPDT Switch).

    http://obrazki.elektroda.pl/3646151600_1371494410.png

    I got mentioned that .. In order to prevent negative voltage from appearing at the output of the inverters, the negative voltage applied to the level shifters must also equal the positive supply voltage. Once +/-5V is set on the circuit, the negative switch bias V- can be set anywhere in the -10 V to -40V range.

    The typical switch Ton/Toff speed is less than 15nSec but their rdriver has Ton/Toff 50nSec and Rise/Fall 35nSec. I thought of tuning the circuit to get a faster Ton/Toff.

    Should I stick into biasing -5V ? , because for example If I am going to replace the BSS84DW with NDC7003P since NDC7003P is faster than BSS84DW .

    Does this Forum allow to attach datasheets ?. I want to attach the datasheet of the Switch and the application Notes for the driver.

    Regarding the resistors R3A , R3B .. Should these resistors limit the current that should be passing to the Switch Voltage Control Lines 1 , 2 ?.

    Do you prefer of using this circuit as is and just tune the values of the resistors even though i am still worried about if the changes to the resistors may affect the current. Does this circuit allows me to bias -50V , because the higher negative , the better isolation at the switch.


    I really appreciate your help
     
    Last edited: Jun 18, 2013
  4. globalone

    Thread Starter New Member

    May 27, 2013
    6
    0
    The maximum rating of the control current at the switch is between -1 to 7.8 mA.

    What do you suggest a value of the resistors at the circuit should be ?.
     
  5. praondevou

    AAC Fanatic!

    Jul 9, 2011
    2,936
    488
    I've redrawn one part of your circuit. I recommend using this method to increase readability.

    You are charging the FETs gate through a resistor and discharging it through the 04 inverter.

    Both are "slow". The 04 inverter provides about 30mA.

    [​IMG]

    What is this application for?
     
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