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Dual MOSFET Switch

Overview

 

In order for a microcontroller to switch large DC loads some form of driver interface is required.

This can either be achieved via an electromechanical relay or though the use of some form of solid state device. Either approach can be used however the use of a solid state design will enable the output to be driven at a much higher frequency then a relay based design would allow.

Here the design of a 12V 3A  MOSFET switch is considered, the design needs to incorporate opto isolation and 3.3V microcontroller control input logic levels.

Circuit Description

 

As outlined above, ideally I would be looking at switching 12V 3A loads via a microcontroller running 3.3V logic and include additional protection in the form of opto isolation.

The circuit layout for the MOSFET switch is depicted below:

 

The transistor element of the opto isolator is connected as a common collector driver so that when the control input is driven to logic 1 the load also energises. Indication of load activation is given via D1.

The calculations that where employed in order to determine the passive component values will now be discussed. 

Design and Stripboard Layout

 

By making reference to the TPL521 data sheet some starting figures can be obtained for the design.

From the IF-VF graph it can be found that at @10mA Vf=@1.15V

This can then be used to calculate the value of R1 as below

       Vcontrol - 1.15V          3.3V - 1.15V

R1 = ----------------------- = ---------------- = 215 Ohms = NPV from the E12 range = 220 Ohms

             10mA                    10mA

Next from the Ic-Vce graph it can be found that when Ic=10mA and If = 10mA then Vce = 0.3V

 

and by consulting the Id - Vgs graph of the IRF530, we can find that at 3A current drain Vgs will be about 5.5V.

 

We can then employ these values to calculate the vales of R3 and R4 as follows.

Since R4 is connected from the gate to the source of the MOSFET and we know that Ic of the opto output transistor is 10mA, then if we ignore the negligible gate current of the MOSFET then IR4 must also be 10mA.

We can then find R4 as follows:

          5.5V

R4 = ------- = 550 Ohms = NPV from the E12 range = 560 Ohms.

       10mA

 

Next if the Vce of the opto output transistor is 0.3V and Ic = 10mA then value of R3 can be found as:

 

      VR3      12V - 0.3V - 5.5V       6.2V

R3 = ----- = --------------------- = ------- = 620 Ohms = NPV from the E12 range = 680 Ohms.

     10mA             10mA             10mA

Finally a suitable value for R2 was arrived at as follows, the typical voltage drop across a red LED is about 1.8V and we need a small forward activation current @10mA.

 

       12v - 1.8v

R2 = ------------- = 1020 ohms = NPV from the E12 range = 1 KOhms.

         10mA

 

The complete circuit for the dual MOSFET driver is depicted below:

The design was then transferred onto stripboard ready for construction.

 

As Built

 

The circuit was then build up for testing, note heat sinks have been attached to the MOSFETS.

 

Testing

 

Finally on load testing was then carried out with the driver board and these are the results:

 

  Measured Calculated
Vsupply 12.2V 12V
Vcontrol 3.35V 3.3V
Vce 0.438 0.3
VR3 6.33V 6.2V
VR4 5.36V 5.5V
LOAD 3A 3A

 

 

Links:

Click here to download the associated project files:

- Dual MOSFET Switch - Stripboard - Rev A

- Dual MOSFET Switch - Circuit - Rev A