Gate drivers — the critical link in power-device performance: part 1

Effective MOSFET/IGBT-device switching depends on the gate driver and its power supply.

From power supplies and motor drives to charging stations and myriad other applications, switching power semiconductors such as silicon (Si), silicon carbide (SiC), and gallium nitride (GaN) MOSFETs, as well as insulated-gate bipolar transistors (IGBTs), are the key to efficient power-system designs. However, to achieve maximum performance from the power device, an appropriate gate driver is needed.

As its name indicates, this component’s role is to drive the power-device gate, quickly and crisply putting it into, or pulling it out of, conduction mode. Doing so requires that the driver can source or sink sufficient current despite internal device and stray (parasitic) capacitance, inductance, and other issues at the load (gate).

Consequently, providing a properly sized gate driver with suitable key attributes is crucial to realizing the full potential and efficiency of the power device. However, to get the most out of the gate driver, the designer must pay special attention to the driver’s DC power supply, which is independent of the power-device DC rail. T

This supply is similar to a conventional supply but has some important differences. It can be the more common unipolar supply, but in many cases, it is a non-symmetrical bipolar supply, along with other functional and structural differences.

Start with switching devices

Figure 1. In cutoff mode, the MOSFET drain-source path looks like an open switch. (Image: Quora)

Understanding the role and desired attributes of the gate-driver dc-dc converter begins with the switching devices. For a MOSFET as a switch device, the gate-source path is used to control the device’s on or off state (IGBTs are similar). When the gate-source voltage is less than the threshold voltage (V_GS < V_TH), the MOSFET is in its cut-off region, no drain current flows, I_D = 0 amperes (A), and the MOSFET appears as an “open switch” as seen in Figure 1.

Conversely, when the gate-source voltage is much greater than the threshold voltage (V_GS > V_TH), the MOSFET is in its saturation region, the maximum drain current flows (I_D = V_DD /R_L), and the MOSFET appears as a low resistance “closed switch” shown in Figure 2.

Figure 2. In saturation mode, the MOSFET drain-source path looks like a low-resistance switch. (Image: Quora)

For the ideal MOSFET, the drain-source voltage would be zero (V_DS = 0 volts), but in practice, V_DS is usually around 0.2 volts due to internal on-resistance R_DS(on), which is typically under 0.1 ohm (Ω) and can be as low as a few tens of milliohms.

While schematic diagrams make it appear that the voltage applied to the gate turns the MOSFET on and off, that is only part of the story. This voltage drives current into the MOSFET until there is enough accumulated charge to turn it on. Depending on the size (current rating) and type of switching drive, the amount of current required to quickly transition into a fully on state may be as little as a few milliamperes (mA) to several amperes (A).

Figure 3. This model of a MOSFET shows the parasitic capacitance and inductance, which adversely affect and challenge the driver performance. (Image: Texas Instruments)

The function of the gate driver is to drive sufficient current into the gate quickly and crisply to turn the MOSFET on, and to pull that current out in a reverse manner to turn the MOSFET off. More formally, the gate needs to be driven from a low-impedance source capable of sourcing and sinking sufficient current to provide for fast insertion and extraction of the controlling charge.

If the MOSFET gate looked like a purely resistive load, sourcing and sinking this current would be relatively simple. However, a MOSFET has internal capacitive and inductive parasitic elements, and there are also parasitics from the interconnects between the driver and the power device, as seen in Figure 3.

The result is ringing of the gate-drive signal around the threshold voltage, causing the device to turn on and off one or more times on its trajectory to being fully on or off; this is somewhat analogous to “switch bounce” of a mechanical switch shown in Figure 4.

Figure 4. Ringing of the driver output due to parasitics in the MOSFET load can cause ringing and false triggering, similar to mechanical-switch bounce. (Image source: Learn About Electronics)

The consequences range from unnoticed or merely annoying in a casual application, such as turning a light on or off, all the way to likely damage in the widely used pulse-width modulation (PWM) fast-switching circuits of power supplies, motor drives, and similar subsystems.

Figure 5. In contrast to the normal MOSFET turn on of Q1 and Q4 (left), or Q2 and Q3 (right), if Q1 and Q2 (or Q3 and Q4) of the bridge are turned on simultaneously due to driver issues or other causes, an unacceptable and possibly damaging short-circuit condition called shoot through will occur between the power rail and ground. (Image: Quora)

It can cause short circuits and even permanent damage in the standard half-bridge and full-bridge topologies, where the load is placed between an upper and lower MOSFET pair, if both MOSFETs on the same side of the bridge are turned on simultaneously, even for an instant. This phenomenon is known as “shoot-through,” as seen in Figure 5.

The next part looks into gate driver details and implications.