ANALYSIS AND COMPARISON OF A FAST TURN ON SERIES IGBT STACK AND HIGH VOLTAGE RATED COMMERCIAL IGBTS
Abstract
High voltage rated solid-state switches such as insulated-gate bipolar transistors (IGBTs) are commercially available up to 6.5 kV. Such voltage ratings are attractive for pulsed power and switch-mode converter applications. However, as the IGBT voltage ratings increase, the rate of current rise and fall are generally reduced. This trade-off is difficult to avoid as IGBTs must maintain a low resistance in the n-epitaxial or drift region layer. For high voltage rated IGBTs with thick drift regions, the high carrier concentrations are injected at turn-on and removed at turn-off, which slows the switching speed. An option for faster switching is to series multiple, lower voltage rated IGBTs. A customized IGBT stack with six, 1200 V rated IGBTs in series has been experimentally tested. The six-seriesed IGBT stack consists of individual, optically isolated, gate drivers and aluminum cooling plates for forced air cooling which results in a compact package. Each IGBT is overvoltage protected by transient voltage suppressors.
The turn-on current rise time of the six-series IGBT stack and a single 6.5 kV rated IGBT has been experimentally measured in a pulsed resistive-load capacitor discharge circuit. The IGBT stack has also been compared to a two seriesed IGBT stack, each rated at 3.3 kV, in a boost circuit application switching at 9 kHz and an output of 5 kV. The six-series IGBT stack results in significantly improved power conditioning efficiency due to a reduced current tail during turn-off. The experimental test parameters and the results of the comparison tests are discussed in the following paper.
I. INTRODUCTION
The IGBT has accrued success as a high power solid state switching device due to its combination of fast switching, low conduction loss, and high impedance gate control. However, there will always be an unrelenting demand for higher performance devices. Manufacturers are therefore motivated to develop switches with extended voltage ratings and current carrying capability. Currently, commercial off-the-shelf (COTS) high voltage IGBTs have achieved ratings up to 6.5 kV from multiple manufacturers. High voltage (>1200 V) IGBTs are commonly sold as modules with ratings from 200 A to over 2000 A, aimed at motorcontrol and traction applications.
Increasing the voltage ratings of IGBTs generally reduces turn-on and turn-off di/dt and increases the switching loss [1]. For systems which require higher switching frequencies such as high voltage switch mode power supplies (SMPS) and pulse power applications, fast switching is essential to the performance of the system with subsequent low turn-on and turn-off losses. As such, a review of the inherent device structure is important to understanding the loss mechanisms.
The IGBT can be modeled as a MOSFET controlled BJT. In a non-punch-through (NPT) IGBT, a PNP BJT is formed with a p+ doped layer added to the basic MOSFET structure's collector. The BJT portion enables conductivity modulation in the lightly doped n- epitaxial or drift region layer with injection of holesfrom the p+ collector. The drift region supports the bulk of the depletion region during the off-state and contributes resistance during the on-state [2]. As the IGBT's voltage capacity increases, the thickness of the drift region generally increases and requires deeper ambipolar diffusion lengths at high injection levels for sufficient carrier concentration across the entire drift region [3]. Thus, high carrier concentration is present in the drift region for high voltage IGBTs. As a result, injection and recombination of large amounts of charge will result in slower current rate of change at turn on and turn off.
The latest generation of IGBT designs such as the fast-stop or soft/light punch-through technology reduces losses compared to similar rated NPT designs [3-5]. These designs use advanced doping profiles, which add a thin type-n doped buffer layer adjacent to the heavily doped p+ collector. This buffer layer provides desired forward on-state voltage during conduction and allows for a thinner drift region to mitigate switching loss. Also, new trench-gate IGBT designs have been shown to lower on-state voltage compared to planar DMOS gate structures by reducing the on-state resistance in the MOSFET portion [4-6]. These technologies are employed in recent 6.5 kV rated COTS, IGBTs designed for rugged industrial and commercial applications. An alternative to high voltage COTS, IGBTs is to assemble a stack of lower voltage rated, IGBTs in series. Theoretically, a stack of IGBTs will switch similar to each individual IGBT. A 7.2 kV rated switch has been built using six, 1200 V rated IGBTs in series. This IGBT stack design divides the total voltage capacity among six IGBTs and eliminates a single large drift region to support the off-state voltage. Therefore, faster switching is achieved. The six series IGBT stack switching performance is experimentally compared to a single 6.5 kV IGBT in a single-shot pulsed resistive load application and two 3.3 kV IGBTs in series in a 5 kV DC output boost converter circuit operating at 9 kHz.
II. DESIGN OF THE IGBT STACK
The University of Missouri Columbia and Loree
Engineering have designed and built the custom IGBT
stack for a 5 kV DC, 1 kW output boost converter as a
primary design application. However, the stack's
switching ability also lends itself well as a pulsed
power solid-state switch, where it can be modularized
for higher voltage or current capability. To characterize.
the switch for pulse power applications, resistive load
pulse tests were utilized to determine the stack's turnon
time.
The IGBT stack uses six International Rectifier (IRF) IRGP30B120KD-E IGBTs rated at 1200 V and 60 A continuous current. This IGBT is a NPT type IGBT, in a TO-247AD package. The NPT type IGBTs have been shown to be nearly ideal for series stack assemblies [7]. The stack can theoretically handle 120 A single-shot pulses according to the IRF datasheet ratings. The IGBT stack is shown below in Figure 1. Each IGBT gate signal is transmitted through fiber-optics to the individual isolated gate driver boards on which the IGBT is connected. Each gate driver board provides +15 V for turn-on, and 0 V for turn-off through a 4.7 Q gate resistor in series with the gate.
The power and optical trigger signals are delivered to the six IGBTs in the stack from a dedicated power conditioning and trigger module at the base. The stack requires a TTL level triggering input and 12 V DC power. From the base, a single loop of wire is fed through a transformer core on each gate driver board, and provides a one-turn primary winding for each board's isolation transformer.
The heat sink for each IGBT is a flat milled aluminum plate (measuring 5.15" x 3" x 0.1875"), with 0.5" radius rounded corners and 3/32" rounded edges. Maximum power dissipation per IGBT has been estimated to be 30 W per IGBT or 180 W total, with approximately 100-200 CFM airflow at 25°C. Cooling can be provided by forced-air between the heat sinks for horizontal cross flow. The switch design currently assumes operation in a controlled laboratory environment. The IGBTs are not isolated from each aluminum cooling plate, so the plates serve as the collector and emitter contacts.
Each IGBT is voltage-protected by six transient voltage suppressors (TVS) diodes in series. The six diodes in series limit each IGBT collector-emitter voltage to 1200 V. This simple method of voltage protection prevents over-voltage of an IGBT that may switch asymmetrically.
Figure 1. The assembled IGBT stack. Located below the switch stack is the power conditioning and trigger input module.
III. EXPERIMENTAL RESULTS
The following experimental analysis demonstrates the switching performance of the IGBT stack in two separate systems. One is a pulsed resistive load circuit that has been designed to allow measurement of the current risetime during turn-on of the IGBT stack. The second circuit is a boost converter, whereby the turn-off speed and energy loss of the IGBT stack is experimentally verified for switch-mode power supply applications.
A. Pulsed Resistive Load Turn-on
The pulsed resistive load circuit is shown below in Figure 2. In this circuit, two, 5 RF capacitors are assembled in series to achieve a total of 2.5 RF of capacitance that is charged to 5 kV. The circuit inductance was not accurately measured, but the total loop inductance was estimated to be around 500 nH. The load impedance is 357 Q, made of two copper sulfate water resistors in parallel for 14 A peak current. The pulse width was arbitrarily set to 10.5 gs, and the turn-on time of the device measured. The turn-on waveform is shown below in Figure 3. The 10-90% risetime of the IGBT stack under the test conditions was 52.4 ns. The voltage monitor was placed directly across the IGBT stack during the measurements to eliminate inductive effects.
Figur 2. Pulsedresistiveloa
Figure 3. The IGBT stack turn-on time is 52.3 ns with a peak current of 14 A.
The IGBT stack was compared to a single 6.5 kV rated IGBT module manufactured be EUPEC (FZ200R65KF1). This module is rated at 200 A, and uses fast-stop technology with trench gate technology. The gate driver applies a gate-emitter voltage of ±15 V without an external gate resistor. Peak gate current is approximately 9.9 A. The risetime of the EUPEC IGBT is shown in Figure 4. The 10-90% risetime was measured to be 179.2 ns.
Figure 4. The 6.5 kV IGBT turn-on time is 179.2 ns, with a peak current of 14 A.
A. Boost Converter Turn-off
A simple boost converter circuit was designed and fabricated to test the performance of the IGBT stack and also the higher voltage devices (Dynex NPT IGBTs (DIM200PHM33). The boost converter steps up 500 V to 5 kV across a resistive load. The simplified circuit schematic is shown in Fig