The MOSFET (metal-oxide semiconductor field-effect transistor) is widely used as an on/off power switch, handling currents from milliamps to hundreds of amps, and single-digit voltages to thousands of volts. While there are small-signal linear MOSFETS which are used very successfully as low-noise front ends for low-level RF signals, and power MOSFETs used as linear power amplifiers (PA) in wireless systems, we'll focus on MOSFET circuits intended for on/off switching.
[A brief note about the IGBT (insulated gate bipolar transistor), a "cousin" of the MOSFET. It is often the preferred choice versus the MOSFET for switching high currents/voltages, especially at lower frequencies. While many of the MOSFET drive issues are similar for IGBTs and some are very different, we'll concentrate on the MOSFET.]
MOSFET Basics and Key Parameters
To understand the MOSFET driver, you need to first understand the MOSFET itself. The MOSFET is a near-ideal switching device, with just three terminals: source, drain, and gate. The gate is insulated by SiO2 from the current-flow channel.
The MOSFET is used in three speed regions:
- Basic load switching, such as a power rail (slow switching speed, tens of Hz)
- Motor control, often in a half-bridge or full H-bridge configuration (medium switching speed, Hz to 10/100 kHz)
- Switching power supplies (highest switching speed, tens of kHz to MHz)
There are two basic types of MOSFETs, P-channel and N-channel. In the N-channel device, when the voltage on the gate is above gate threshold voltage, electrons from the source pass through the device, and exit at the drain; a “positive” gate voltage widens channel. An appropriate signal applied to gate turns the MOSFET on, resulting in low drain-source resistance, on the order of milliohms. When the gate signal is removed, the MOSFET turns off, and has high drain-source resistance. In the P-channel MOSFET, a more negative voltage widens channel. The two versions are complementary mirror images in many ways – but not exactly so for some key parameters – and are often used as “push-pull” pairs.
There is a lot of confusion regarding the MOSFET schematic symbol. There are many specifically defined MOSFET symbols, depending on specific MOSFET type and construction (there are many sub-varieties). Often, however, designers just use a generic MPOSFET symbol due to sloppiness or the fact that the internal fabrication details are not relevant on the schematic. Figure 1 shows the most commonly used symbols.
To switch a load using the simplest arrangement, the MOSFET is connected between the load and ground, Figure 2. Therefore, the MOSFET is grounded while the load is not. This is a non-issue in some applications, but a major problem in many others.
Driving the MOSFET
The MOSFET driver is the electrical interface between a low-voltage, low-current signal source (such as from a microcontroller) which wants to control the MOSFET (turn it on and off) and the higher voltage/current of the MOSFET itself, Figure 3. Like all electronic components, MOSFETs have several top-tier parameters, numerous second-tier ones, and an even-larger number of third-tier factors.
In addition to the basic current and voltage rating of the MOSFET which determines which ones can even be considered in the application, two important MOSFET parameters are its on-resistance RDS(ON), and gate-source capacitance CGS. On-resistance determines the ohmic loss through the MOSFET when it is turned on, while the capacitance determines many of the characteristics of the driver which must turn the MOSFET on and off.
For low-power, low-voltage MOSFETs, the driver is usually a fairly simple circuit and only needs modest current-drive capability. However, for MOSFETs which are controlling higher power, on the order of half to one amp or more, and more than several volts, the driver is a critical and challenging device. It must provide both the needed magnitude voltage and the current to charge or discharge the input capacitance, and do so fast enough (slew rate). Mathematically, the current is equal to the gate charge Qg divided by switching time. Therefore, the driver must, at its core, be a well-behaved current source, with enough drive voltage and a fast slew rate while sourcing/sinking the current to or from a capacitive load.
How fast is "fast enough" depends on the application. During turn on and turn off transition period, the MOSFET looks like a resistor and so dissipates power. For MOSFETs, which are being used to turn a power rail on and off infrequently, such as for idling/activating a subsystem, the turn on/off time can be relatively long, since this action only occurs occasionally.
However, for MOSFETs used for motor control or switching power supplies, where the turn-on/off rate can be in the hundreds of kHz and even approach MHz rates for some switchers, the turn on/off time must be very fast. This means the driver must be able to source/sink considerable current at a fast slew rate with respect to the MOSFET’s stray and internal capacitance.
In general, the driver must be able to drive the gate-source voltage Vgs to at least 10 V (typically) to be above turn-on threshold. The driver must also be designed to not oscillate as a result of parasitic inductance, which would cause false turn-on, so a series resistor is often added to damp any potential oscillations.
Isolation and Driving MOSFET Bridges
In the simple MOSFET and load of Figure 2, the topology was simple and the MOSFET was grounded. This is called a low-side switch, where the MOSFET sinks current from the load. However, for many applications, low-side switching is not acceptable, as the load must be grounded either for safety or mechanical-mounting reasons. For example, the many small motors in a car operate from the 12-V supply, and the motor frames or enclosures are connected to ground (the car chassis).
For these situations, the MOSFET must be used in a high-side switch topology, with the MOSFET between the power rail and the load while the load is grounded; thus, the MOSFET sources current to the load rather than sinking it from the load. That sounds straightforward, but has a significant consequence, as the MOSFET and its driver can no longer have any ground connection.
The solution is to employ electrical isolation between the low-voltage control signal from the processor and the MOSFET driver/MOSFET pair. By doing so, the driver/MOSFET pair is "floating" with respect to ground. This galvanic isolation (meaning there is no ohmic path between input and output) can be achieved in two ways: by using a transformer to couple the control signal to the driver, or with an optoisolator (optocoupler), Figure 4. Both components can provide isolation and are used successfully in isolated driver designs, but each has different attributes. In general, a transformer is more costly, larger, and rugged, while the lower-cost, easier-to-use optoisolator has some longer-term reliability issues. Of course, there are many other considerations when making the choice between the two.
The isolated driver problem also appears in the standard "H-bridge" configuration, Figure 5, which is frequently used to control a motor and its direction. By switching on/off the crisscrossed pairs of MOSFETs, the direction of the current to the motor can be reversed across the windings and so the motor's direction can be reversed, even when operating from a unipolar supply.
In the H-bridge, the two upper MOSEFTs are not grounded, and so must have isolated drivers. Even in a simplified version of the H-bridge called the half-bridge, which is suitable for some motor designs, there is only a single low-side/high-side MOSFET pair, but it still requires an isolated driver for the high-side MOSFET.
Careful control of the MOSFET turn on/off timing via the drivers is critical in either bridge. There can be brief periods during the turn on/off time of the pairs when the turn-off of one MOSFET takes longer than the turn-on of the other MOSFET. For that instant, both MOSFETs on the left (or right) side will be on, resulting in a direct short circuit between the power rail and ground. This can destroy the MOSFETs due to excessive current flow or damage the power souce. Therefore, the designer choosing the MOSFET drivers and MOSFETs must look at worst-case and best-case turn on/off scenarios, to ensure that this will not happen.
There is another important reason to use isolation, which is why it is often used even when the driver does not have to float with no connection to ground. MOSFETs often handle high voltages and currents; a failure of the driver or MOSFET could place these currents and voltages directly on the electronic circuitry which is controlling the driver and likely destroy it. Further,the driver or MOSFET failure may also result in a shock hazard to users (depending on the application and country, source voltages above 48 to 60 V are considered potentially dangerous). By using isolation, any failure on the driver/MOSEFT side cannot ripple-through back to the electronics.
The choice of a MOSFET as a switch to source and sink current via high-side and low-side topologies involves many load-related parameters. However, it also requires careful consideration of the MOSFET driver, which is the critical interface between the safer low-voltage/current world of the control processor and the harsh, high-voltage/curent domain of the MOSFET itself. Matching the MOSFET driver to the MOSFET and applcation requires careful study of current sourcing/sinking, transients, MOSFET capacitance, isolation, and driver slew rates, plus other factors.