Hi today I will discuss how to parallel power MOSFETS.
What is affecting current sharing and best practices to optimize your design using an Xperia power mosfets.
Welcome to this quick learning video
I'Christian radicchi and I'm an application engineer working at nexperia in the automotive department.
There are two main types of applications that require mosfets to be paralleled load switch and switch mode.
Switch mode applications now also known as pwm applications include motor drive and switch mode power converters .
This is the topology that I will discuss today.
As you can see three mosfets are paralleled both at the low side and the high side of a half Bridge ,driving an inductive load.
In this case the current will share among the three devices.
Therefore are equal current sharing is the main target.
Current sharing can be influenced by both internal and external parameters to the mosfet.
Internal ones are related to certain data sheet parameters and their part-to-part variation also known as spread.
We individuated three parameters on state resistance RDS on,input charge qg,and the threshold voltage.
Now the spreads are unfortunately inevitable during the manufacturing process of silicon dye.
The lower the spread the more synchronized the operation of the mosfets.
Now let's start with the RDS on,RDS on has the lowest impact of all three and will affect mainly the conduction or static operation.
In this case mosfets behave as simple resistors therefore current will flow mainly in the lowest resistance one.
However the positive temperature coefficient of the resistance will help to balance out the current.
Input charge instead has more of an influence on current sharing mainly during switching.
However this can be counteracted effectively by splitting the gate resistance.
Between one smaller at the mosfet side and one larger and common to the three mosfets at the driver's side
by doing this the operation of the mosfet will be synchronized.
And they will share current evenly.
However threshold voltage is the most problematic one.
In this case M1 has a lower threshold voltage than the others and will turn on.
First and turn off last thus and Link more current and increasing its dissipated power.
Moreover the negative temperature coefficient of the threshold voltage will decrease even further the threshold voltage of M1.
However in Xperia's power mosfets we often observe more than three times
less spread in threshold voltage when considering batch to batch variation.
Now external factors to the mosfets are instead related to the PCB layout.
And in this case a tighter layout will help both the electrical and The Thermals.
Electrical by reducing the parasitic resistance and inductance and thermals to make so that mosfets can operate at a similar Junction temperature.
Moreover a single driver driving all the parallel mosfets is usually recommended to synchronize the operation of the mosfets.
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