A transistor (Field Effect Transistor (FET), in modern Integrated Circuits) neve
ID: 1715036 • Letter: A
Question
A transistor (Field Effect Transistor (FET), in modern Integrated Circuits) never switches instantly from full OFF to full ON. There is a period while it’s turning on or off where the FET acts like a resistor (even when fully ON it still has a resistance).
As you know, passing a current through a resistor generates heat.
The more the transistors switch the more time they spend in that resistive state, so the more heat they generate. So the amount of heat generated can be directly proportional to the number of transistors - but it is also dependent on which transistors are doing what and when, and that depends on what the chip is being instructed to do.
If T(n) represents the temperature of a integrated circuit with n (active) transistors.
What does the limit
lim h0 (T (5000 + h) T (5000))/(h)
represent?, can we change 5000 for any other number?.
Explanation / Answer
When used in switch mode, the resistance drain to source when the device is turned all the way on. This number is lower, the less power will lose across the device as heat. The voltage across the device will be the current times resistance of drain to source, and the power dissipated in heat will be this voltage times the current through the device.
Note that source –to-drain resistor in the transistor does not behave like a normal resistor. There is also small capacitance between the gate and the source. This capacitance is in parallel with the detector capacitance and it should be added to it.
Turning-on or off the MOSFET involves charging or discharging the capacitor. When the voltage across a capacitor is changing, a certain amount of charge has to be transferred. The amount of charge required to change the gate voltage between 0V and the actual gate drive voltage is characterized by the typical gate charge vs. gate-to-source voltage.
In normal operation when turn-off dv/dt is forced across the drain-to-source terminals of the power switch while it is off. This situation is more common than one may originally anticipate. Since these dv/dt’s are significantly higher than during power up and VTH is usually lower due to the higher operating junction temperature, protection must be provided by the low output impedance of the gate drive circuit.
The first task is to determine the maximum dv/dt which can occur under worst case conditions.
The next step in evaluating the suitability of a particular device to the application is to calculate its natural dv/dt limit, imposed by the internal gate resistance and the CGD capacitance of the MOSFET.
Once the maximum pull down resistor value is given, the gate drive design can be executed. It should be taken into account that the driver’s pull down impedance is also temperature dependent. At elevated junction temperature the MOSFET based gate drive ICs exhibit higher output resistance than at 25°C where they are usually characterized. Turn-off speed enhancement circuits can also be used to meet dv/dt immunity for the MOSFET since they can shunt out at turn-off and during the off state of the device.
They also have to take into account power distribution within the chip, so they have to come to a compromise.
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