ADI In-depth: Demystifying Isolated Gate Drivers

The structure of an IGBT/power MOSFET is such that the gate forms a nonlinear capacitor. Charging the gate capacitance turns the power device on and allows current to flow between its drain and source pins, while discharging turns the device off and large voltages can be blocked across the drain and source pins. The minimum voltage at which the gate capacitance is charged and the device can just turn on is the threshold voltage (V TH ). To use an IGBT/power MOSFET as a switch, a voltage sufficiently greater than V TH should be applied between the gate and source/emitter pins .

Consider a digital logic system with a microcontroller that can output a 0 V to 5 V PWM signal on one of its I/O pins. This PWM will not be sufficient to fully turn on the power devices used in the power system because their overdrive voltages typically exceed the standard CMOS/TTL logic voltages. Therefore, an interface is required between the logic/control circuitry and the high-power device. This can be achieved by driving a logic-level n-channel MOSFET, which in turn drives a power MOSFET, as shown in Figure 1a.

Figure 1. Driving a power MOSFET with inverting logic.

As shown in Figure 1a, when IO ₁ sends a low signal, V GSQ ₁ < V THQ ₁ , so MOSFET Q ₁ remains off. As a result, a positive voltage is applied to the gate of power MOSFET Q _2. The gate capacitance (C GQ2 ) of Q ₂ is charged through the pull-up resistor R 1 , and the gate voltage is pulled to the rail voltage of VDD. If V _DD > V THQ ₂ , Q ₂ is turned on and can conduct current. When IO ₁ outputs a high level, Q ₁ is turned on and C GQ ₂ is discharged through Q _1. V DSQ ₁ ~ 0 V, making V GSQ ₂ < V THQ2 , so Q ₂ is turned off. One problem with this setup is the power dissipation of R ₁ when Q ₁ is on .

To solve this problem, pMOSFET Q ₃ can be used as a pull-up device, which works in a complementary manner to Q _{1 , as shown in Figure 1b. PMOS has a low on-resistance and a very high off-resistance, which greatly reduces the power consumption in the drive circuit. To control the edge rate during the gate transition, a small resistor is added between the drain of Q 1} and the gate of Q _2. Another advantage of using MOSFETs is that they are easy to fabricate on a die, while resistors are relatively difficult to fabricate. This unique interface for driving the gate of a power switch can be created in the form of a monolithic IC that accepts a logic level voltage and produces a higher power output. This gate driver IC will almost always have other internal circuitry to achieve more functions, but it is mainly used as a power amplifier and level shifter.

Drive Strength

The problem of providing the proper gate voltage is solved by the gate driver, which performs the level shifting task. However, the gate capacitor cannot change its voltage instantaneously. Therefore, the power FET or IGBT has a non-zero finite switching interval. During switching, the device may be in a high current and high voltage state, which generates power dissipation and converts into heat. Therefore, the transition from one state to another needs to be fast to minimize the switching time. To achieve this, high transient currents are required to quickly charge and discharge the gate capacitance.

Figure 2. MOSFET conduction transition without gate driver

Drivers that can source/sink higher gate currents for longer periods of time will have faster switching times and, therefore, lower switching power dissipation in the transistors they drive.

Microcontroller I/O pins are typically rated for sourcing and sinking currents of tens of milliamps, while gate drivers can provide much higher currents. In Figure 2, a long switching interval is observed when the power MOSFET is driven by the microcontroller I/O pin at its maximum rated source current. As shown in Figure 3, the transition time is greatly reduced when the ADuM4121 isolated gate driver is used; the driver is able to provide much higher drive current than the microcontroller I/O pin when driving the same power MOSFET.

Figure 3. MOSFET conduction transition with gate driver

In many cases, driving a larger power MOSFET/IGBT directly with a microcontroller may cause the controller to overheat and be damaged because the digital circuit may overdraw the current. Gate drivers have higher drive capabilities and support fast switching, with rise and fall times of only a few nanoseconds. This can reduce switching power losses and improve system efficiency. Therefore, drive current is often considered an important indicator for selecting gate drivers.

Corresponding to the drive current rating is the gate driver's drain-source on-resistance (RDS(ON)). Ideally, the RDS(ON) value of a MOSFET when fully on should be zero, but due to its physical construction, it is typically in the range of a few ohms. This takes into account the total series resistance in the current path from drain to source.

RDS(ON) is the true basis for the maximum drive strength rating of the gate driver, as it limits the gate current that the driver can provide. The RDS(ON) of the internal switch determines the sink and source current, but the external series resistor is used to reduce the drive current and therefore affects the edge rate. As shown in Figure 4, the high-side on-resistance and the external series resistor EXT form the gate resistance in the charging path, and the low-side on-resistance and REXT form the gate resistance in the discharge path.

Figure 4. RC circuit model of a gate driver with a MOSFET output stage and the power device acting as a capacitor

RDS(ON) also directly affects the power dissipation inside the driver. For a specific drive current, the lower the RDS(ON) value, the higher the REXT value that can be used. The power dissipation is distributed across REXT and RDS(ON), so a higher REXT value means more power dissipation outside the driver. Therefore, for a given chip area and size of the IC, the lower the RDS(ON) value, the better, in order to improve system efficiency and relax thermal regulation requirements within the driver.

Timing

Gate driver timing parameters are critical to evaluating their performance. A common timing specification for all gate drivers, including the ADuM4120, as shown in Figure 5, is the driver’s propagation delay (tD), which is defined as the time required for an input edge to propagate to the output. As shown in Figure 5, the rising propagation delay (tDHL) can be defined as the time from the input edge rising above the input high threshold (VIH) to the time the output rises to more than 10% of its final value. Similarly, the falling propagation delay (tDHL) can be stated as the time from the input edge falling below the input low threshold VIL to the time the output falls below 90% of its high level. The propagation delay of the output transition can be different for rising and falling edges.

Figure 5. ADuM4120 gate driver and timing waveforms

Figure 5 also shows the rise and fall times of the signal. These edge rates are affected by the drive current that the device can provide, but they also depend on the load being driven, which is not accounted for in the propagation delay calculation. Another timing parameter is pulse width distortion, which is the difference between the rising and falling propagation delays of the same device. Therefore, pulse width distortion (PWD) = |tDLH – tDHL|.

Due to transistor mismatches within different devices, the propagation delays of two devices will not be exactly the same. This results in propagation delay skew (tSKEW), which is defined as the time difference between the output transitions of two different devices in response to the same input under the same operating conditions. As shown in Figure 5, propagation delay skew is defined as device-to-device skew. For devices with multiple output channels, this specification is stated the same way but is referred to as channel-to-channel skew. Propagation delay skew cannot usually be compensated in the control circuitry.

Figure 6 shows a typical setup of the ADuM4121 gate driver, used in conjunction with power MOSFETs in a half-bridge configuration for power and motor drive applications. In this setup, if Q1 and Q2 are turned on at the same time, there is a chance that shoot-through will occur due to the shorting of the power and ground pins. This can permanently damage the switches and even the drive circuitry. To avoid shoot-through, a dead time must be inserted into the system to greatly reduce the probability of both switches being turned on at the same time. During the dead time interval, the gate signals of both switches are low, so ideally the switches are in the off state. If the propagation delay skew is low, the required dead time is shorter and the control becomes more predictable. The lower the skew and the shorter the dead time, the smoother and more efficient the system operation.

Timing characteristics are important because they affect the speed at which the power switches can operate. Understanding these parameters can make control circuit design simpler and more accurate.

isolation

Isolation refers to the electrical separation between the various functional circuits in a system so that there is no direct conduction path between them. This allows different circuits to have different ground potentials. Signals and/or power can still pass between isolated circuits using inductive, capacitive or optical methods. For systems using gate drivers, isolation may be necessary to perform the function and may also be a safety requirement. In Figure 6, VBUS may be hundreds of volts and tens of amps may be flowing through Q1 or Q2 at a given time. In the event of a fault in this system, if the damage is limited to the electronic components, safety isolation may not be necessary, but if human activity is involved on the control side, then galvanic isolation is required between the high power side and the low voltage control circuit. It protects against any faults on the high voltage side because the isolation barrier will prevent power from reaching the user despite component damage or failure.

Figure 6. Isolation barrier in a half-bridge setup using the ADuM4121 isolated gate driver.

Isolation is a mandatory requirement for regulatory agencies and safety certification bodies to prevent electric shock hazards. It also protects low-voltage electronics from any damage caused by faults on the high-power side. There are many ways to describe safety isolation, but at a basic level, they all relate to the breakdown voltage of the isolation barrier. This voltage rating is generally given for the lifetime of the driver and voltage transients during specific periods and situations. These voltage levels are also related to the physical size of the driver IC and the minimum distance between pins on the isolation barrier.

In addition to safety reasons, isolation may also be essential for proper system operation. Figure 6 shows a half-bridge topology commonly used in motor drive circuits, where only one switch is on at a given time. On the high-power side, the source of the low-side transistor Q2 is grounded. The gate-source voltage of Q2 (VGSQ2) is therefore directly referenced to ground, and the design of the drive circuit is relatively simple. The situation is different for the high-side transistor Q1, as its source is the switch node, which will be pulled to the bus voltage or ground, depending on which switch is on. To turn Q1 on, a positive gate-source voltage (VGSQ1) that exceeds its threshold voltage should be applied. Therefore, when the source is connected to VBUS, Q1 will have a gate voltage higher than VBUS when it is on. If the drive circuit does not have isolation for ground reference, a voltage greater than VBUS will be required to drive Q1. This is a cumbersome solution and is not practical for an efficient system. Therefore, one needs a control signal that is level-shifted and referenced to the source of the high-side transistor. This is called functional isolation and can be achieved using isolated gate drivers such as the ADuM4223.

Noise Immunity

Gate drivers are used in industrial environments with a large number of noise sources. Noise can corrupt data, make the system unreliable, and cause performance degradation. Therefore, gate drivers must have good noise immunity to ensure data integrity. Noise immunity is related to how well the driver rejects electromagnetic interference (EMI) or RF noise and common-mode transients.

EMI is any electrical noise or magnetic interference that disrupts the intended operation of an electronic device. EMI, which affects gate drivers, is a result of high-frequency switching circuits, primarily caused by the magnetic fields of large industrial motors. EMI can be radiated or conducted, and can couple into other nearby circuits. Therefore, EMI or RF immunity is a measure of the gate driver's ability to reject electromagnetic interference and maintain robust operation without errors. High immunity allows the drive to be used near large motors without causing data transmission failures.

As shown in Figure 6, the isolation barrier is expected to provide high voltage isolation between ground points at different potentials. However, high frequency switching results in short edges for voltage transitions on the secondary side. These fast transients are coupled from one side to the other due to parasitic capacitance across the isolation boundary, which can lead to data corruption. This can manifest itself in the form of jitter introduced into the gate drive signal, or inverting the signal completely, leading to inefficiencies or even shoot-through in some cases. Therefore, a decisive specification for gate drivers is common-mode transient immunity (CMTI), which quantifies the ability of an isolated gate driver to reject large common-mode transients between input and output. If the slew rate in the system is high, the driver needs to have high immunity. Therefore, the CMTI number is particularly important when operating at high frequencies and large bus voltages.

This article is intended to be a brief introduction to gate drivers, so the parameters discussed so far do not fully reflect the characteristics of isolated gate drivers. Drivers have other specifications such as supply voltage, allowable temperature, pinout, etc., which are common considerations for every electronic device. Some drivers, such as ADuM4135 and ADuM4136, also include protection features or advanced detection or control mechanisms. There are many types of isolated gate drivers on the market, and system designers must understand all of these specifications and characteristics in order to make an informed decision on the appropriate driver to use in the relevant application.