+ Post New Thread
Results 1 to 4 of 4
  1. #1
    Newbie level 4
    Points: 114, Level: 1

    Join Date
    Nov 2018
    Posts
    5
    Helped
    0 / 0
    Points
    114
    Level
    1

    IRFZ48 N and FAN7392 MOSFET Switch

    Hello,
    I'm trying to build a high current MSFET switch to control the temperature of a 400 W heater bed. Instead of bying a board, i thought I would make one. I got following components from nearest store,

    IRFZ48N and IRFZ48N MOSFET driver.
    I have never used a MOSFET driver before and I'm little confused about the circuit

    Following is the schematics of the circuit that I made. I haven't actually made the circuit in board since I don't know whether circuit is correct or not.

    Click image for larger version. 

Name:	circuit.png 
Views:	2 
Size:	29.3 KB 
ID:	152606
    I have some general doubt about understanding the MOSFET data sheet. Load is 12 V 250 W heater. So current of 21 A.
    In the data sheet of IRFZ48, Gate threshold voltage is 2-4 V, that is the minimum volatge required to turn ON th channel, but it doesn't have enough channel width for 20 A, so I have to choose a gate voltage that is enough to create a channel for 20 A it should be less than maximum allowed Gate voltage, which 20 V from data sheet.

    Click image for larger version. 

Name:	mosfet.PNG 
Views:	2 
Size:	81.8 KB 
ID:	152608
    Click image for larger version. 

Name:	mosfet1.PNG 
Views:	1 
Size:	38.2 KB 
ID:	152607
    To know how much Gate current, I should check the output characteristics.
    Click image for larger version. 

Name:	mosfet2.PNG 
Views:	3 
Size:	42.6 KB 
ID:	152609

    I have a 12 V supply, so giving a Vgs of 12 V will create enough channel.

    Am I right up to this point? Is there any other general things that should consider for MOSFET switches??

    Thanks

    •   AltAdvertisment

        
       

  2. #2
    Super Moderator
    Points: 73,982, Level: 66
    Achievements:
    7 years registered
    Awards:
    Most Frequent Poster 3rd Helpful Member

    Join Date
    Apr 2014
    Posts
    15,006
    Helped
    3419 / 3419
    Points
    73,982
    Level
    66

    Re: IRFZ48 N and FAN7392 MOSFET Switch

    Hi,

    to control the temperature of a 400 W heater bed
    Power is not that important. For Calculations you mainly need voltage and current.

    --> and donīt post confusing informations:
    Load is 12 V 250 W heater. So current of 21 A.
    What now? 250W or 400W?

    IRFZ48N...
    I have never used a MOSFET driver before and I'm little confused about the circuit
    If you buy a so called "logic level MOSEFT" then you may omit the MOSFET gate driver circuit.

    In any case: You need to have all the datasheets by hand and read through them.
    If the datasheet doesnīt show "typical application schematics" then go to the IC manufacturerīs intenet site and look for related documents. Like application notes.
    I donīt recommend to use random schematics from the internet, they often are not reliable and may contain mistakes. Use reliable ones from IC manufacturers, universities...

    To your circuit:
    Use example circuits form reliable documents and modify them for your needs. Always draw complete schematics. Hand drawn are OK.

    Your schematic contains a lot of issues:
    * missing power supply capacitors
    * unconnected COM pin
    * Check that no unused input is left floating (general rule: donīt let (unused) inputs floating, neither for digital circuits, nor for analog circuits)
    * protect your MOSFET from overvoltage spikes with a zener. Especially with high speed switching you may expect high voltage peaks on drain-source due to stray inductance in wiring and/or load.

    In the data sheet of IRFZ48, Gate threshold voltage is 2-4 V, that is the minimum volatge required to turn ON th channel,
    No. This is not ON state. This is the edge where the MOSFET becomes conductive. See the tiny current of 250uA!

    To know how much Gate current, I should check the output characteristics.
    Static gate current is negligible with MOSFETS. Just care about V_GS.

    I have a 12 V supply, so giving a Vgs of 12 V will create enough channel.
    This is not the correct chart to look at. (mind the 20us pulse comment)
    Look for the R_DS_ON vs _V_GS chart.

    But in this case just the table value V_GS_ON (@10V V_GS = 14mOhms) is suffcient.
    Calculate voltage drop and static power dissipation with the use of 14mOhms.

    Klaus
    Please donīt contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



    •   AltAdvertisment

        
       

  3. #3
    Newbie level 4
    Points: 114, Level: 1

    Join Date
    Nov 2018
    Posts
    5
    Helped
    0 / 0
    Points
    114
    Level
    1

    Re: IRFZ48 N and FAN7392 MOSFET Switch

    There were mistakes I made in post, load is a 250 W. I had a 400 W , 24 V heat bed besides me when I was writing this post, sorry above that.

    To know how much Gate current, I should check the output characteristics.
    Static gate current is negligible with MOSFETS. Just care about V_GS.
    what I meant was gate voltage.

    This is not the correct chart to look at. (mind the 20us pulse comment)
    Look for the R_DS_ON vs _V_GS chart.
    I couldn't find any Rds ON vs Vgs chat in the data sheet,
    but in the electrical characteristics table

    RDS(on) Static Drain-to-Source On-Resistance ––– ––– 14 mΩ VGS = 10V, ID = 32A

    That mean 10 Vgs will be enough for 32 A, right?

    voltage drop = 0.014 * 32 = 0.448 V
    and power dissipation = 32*32*0.014 = 1.334 W
    I donīt recommend to use random schematics from the internet, they often are not reliable and may contain mistakes. Use reliable ones from IC manufacturers, universities...
    This schematics is drawn by me, I couldn't find any thing online, I have drawn this based on the example circuit found in the datasheet.

    Click image for larger version. 

Name:	example circuit.PNG 
Views:	0 
Size:	46.9 KB 
ID:	152636

    When I made the circuit on PCB, I connected COM to ground and when tested with LED as load, it's working

    Click image for larger version. 

Name:	circuit V1.png 
Views:	0 
Size:	29.5 KB 
ID:	152637

    And to solve some issues that you have mentioned

    Click image for larger version. 

Name:	circuit V2.png 
Views:	1 
Size:	32.0 KB 
ID:	152638

    Would this be fine?
    I think I can pull down the SD and HIN to ground using 10 K pull down resistor though not sure about HO< VB and VS pins.
    Datasheet mentions about bootstrap resistor and diodes, what are those?



    •   AltAdvertisment

        
       

  4. #4
    Advanced Member level 5
    Points: 9,591, Level: 23
    d123's Avatar
    Join Date
    Jun 2015
    Posts
    1,692
    Helped
    398 / 398
    Points
    9,591
    Level
    23

    Re: IRFZ48 N and FAN7392 MOSFET Switch

    Quote Originally Posted by Athul View Post
    bootstrap resistor and diodes, what are those?
    The Internet, and expecting other people to do incredibly basic research oneself should do - what are those? ;)



--[[ ]]--