Are specialized gate drivers required to provide positive and negative voltages?

If a particular power device requires both positive and negative gate drive, the circuit designer does not need to look for a special gate driver that can operate in bipolar mode. Using a simple trick, a unipolar gate driver can be made to provide bipolar voltages!

When driving medium/high power MOSFETs and IGBTs, there is a risk of Miller effect turn-on as soon as the voltage change rate on the power device is high. Current is injected into the gate of the power device through the gate-drain capacitance or the gate-collector capacitance. If the current injection is large enough to make the gate voltage higher than the threshold voltage of the device, parasitic turn-on effects can be observed, resulting in reduced efficiency or even device failure.

Miller effect can be mitigated by using an ultra-low impedance path from the power device gate to the source or drain, or by providing a negative drive voltage to the gate relative to the source or drain. The goal of Miller effect turn-on mitigation techniques is to keep the gate voltage below the desired threshold when the current through the Miller capacitance reaches a peak.

Some power device types even require negative voltages to be fully turned off, which necessitates a negative voltage drive from the gate driver. Devices that device manufacturers recommend using negative gate drive voltages include standard silicon-based MOSFETs, IGBTs, SiC, and GaN devices.

There are a wide variety of isolated gate drivers that can operate from a unipolar supply on the secondary side (power device driving side), but there are far fewer gate driver devices that allow explicit bipolar voltage drive. One way to overcome the lack of negative gate drive devices is to offset the gate driver device relative to the power device, thereby forming a negative gate drive voltage relative to the source or drain of the power device, while the gate driver IC still only sees a unipolar supply. Examples of unipolar and bipolar gate drive waveforms are shown in Figure 1.

A schematic with ideal voltage sources is shown in Figure 2. In this example, the driver IC is powered by a voltage equal to the sum of V1 and V2, while the gate drive voltage to the MOSFET is +V1 in the on state and –V2 in the off state (relative to the source node of the MOSFET). Note that in this example, both voltage sources have been decoupled using separate capacitors. The effective decoupling of the gate driver IC is the series combination of the capacitors, which is less than the capacitance of each individual capacitor. Additional decoupling can be added between VDD and GND if desired, but the most important thing is to keep C1 and C2 as capacitors that provide a low impedance path for the gate current, independent of each other during turn-on and turn-off.

Isolated gate driver ICs usually have undervoltage lockout (UVLO) to prevent the power device from being weakly driven when the gate voltage of the gate driver is too low. As shown in Figure 2, when driving a unipolar gate driver, attention must be paid to the expected operation of the UVLO because the UVLO is usually referenced to the ground of the gate driver. Consider the case where V1 = 15 V, V2 = 9 V, and the gate driver UVLO is about 11 V, which is a common IGBT operating condition. If V1 drops more than 4 V, the UVLO will not be triggered, but the IGBT will be driven below 11 V during the on period, so the IGBT is under-driven.

To address this issue, two isolated power supplies can be used to create two separate voltage sources, but the cost of this approach is often a concern. If a flyback topology is used, multiple winding taps can be used, making it relatively easy to obtain multiple voltages.

There are isolated power modules that can provide isolated power, and some manufacturers are selecting voltages that are suitable for power devices. One example is RECOM, whose device product line for IGBTs generates isolated power rails of +15 V and -9 V.

For such a large voltage swing, the gate driver must be able to withstand a wider voltage range than other devices. ADI's ADuM4135 and ADuM4136 IGBT gate drivers with iCoupler® technology can operate normally in this voltage range, and their recommended voltage range can be up to 30 V. Both devices provide a dedicated ground pin on the output side, allowing the driver's UVLO to be referenced to the positive supply rail. The ADuM4135 also includes an integrated Miller clamp to further suppress the gate voltage abrupt change caused by the Miller effect.

A simple method of generating a bipolar supply from a single voltage source is to use a biased Zener diode to generate a second voltage source. Although the gate driver provides high current during the turn-on and turn-off of the power device, the actual average current required from the power supply is relatively low, typically in the tens of milliamps range for most applications.

Using a Zener diode allows both positive and negative voltages to be regulated, and the choice can be made based on which voltage rail requires greater accuracy. The example setup shown in Figure 3 regulates the positive voltage, but not the negative voltage. One possible reason for requiring positive voltage regulation is if the gate driver has tight tolerance requirements on the gate voltage (such as is the case with some GaN devices). Regulating the positive supply has the added advantage of allowing the gate driver’s UVLO to work as intended, as any fluctuations in V3 are attenuated by the Zener diode until V3 is too low to provide the Zener diode’s operating voltage.

Using a Zener diode to generate two supplies from one also offers the advantage of saving layout space. Not only does the Zener diode and resistor effectively replace the entire isolated voltage source, but by using a unipolar isolated gate driver, a six-pin device (such as ADI’s ADuM4120 with iCoupler technology) can be used, saving even more space in the isolated creepage area near the gate driver IC.

A reference example of a Zener diode bipolar setup created using ADI’s ADuM4121 and GaN Systems’ GS66508T to create a half bridge. This example is designed to generate a +5 V and –4 V drive referenced to the source of the device. This example can easily be modified to +6 V and -3 V drive using a different Zener diode and the same 9 V isolated supply. The large dead time is used to visually distinguish the Miller jump from other turn-off transients, but in practice, the ADuM4121 can achieve much shorter dead times (in the tens of nanoseconds range), which is an important metric for efficient GaN designs.

Creating a negative gate voltage driver that can mitigate Miller effect parasitic turn-on doesn’t have to be complicated. Many existing gate drivers that work unipolarly can easily drive negative gate voltages with minimal external circuitry. There are some additional considerations (such as the effective UVLO voltage) that need to be taken into account, but the advantages of this approach are greater. Do you get it?