Mosfet

[Electronics4you]

Can a MOSFET IRFZ34 drive a 10A motor with only a 5V/20mA TTL signal on the gate?

 

[denikar]

The part can provide a max steady state current of about 6.3A at room temperature with Vgs = 10V, so no.

 

[phalanx]

Go to digikey. Search for FET. Click on the FET link under Discrete semiconductor products. Select a current level that you want and change transistor type to N-Channel Logic. Choose part from the list. Umm.... PROFIT!

 

[Roff]

The part can provide a max steady state current of about 6.3A at room temperature with Vgs = 10V, so no.

I don't know where you got 6.3 Amps. The IRF datasheet says 30 Amps max, with Ron=0.06 ohms max at 10V. This would be a power dissipation of 6 watts, which would require a heat sink. The problem is you can't do this with TTL drive. You need 10 volts of gate drive.

 

[Electronics4you]

I use a TC426 1.5A Dual High-Speed Power MOSFET driver and a Vdd supply of at least 20V, which should be able to drive at least a 10A motor.

 

[audioguru]

I use a TC426 1.5A Dual High-Speed Power MOSFET driver and a Vdd supply of at least 20V, which should be able to drive at least a 10A motor.

No, the max voltage for the TC426 is only 18V and the Mosfet's gate needs only 10V, so use a 10V supply for the TC426.

 

[Electronics4you]

OK the TC426 can only support 18V-25mV, but the IRFZ34 can support +/-20V, so I just use 18V

 

[denikar]

I don't know where you got 6.3 Amps. The IRF datasheet says 30 Amps max, with Ron=0.06 ohms max at 10V. This would be a power dissipation of 6 watts, which would require a heat sink. The problem is you can't do this with TTL drive. You need 10 volts of gate drive.

I could be mistaken, but here is where the 6.3A came from:

The junction-to-ambient thermal impedance is listed at 62 C/W with no heatsink. The maximum Rdson at room temperature is given at 50mhm: with a Vgs of 10V.

At room temperature, the part can safely dissipate about 2W with no heatsink (150C - 25C)/(62 C/W).

Imax = sqrt(P/Rdson) = sqrt(2W/50mhm = 6.32A.

 

[Roff]

I could be mistaken, but here is where the 6.3A came from:

The junction-to-ambient thermal impedance is listed at 62 C/W with no heatsink. The maximum Rdson at room temperature is given at 50mhm: with a Vgs of 10V.

At room temperature, the part can safely dissipate about 2W with no heatsink (150C - 25C)/(62 C/W).

Imax = sqrt(P/Rdson) = sqrt(2W/50mhm = 6.32A.

You may be right, but why would you restrict yourself to no heatsink on a power MOSFET sinking 10 Amps?

 

[Electronics4you]

Is it really right that the voltage through drain-source can only be around 2.1V less the gate voltage? Then it can't be directely controlled with a TTL circuit. Is it the threshold voltage (what is that anyway)

 

[audioguru]

The drain-source voltage of a turned on Mosfet with 10V between its gate and source is extremely low and depends on the load's current and the temperature. Look at the datasheet.

The output high voltage from a TTL gate is only 3.5V, isn't it? I haven't used old TTL for over 30 years.

Most Mosets are spec'd to turn on very hard with a gate to source voltage of 10V so few Mosfets will work from a TTL signal.
Some "logic level" Mosfets work from the 5V of a Cmos gate or Cmos microprocessor.

 

[kchriste]

Then it can't be directely controlled with a TTL circuit. Is it the threshold voltage (what is that anyway)

The FETs threshold voltage is the lowest voltage on the gate at which the FET will start to conduct. To drive a FET from a TTL gate you'll need a 4K7 pull up resistor and a logic level FET. If it is driven from 5V CMOS logic, no pullup is required. Something like the IRLZ44 will work.

 

[Oznog]

Do you actually intend to draw 10A?
The max rating of a motor and its usage are only loosely related. You may have an application in which your 10A motor would never draw more than 1A in normal use. Also, that 10A motor might draw 30A if it were stalled.

 

[Electronics4you]

If I need to get 12V to the motor in the mittle of the H-bridge, what would the base input voltage and current then be - can it be driven by the TC426? If the drain-source voltage of a turned ON MOSFET would be extreemely low with a 10V base voltage, how can they be used to drive motors?

 

[audioguru]

When the voltage across a turned on Mosfet driver is extremely low then less power is wasted as heat in it and more power goes to the load.

 

[Hero999]

The current a MOSFET can pass depends on the gate voltage. 3.5V might be enough for small loads but 10A will require a higher voltage. There should be graphs of Vgd vs Id on the datasheet.

 

[Electronics4you]

I'm just lost now...
What should the base and collector voltages and currents be to get 12V to the motor - please explain in details. I use the IRFZ34 and a TC426 MOSFET driver and need 12V and around 8A would be very nice if possible

 

[dknguyen]

In your previous post, you are talking about base and collector voltages. This is a BJT transistor. Not a MOSFET. MOSFETs work differently than BJTs. For your purposes, a BJT's current is controlled by the gate current, but...for MOSFETs:

A MOSFET (for your purpose) is a resistor whose resistance can be controlled by gate-source voltage. The higher the voltage, the lower the MOSFET's resistance. If you are using it as a switch for the motor (which you are) you want the lowest resistance possible. Just ram as much gate voltage into the MOSFET as you can without exceeding the limits to get the lowest MOSFET on-resistance as possible. If you want to get 12V to the motor just stick 12V across the motor and MOSFET in series (or the H-bridge).

Go to your MOSFET datasheets, and look at the graph for Drain Current (Id) vs. Source-Drain Voltage (Vds). There should be multiple curves on the graph, each one for a different gate-source voltage (Vgs).