To understand power MOSFET and their driver circuits it is useful to first know a little bit about how MOSFETs are constructed and operate. The power MOSFET, like other MOSFETs, is basically a voltage controlled device, that is the gate-source voltage controls the drain current.
Below picture shows the two full power MOSFET symbols that includes the parasitic diode which is an intrinsic part of the MOSFET’s structure. This diode is quite often not included in schematics, the basic MOSFET symbols being used instead.
Conduction between source and drain in an ordinary MOSFET takes place in a narrow channel region under the gate. The term lateral MOSFET is used to describe this structure of the standard low power MOSFET, as the current flows entirely through a horizontal plane.
The basic operation of the N-channel MOSFET is as follows. If we apply zero, low or negative gate-source voltage, the device is off because the N-P-N regions act as two back-to-back diodes. Only a very small leakage current can therefore flow from drain to source (or vice versa). Here, N and P refer to the type of chemical used to ‘dope’ pure silicon to create an interesting semiconductor behaviour.
N-type silicon has more electrons free to take part in conduction than in pure silicon. P-type has fewer electrons, but these gaps can he regarded as mobile ‘holes’ which act like positively charged versions of the electrons in the N region.
Thus both P and N-type silicon conduct to some extent. Placing an N region next to a P region creates a PN junction, also known as a diode junction, through which current will usually flow in only one direction.
If we apply a positive gate-source voltage the electrostatic attraction of this gate voltage will pull (negatively charged) electrons from the nearby silicon to the P-type region just under the gate. If sufficient electrons accumulate here there will eventually be an excess of electrons so the area just under the gate will behave as if it is N-type silicon.
At this point there will have been created an N-type channel connecting the N-type drain and source regions, thus we have an NN-N path from source to drain, rather than the N-P-N back-to-back diodes previously described. Conduction can now take place from source to drain. The transistor is on and the gate-source voltage at which this occurs is called the threshold voltage.
Power MOSFET Physical structure
The approach to the physical structure of the MOSFET device shown in before picture cannot readily be extended to produce high power devices – the cross-sectional area of the conducting region simply cannot be made big enough (to make the on-resistance, RDS on, small) without using an unreasonably large area of silicon.
Furthermore, the large gate area would make such a device very slow due to the high capacitance of a very large gate area. The structure of a basic power MOSFET is shown in below picture. The channel is still horizontal under the gate, but it is much shorter than in the conventional MOSFET, and the current flow between channel and drain is vertical.
The short channel means a low on resistance, a property required by power devices. The actual structures of real power MOSFETs are more complex than those shown in picture (and a variety of other structures, including ‘trenches’, are used).
The vertical nature of power MOSFETs means that they can readily be repeatedly wired in parallel connection to increase current handling capacity. Some power devices have over 20,000 parallel transistor cells. MOSFETs work happily in parallel because they do not suffer from current hogging and thermal runaway like bipolar transistors.
Power MOSFET Device types
The variety of device structures and parallel layout plans of power MOSFETs lead to a variety of commercial brand names such as DMOS, VMOS, TMOS, HEXFET, TrenchFET and PowerTrench. The power MOSFET market can probably be divided into the ‘heavy duty’ area dealing with very high voltages and currents, and the ‘high efficiency’ area at low voltages and moderate currents, where devices are typically targeted at applications such as the switch mode power supplies in portable systems like laptops.
For heavy duty use, MOSFETs capable of handling 1000V drain-source voltage or drain-source currents of over 150A are available. In terms of choosing a device to use, first understand that the various names given to power MOSFETs relate to each company’s promotion of their technology, and that all the devices are basically power MOSFETs.
Identify your key need high efficiency, high speed, high voltage, high current, etc, and then select a device optimised for this that meets all your other requirements in terms of voltages, currents, power and speed. Manufacturers’ web sites often have ‘product selection’ systems that allow you to input or set the specification you need; then you get a list of devices that match that. Once you have selected a likely device, have a good look at the datasheet, which will usually be available as a PDF download.
Power MOSFET drivers
Now we have covered the MOSFETs, let’s look at the drivers. The term MOSFET driver usually refers to switched control of the MOSFET, where it is switched between fully on and fully off, by switching the gate-source voltage between 0V and some voltage well above the threshold. Use of voltages well above threshold ensures saturated operation, in which the on-resistance (RDSon) voltage drop across the device, and power dissipation are minimised.
We can consider the device to be either in the off state, where little or no power is dissipated, or the on state, where power dissipation depends on RDSon and the drain source current. Of course, there are circuits, such as audio power amplifiers, in which MOSFETs are driven by a continuous gate voltage rather than switched.
Typically in these circuits the MOSFETs will be embedded in bias and feedback circuits rather than having a simple forward connection from driver to gate. Our discussion of drivers here is limited to switching circuits. In order for power MOSFETs to switch quickly and efficiently, sufficient current must be available to quickly charge or discharge the gate capacitance of the device.
The driver circuit’s source resistance and the resistance of the wiring both inside and outside the device cause the gate voltage to follow an RC charging curve, so the MOSFET will spend some time in between being fully on and fully off. During this time the device may dissipate a lot of power, a problem referred to as switching losses. The drive circuit therefore must be able to supply enough transient current to charge the gate capacitance at the required rate.
In some cases this current may be quite substantial, particularly for large very high power devices, or where paralleled MOSFETs are being used. The effective capacitive of the MOSFET gate and hence the drive current required is increased by the Miller effect. The Miller effect occurs when a capacitor is connected to produce negative feedback in an amplifier – the gate-drain capacitance in this case.
The capacitance is multiplied by a factor related to the amplifier gain to get effective capacitance. The dynamic capacitance of a power MOSFET gate during switching is complex and can be difficult to analyse. Basically, all this means is that driving the gate is probably harder than it first looks, hence the need for good driver circuits.
Power MOSFET Source current
Many low-power circuit outputs, such as those of logic gates and microcontrollers simply cannot deliver enough current to drive the gate of a power MOSFET correctly. A power MOSFET driver is therefore a power amplifier that accepts a low-power input from a microcontroller (e.g. PIC) or other circuit and delivers the required highcurrent gate drive to the MOSFET.
Gate drivers may be implemented as dedicated ICs, discrete transistors, or transformers. The circuits can be quite complex, particularly for high-side drivers (see later) and bridges, so the use of dedicated ICs can save a lot of effort. At first, the complexity of drivers may seem unnecessary, but seemingly small imperfections in the control of devices switching very large currents or high voltages can have significant consequences.
Power MOSFET threshold voltages are typically 4V, but in order to fully turn on many of these devices for use at their full current rating, drive voltages of 10V or more may be needed. In some cases the driver circuit will translate the voltage levels in the control circuit (3V logic) to those required by the gate (10V) as well providing the high current drive – level shifters.
As well as being too slow, it is also possible for power MOSFET circuits to switch too fast, or put more accurately, for voltage or currents within the circuit to change too fast. Very fast current and voltage changes can damage devices and also cause more interference radiation than slower switching. Careful design of the driver circuit may be required to get the switching behaviour correct, particularly in high speed and very high power applications.