20.2.1 Current-driven Devices

A current-driven device is a device that requires a constant current drive for a period of time in order to initiate and/or remain in conduction. Two popular types of current controlled devices are the bipolar power transistor and the thyristor (SCRs). The SCR is mainly used in AC–DC converters such as controlled rectifiers where the input AC voltage helps to commutate (turn-off) the devices by polarity reversal. In DC–AC inverters, the gate turn-off thyristors (GTOs) are generally used due to their ability to be both turned on and off by a gate control signal. This eliminates the need for forced commutation circuitry, needed to turn-off the thyristor in these applications due to the absence of a reversing polarity AC input voltage. Thyristors and GTOs still dominate the high voltage and high current applications, like DC transmission, requiring converters up to the megawatt range. The evolution of the IGBT power semiconductor device has resulted in thyristors and GTOs being used less in conventional power converters these days. IGBTs are readily available with on-state currents of several hundred amperes and blocking voltages in excess of 1.7 kV. The IBGTs offer much faster switching speeds than thyristors and have much lower gate drive power requirements than SCRs or power bipolar junction transistors (BJT). The IGBTs have already replaced power BJTs in most applications due to their superior performance. The next generation of silicon carbide technology could realize IGBT devices with blocking voltages in excess of 4 kV and may one day even replace SCR and GTO devices completely.

20.2.2 Voltage-controlled Devices

These devices are semiconductors which require a constant voltage drive on the gate control terminal in order to remain in conduction. The input drive requirements of these devices are substantially lower than their current-driven counterparts and are the preferred choice in modern power electronics. Two such devices are the power MOSFET and the IGBT which are forced commutated switching devices being fully controlled at the gate terminal under normal operating conditions. These devices do not latch into conduction, and therefore do not require special commutation circuits. The gate input junction of a MOSFET and IGBT is purely capacitive, so, no gate drive current is needed in the steady state, unlike transistors. A minimum gate drive voltage however must be maintained (above the gate threshold voltage) at the device gate in order for it to remain in conduction. A high current low impedance drive circuit is needed to inject, or remove current, from the gate at high slew rates in order to switch the device rapidly. The gate drain capacitance, although small, can also require significant charge as at high drain voltage slew rates (the Miller effect) [1].

The process of supplying the necessary power for the efficient driving of voltage-controlled devices is an ongoing area of research. This chapter will however only focus on the gate driver circuitry required to drive power MOSFETs and IGBTs in the bridge circuit configuration. The basic symbolic representation of the power MOSFET and IGBT is shown below. Figure 20.2 represents the static model of these devices. Unlike the power MOSFET, the IGBT can be manufactured without the integral body diode. Often power MOSFETs are represented as shown below, omitting the integral body diode which is always present in the MOSFET device. Care should be taken when reading circuits, not to forget this extra unseen component.

FIGURE 20.2 Static model representation of the power MOSFET and IGBT power semiconductor devices.

The dynamic model for a power MOSFET is shown in Fig. 20.3 [1, 2]. It can be noted that the integral body diode situated in the power MOSFET structure is actually a parasitic bipolar transistor. This parasitic element is the result of a fabrication process used when manufacturing power MOSFETs. The base of the BJT is usually floating, and a pull down resistor Rbe is included in the chip structure in order to keep the BJT base shorted to the MOSFET source terminal at all times. This is very necessary, since a floating BJT base will result in uncontrolled turn-on of the BJT device. By shorting the BJT base to the MOSFET's source terminal, a diode structure is now created, yielding the dynamic model with the integral body diode formed by the BJT base collector junction as shown in Fig. 20.3 [1, 2]. The turn-on of the BJT element during transients substantially degrades the performance of the power MOSFET. When the MOSFET is exposed to a fast changing transient voltage (dv/dt) across its drain to source terminals, enough current flows in Rbe, thereby biasing the parasitic bipolar into conduction. This transient can occur during the turn-on or turn-off of the device. To avoid transient susceptibility, the device dv/dt rating must be adhered to at all times. The input capacitance of the power MOSFET comprises the gate to source capacitance (Cgs) and the gate to drain capacitance (Cgd) also known as the Miller capacitance. The input gate structure for the IGBT is essentially the same as for the power MOSFET. A gate driver must be capable of supplying sufficient charge to both Cgs and Cgd when switching power MOSFETs and IGBTs in power converters.

FIGURE 20.3 A dynamic model of the power MOSFET shows the presence of the parasitic BJT element found in the MOSFET structure [1, 2].

A dynamic model of the IGBT is shown in Fig. 20.4. The IGBTs are essentially bipolar devices like BJTs with a MOSFET structured high impedance input. A parasitic transistor is also found in the IGBT chip structure as part of the manufacturing process. Combined with existing bipolar power transistor, it forms a parasitic thyristor, which once latched into conduction device, turn-off is not possible via the gate terminal. To avoid this uncontrolled behavior, the maximum allowable peak current and dv/dt ratings of the device must be adhered to at all times [1].

FIGURE 20.4 Dynamic model of an IGBT showing the location of the parasitic thyristor responsible for uncontrolled device latch up [1].

Both MOSFETs and IGBTs require sufficient charge deposited into their gate junctions, whilst maintaining a minimum gate threshold voltage in order to remain in conduction. When designing a gate driver, it is always important to understand both the static and dynamic behavior of the semiconductor device used as it aids the effectiveness of the design for a given gate driver system.

20.3.1 Floating Supply

Power converters employing bridge configurations function by virtue of a high-side switch as shown in Fig. 20.5 below.

FIGURE 20.5 Simple structure of a bridge topology and driving circuit illustrating the concept of a high-side switch.

In order to drive a device like an IGBT or power MOS–FET into conduction, the gate terminal must be made positive with respect to its source or emitter. A common misconception made by many newcomers to the field of power electronics is that, because the emitter or source terminal is usually at some ground potential, it must be made positive with respect to ground. It can be noted that the emitter terminal of IGBT1, in the circuit above, can be floating anywhere from ground up to the DC bus potential, depending on the operating states of IGBT1 and IGBT2, and is therefore not referenced to the system ground potential. A supply is therefore needed in order to provide power to any circuitry associated with this floating midpoint potential. This type of supply is commonly referred to as a floating supply. Three of the most common methods of generating a floating supply are described next.

A simplified schematic of a bootstrap circuit is shown in Fig. 20.6. When the low-side switch M2 is on, the bootstrap diode, D, conducts and charges the storage capacitor. If we assume the saturation voltage drop of the low-side power device M2 and the forward volt drop of diode D to be negligible, the capacitor will charge to approximately the low voltage supply potential. When the high-side switch M1 is on low side switch M2 is off, D is reverse-biased and the high-side circuitry is powered from C. In this condition, the voltage on C droops as it discharges when supplying the high-side circuitry.

FIGURE 20.6 Bootstrap technique employed for creating a floating supply.

The amount of voltage droop is a function of the current drawn, the size of the capacitor, and the operating frequency of the converter [3]. Detailed design and application literature on bootstrap supplies can be found in [4].

A summary and comparison of the various methods of generating a floating supply is given in Table 20.1 [5].

TABLE 20.1 Comparison of the various techniques of creating a gate driver floating supply [5]

20.3.2 Level Shifting

The second requirement for driving a high-side switch, is that the control signal fed from the PWM electronics needs to be conveyed to the floating driver circuitry. A level-shifting circuit is required in order to achieve this.

The GD1 and GD2 are the respective high- and low-side gate drive circuits. In order to signal the high-side gate driver circuitry to commence turn-on of the high-side IGBT switch, the control signal V1 which is referenced to the control circuit ground potential needs to be referenced to the floating potential V_out at the IGBT1 emitter. This implies that the control signal V1 will be level shifted to V_g1 as shown in Fig. 20.7. A level shifter can be thought of as an isolating black-box that transfers a signal across a potential barrier [6]. The maximum level attained by V_g1 will be V_out + V1. In power converters, operating off mains voltage, these levels can be in excess of 500 V. Level shifting is achieved in one of the following ways:

FIGURE 20.7 The concept of level shifting and the placement of the switching control signal on top of the inverter output voltage.

FIGURE 20.8 Combined transformer level shifter and gate driver. Capacitor C1 removes any DC offset in V1 thereby maintaining a zero average volt–time product across the transformer windings.

FIGURE 20.9 Operating waveforms for the pulse transformer circuit in Fig. 20.8 under different duty cycles.

FIGURE 20.10 Fiber optic level shifting circuit used for high speed, extremely high noise immunity, and very high isolation voltage capability.

FIGURE 20.11 Electronic level shifting employed to convey the switching signal to the high-side gate drive circuitry. This technique does not offer any isolation but has widespread applications as an HV interface IC for power electronics applications.

A summary and comparison of the various methods of level shifting is given in Table 20.2 [5].

TABLE 20.2 Comparison of the various techniques of level shifting for gate drivers

20.4.1 Isolated Gate Drivers

Gate driver circuits incorporating electrical isolation, provide the benefit of good noise interference immunity between power and control circuits. This is as a result of the separated ground return paths. Several types of isolated driver circuits exist namely:

FIGURE 20.12 Mains frequency transformer employed to generate a floating supply. Simple and reliable, but larger than other solutions due to its mains frequency transformer. Works well for a half-bridge but would require more isolated power supplies if a full-bridge topology was employed. A 3-phase inverter would require either six separate floating supplies or three separate and one shared supply for the low-side devices.

FIGURE 20.13 Switching waveforms for the circuit in Fig. 20.12.

FIGURE 20.14 Transformer coupled gate driver used to supply both the control signal and the gate drive power to the device.

FIGURE 20.15 Transformer coupled gate driver with a large duty cycle operating range.

FIGURE 20.16 Operating waveforms of the transformer coupled gate driver in Fig. 20.15.

FIGURE 20.17 Signal modulated carrier used for level shifting and generation of a floating supply.

FIGURE 20.18 Signal modulated carrier used for level shifting and generation of a floating supply.

FIGURE 20.19 Low cost high frequency switch-mode floating supply.

20.4.2 Electronic Gate Drivers

These drivers utilize electronics in order to store energy in capacitors. This is used to produce floating referenced potentials. All circuits are referenced to a common ground potential. This technique provides a very cost-effective gate driver solution over the isolated versions, and are becoming increasingly popular in industries.

FIGURE 20.20 Optically isolated level shifter with an un-isolated bootstrapped floating supply derived from the low-side power supply.

FIGURE 20.21 Level shifted gate driver with floating supply derived from the DC bus.

20.5.1 Transformer Coupled Isolated Drivers

Semikron Corporation has developed a gate driver module that employs integrated floating supplies (Fig. 20.22) [9]. The level shifting is accomplished through transformer coupling, which is used as a bi-directional communications link for the transmission of the gate drive and fault signals.

FIGURE 20.22 SKHI22 integrated half-bridge IGBT module capable of operation at 1200 V. A product of the Semikron Corporation.

Each module is configured for a half-bridge inverter configuration and one additional driver module can be added for each inverter leg needed. The modules come in a variety of output current capabilities, depending on the IGBT drive requirements, with usable operating frequencies ranging from the low kHz range up to 30 kHz.

Another manufacturer of isolated gate driver modules is Eupec semiconductor [10]. These modules employ a mono-lithically integrated coreless planar transformer design in a gate driver IC. This allows it to be substantially smaller and cheaper [6]. These modules are operational to 60 kHz with output current capabilities in excess of 2 A peak. The EUPEC EICE-Driver (2ED020I12-F) is shown in Fig. 20.23.

FIGURE 20.23 EUPEC EICE-Driver incorporating monolithic coreless transformer technology allowing for a substantial reduction in the device size.

20.5.2 Non-isolated Electronic Level Shifted Drivers

The International Rectifier Corporation has developed a family of low cost gate driver ICs for the power electronics industry [11]. International Rectifiers, gate driver ICs (Fig. 20.24) utilize electronic level shifting for conveying the switching signal to the high-side switch and a bootstrap technique for the generation of a floating supply. These ICs are relatively inexpensive and are being used mainly in the low cost consumer electronics industry. Application include Electronic Ballasts and low to medium power inverter applications such as variable speed drives and DC to single-phase AC backup inverter systems. Available in a single-channel, half-bridge, or three-phase inverter driver IC, these devices require a minimum amount of external components. Intelligent features like under voltage lockout, shut down, and current sensing inputs are included in the driver IC.

FIGURE 20.24 International Rectifiers IR2113 (Left) and the IR2233 (Right) high voltage gate driver ICs.

Functional diagrams of the IR2113 half-bridge driver and the IR2233 (Three-phase full-bridge) driver are shown in Fig. 20.25. The driver section of the IR2233 comprises a similar structure as the IR2113's bootstrap and level shifting circuitry. Even though the IR range of driver ICs do not provide isolation between the control and driver electronics, this device is a low cost and very compact solution for low to medium power applications where electrical noise and interference can be managed by PCB circuit layout techniques. The IR2113 half-bridge driver is capable of sourcing and sinking 2 A peak current to a capacitive load without external buffer circuitry. This feature coupled with low propagation delay times makes this device capable of operating at frequencies in excess of 100 kHz.

FIGURE 20.25 Functional diagrams of the IR2113 half-bridge driver and the IR2233 three-phase full-bridge driver.

The IR2113 and IR2233 are capable of driving floating loads (high-side switches) at 600 and 1200 V DC bus voltages, respectively, making them suitable for most direct off-line single-phase and three-phase inverter applications.

Other driver ICs available include POWEREX's M57959L and INTERSIL'S HIP2500 bridge driver.

20.5.3 High-speed Gate Drivers

In the quest for high conversion efficiency and high power densities, the trend in power electronics is the move towards high converter switching frequencies at increasingly higher power levels. The main reason for this is to reduce the size of the energy transfer and storage components such as transformers, capacitors, and inductors. This results in a substantial reduction in component size and to a certain extent also cost. Achieving the objectives of high speed and high power switching requires fast switching power semiconductors. These devices are readily available, but as the device die-size increases with voltage and current requirements, so does its gate drive power requirements. This is due to an increase in device input capacitance as the die-size is increased. Low impedance gate driver electronics capable of delivering gate peak drive currents to large die-size IGBTs and power MOS-FETs in excess of 8 A are often required. In order to meet these requirements, driver ICs must be able to deliver these large pulse currents efficiently at high switching frequencies. This is mainly due to a limitation in the maximum allowable device power dissipation. Device package power dissipation is typically limited to around 1.5 W for a DIP-14 IC without heat sinking. Many manufacturers have high speed driver solutions which realize drivers with low output impedances such as the TC4427 and the TC4422. These devices are available as single or dual channel drivers capable of delivering up to 9 A peak to a 1–10 nF capacitive load with a typical rise time of 30–50 ns. A table of available high-speed drivers from TelCom Semiconductor Inc. is shown in Table 20.3 [12].

TABLE 20.3 High current high-speed driver ICs available from TelCom semiconductor

20.5.4 Resonant Gate Drivers

High speed gate driving requires the driver and power semiconductor device impedance to be as low as possible. It is also adversely affected by parasitic inductance and capacitance caused by incorrect layout of the gate driver circuit. Sometimes these unavoidable parasitic elements tend to increase the gate drive loop impedance, (between the driver and the power semiconductor device) resulting in a limitation of the peak current drawn by the device gate. This limits the maximum achievable switching speed of the device. It also results in uncontrolled voltage excursions on the device gate, making high-speed driver circuit design quite challenging.

Resonant gate drivers utilize these parasitic elements in the gate drive loop by virtue of a controlled series resonant mode of operation. The resonant circuit comprises the driver resistance (which is small), the device input capacitance, and its internal gate electrode resistance. By the addition of a defined amount of extra inductance to the gate drive loop, the voltage swing and switching speed of the gate driver can be controlled. A typical resonant gate driver for large die-power semiconductors has been developed by Turboswitchers Inc. A resonant gate driver called the TD-000, employs a patented low loss capacitance driver circuit topology, that reduces the driver power losses to less than half that of conventional high-speed drivers, at all switching frequencies. Operational from a 5 V supply, the driver is capable of high-switching speeds and generates a gate drive voltage swing of up to + 24 to –19 V, depending on the inductance values used. Performance tests show that the TD-000 driver is capable of switching a 500 V/32 A power MOSFET supplied from a 400 V DC bus, in less than 3 ns. This switching speed allows for power electronic converters operating well into the MHz range. Detailed device application information can be found in [13].