Correction For ALL my Logic FET Posts

Status
Not open for further replies.

gramo

New Member
If there’s anyone out there that has used the IRL2203 FET in the past because of my guidance, I'm sorry!

I have recommended it in the past with Micro Controller projects, but things may not work as they intended too

Just tonight in a practical circuit I ran into some hick-ups. The IRL2203 is not logic voltage driven - as I have suggested in the past

However - The following available from digikey.com is a logic voltage driven FET;

RFD14N05L-ND
https://www.electro-tech-online.com/custompdfs/2007/03/RFD14N05LSM.pdf

The above model is a TO-251AA package (similar to the TO-220, but without the heat sink hole on top)
 
Just on that, If you want to put a heat sink on the RFD14N05L-ND then simply leave enough room to bend the component backwards on the PCB so that it lays flat, and with some heat compound glue, throw your heat sink under it so that its flush between the component and the PCB
 
Heres how you would control the IRL2203/RFD14N05L-ND

**broken link removed**

**broken link removed**
 
gramo said:
When does that happen
Click to expand...

When things go badly wrong! - it might NEVER happen, but there have been plenty of commercial examples where 'non-essential' components have been left out, only to cause much more damage when things do go wrong.

In this particular case the FET could easily go S/C, this could very possibly damage the top transistor, which in turn could stick 12V down the PIC pin. Not a problem in this case, as a PIC is cheap - but if this was a commercial product, with a protected PIC, and the manufacturer has gone bust - then your entire device is scrap, all for the cost of a simple resistor?.

It's good practice, it costs very little, and it 'might' prove of great value?.
 
Ahh, I see

Well the whole idea about not having a resistor there is to improve switching speeds of the FET. If the FET becomes biased (not fully on or off) for any period of time, it develops a lot of heat in high current circuits.

So, keeping that in mind, you would need a 480ohm resistor to limit the sink current (max 25mA) on the PIC from a 12V potential, should the transistor break. But that would decrease the bias current on the transistors down to 10mA.

Would that still be suffice to switch the FET's fast enough?
 
You wouldn't need that high a resistor to stop the PIC getting killed, they stand a fair amount of abuse - but I wouldn't expect 480 ohm to be a problem?.
 
The FET's gate acts like a capacitor, and charges to its potential, with the transistors only being driven by 1/2 the PIC's maximum, wont that decrease the amount of current they will switch to the FET's gate?
 
gramo said:
The FET's gate acts like a capacitor, and charges to its potential, with the transistors only being driven by 1/2 the PIC's maximum, wont that decrease the amount of current they will switch to the FET's gate?
Click to expand...

How much current do you need? - probably not as much as you think?.
 
The totempole circuit shown in gramo's post isn't going to help a non-logic level MOSFET since it doesn't increase the gate drive voltage to the MOSFET anyway.

In fact it's slightly worse than driving it directly from the PIC output because you get the 0.6v base-emitter drop as well.

You need another NPN transistor and resistor to 12V rail between the PIC and the totempole transistors to make it work.
 
geko said:
The totempole circuit shown in gramo's post isn't going to help a non-logic level MOSFET since it doesn't increase the gate drive voltage to the MOSFET anyway.
Click to expand...

The PIC's 5V output will bias the transistors on and off, allowing much more voltage (in this case 12V) and current to switch the FET's Gate.

This allows the Gate to charge and discharge much faster, as the Gate acts like a capacitor, requiring time to charge/discharge.
During this time the FET becomes biased, and will drop a voltage across it, instantly dissipating much more heat as P = V * I

Now you can use PWM signals to control the output safely, or for switching of high current loads without burning out your FET.

geko said:
In fact it's slightly worse than driving it directly from the PIC output because you get the 0.6v base-emitter drop as well.
Click to expand...

The base-emitter voltage drop is not the issue, infact, its across Vce. Ideally Vce = 0, and the FET's gate can charge at 200mA (Ic max of the 3904/3905, 800% more than the PIC's maximum current output). And the opposite with a low output from the PIC - when the PNP ties the FET's gate to earth.

geko said:
You need another NPN transistor and resistor to 12V rail between the PIC and the totempole transistors to make it work.
Click to expand...

I don’t understand why, as the transistors will drive hard on with 5V and no base resistor wont they?
 
Last edited:
I don’t understand why, as the transistors will drive hard on with 5V and no base resistor wont they?
Click to expand...

The 2N3904 is an NPN transistor and you're using it as an emitter follower. If you drive its base to 5V, its emitter has to be 0.6V less. (unless its failed with an emitter-collector short)

The PIC's 5V output will bias the transistors on and off, allowing much more voltage (in this case 12V) and current to switch the FET's Gate
Click to expand...

Not with the circuit shown. Put it on a bread board and try it.
 
Now I see what you mean, so with another NPN driving the other two, the FET would be driven by 11.4 Volts instead of 4.4?

But wouldnt that make the voltage at the base of the totempole 4.4 volts, and thus making the voltage at the gate of the FET 3.8 volts?

Hmm, perhaps a quick piccy is in order
 
The PNP won't ever turn off will it.

The simulation is flawed because you're not simulating the output of a PIC. Except for the open drain outputs, a PIC will drive the output either to near Vss or Vdd. Your simulation is opening a switch, leaving the base of the transistor floating with no signal on it. It needs to drive it like a real PIC will.

You need an NPN transistor like the attached schematic shows
 

Attachments

  • driver.jpg
    6.4 KB · Views: 149
Last edited:
Sorry, that schematic has the PNP transistor the wrong way round.

The attached one here is correct.
 

Attachments

  • driver.jpg
    7.3 KB · Views: 142
Status
Not open for further replies.

Similar threads

P
Checking my logic.
Replies
12
Views
3K
Pommie
P
  • Question
cycling all the ports for testing
Replies
3
Views
759
MrDEB
Need some heads.. Logic problem I have spent days on
Replies
12
Views
2K
Nigel Goodwin
T
  • Question
Basic programming for a cubloc
Replies
4
Views
487
rjenkinsgb
D
Microchip C for dsPICs
Replies
2
Views
501
rjenkinsgb
Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…