3-SERIES CONNECTION OF POWER MOSFETS

3-Series Connection of Power MOSFETS

Table of Content

INTRODUCTION3

PARAMETERS AND DIALOG BOX5

INPUTS AND OUTPUTS6

ASSUMPTIONS AND LIMITATIONS7

Example7

SOURCE/DRAIN ON INSULATOR STRUCTURE AND FABRICATION12

MOSFET AND POWER ELECTRONICS15

DEVICE CHARACTERISTICS AND DISCUSSIONS18

FINGERED DEVICES24

THRESHOLD VOLTAGE24

VOLUME CONDUCTION25

DEVICE SCALABILITY26

MOBILITY DEGRADATION29

VELOCITY OVERSHOOT32

CIRCUIT DESIGN OPPORTUNITIES37

CONCLUSION38

REFERENCES41

APPENDIX44

Figure 1: Contribution of the different parts of the MOSFET to the on-state resistance.44

Figure 2: Cross section of a VDMOS, showing an elementary cell. Note that a cell is very small (some micrometres to some tens of micrometres wide), and that a power MOSFET is composed of several thousand of them.44

Figure 3: Location of the intrinsic capacitances of a power MOSFET.45

Figure 4: Simple Linear Regulator46

3-Series Connection of Power MOSFETS

Introduction

The Metal-Oxide-Semiconductor-Field-Effect-Transistor (Mosfet) block is a semiconductor device controllable by the gate signal (G > 0) if its current Id is positive (Id>0). The Mosfet block is connected in parallel with an internal diode which turns on when the Mosfet block is reverse biased (Vds < 0). The model is simulated as a series combination of a variable resistor (Rt) and inductor (Lon) in series with a switch controlled by a logical signal (G>0 or G=0).

The Mosfet block turns on when the drain-source voltage is positive and a positive signal is applied at the gate input (G >0).

With a positive current flowing through the device, the Mosfet block turns off when the gate input becomes zero. If the current Id is negative (Id flowing in the internal diode) and the gate signal is zero (G = 0), the Mosfet block turns off when the current Id becomes zero (Id = 0).

Note that the on-state resistance Rt depends on the drain current direction:

Rt= Ron if Id > 0, where Ron represents the typical value of the forward conducting resistance of the Mosfet block

Rt= Rd if Id < 0, where Rd represents the internal diode resistance

The Mosfet block also contains a series Rs-Cs snubber circuit, which is usually connected in parallel with the Mosfet block. You can specify a snubber that is purely resistive (Cs = Inf) or purely capacitive (Rs=0). If you specify either Rs=Inf or Cs=0, the snubber is eliminated and it disappears on the Mosfet icon

The initial current Ic flowing in the Mosfet block is usually set to zero, so that the simulation is started with Mosfet blocked. However, you may specify an Ic value corresponding to a particular state of the circuit. In such a case, all states of the linear circuit must be set accordingly. Initializing all states of a power-electronic converter is a complex task. Therefore, this option is useful only with simple circuits.

Parameters and Dialog Box

Inputs and Outputs

The first input and output are the Mosfet connection to drain (d) and source (s). The second input (g) is a logical Simulink signal applied to the gate. The second output is a Simulink measurement vector [Id, Vds] returning the Mosfet current and voltage.

Assumptions and Limitations

The Mosfet block implements a macro-model of the real Mosfet device. It does not take into account either the geometry of the device or the ...