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Posted on 01 October 2019

One for All - Plug & Play Drivers for High-Power IGBTs

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High-performance gate driver for IGBT modules from 1200V to 3300V.

Growing power-system complexity calls for the utilization of ready-to-use building blocks. This approach is particularly suited to IGBT gate drivers, where fully customized Plug & Play components are an attractive option for successful design.

By Georg Näf and Sascha Pawel, CT-Concept Technologies AG

 

CONCEPT offers outstanding driver solutions based on 20 years experience to meet the specific needs of power system designers. Our award-winning SCALE technology provides a versatile and highly scalable platform to set up power conversion systems quickly and easily. SCALE is based on CONCEPT’s tried and tested integrated chip set. When used in SCALE cores, a broad range of IGBT modules can be served by each driver core in its respective voltage and current range. SCALE Plug & Play drivers, on the other hand, are highly specialized designs, dedicated to individual IGBT modules. All necessary monitoring and protection functions as well as module-specific control of turn-on and turn-off switching speeds are already built into the Plug & Play driver. This allows system designers to avoid time-consuming design iterations at the performancecritical interface between power switch and digital intelligence.

1SD536F2 mounted onto high-power modules

Overview

CONCEPT offers Plug & Play drivers for a large number of widely used IGBT modules. Its newest member, the 1SD536F2, continues the ongoing extension of our Plug & Play family.

The 1SD536F2 is a new development based on the successful 1SD418F2 with its 6-year history of industry design-ins. It is available for insulation voltages between 1200V and 3300V. Built around the same reliable bidirectional signal transmission system and high-voltage insulation, the 1SD536F2 doubles the output driver current from 18A to 36A. The total drive power is raised from 4W to 5W. Both features facilitate fast switching of existing and future high-power modules.

Figure 2 gives an overview of the internal structure of the 1SD536F2. The input of the user-generated control signals as well as the error and status feedback are transmitted via fiber optic links (FOL) that are highly insensitive to EMI. Three different types of fiber optic interfaces are available to choose from. After having run through the input interface, the signals enter the central control logic. Here, all internal monitoring stages report to status control. In normal operation, the signals are then passed on to an analog summing point. This circuit block connects the signals to the driver’s feedback loops for active clamping and di/dt-control. After final amplification in the output stages, the signals are driven to the gate of the power device.

Block diagram of the 1SD536F2

In addition to common “intelligent” monitoring and protection features such as undervoltage lock-out, desaturation detection and conventional active clamping, the driver provides an advanced active clamping scheme. As can be seen from Figure 2, the active clamping feedback not only influences the IGBT gate directly but also uses a second path inside the driver. The benefits of this second loop are reduced thermal stress in the driver output stage and the clamping diodes during clamping as well as reduction of the total phase lag inside the feedback loop. This significantly enhances the stability of the active clamping configuration with reduced oscillation of the terminal voltages and therefore lower losses. An optional capacitive feed-forward path (not shown) speeds up the dynamic response of the active clamping in applications with steep and rapidly varying voltage transients.

Like all SCALE Plug & Play drivers, the onechannel 1SD536F2 is a compact component that provides high system-design flexibility because each IGBT can be controlled completely individually. All our Plug & Play drivers are ready-to-use products. Putting them to work literally is as simple as 1 (placing the driver onto the module), 2 (tightening three screws) and 3 (turning on), because the drivers are mechanically fitted and electrically optimized for a specific IGBT module. Secondary-side power is transferred via an internal DC/DC converter integrated into the driver. This converter provides the bipolar output voltage of +/-15V as well as an auxiliary voltage level of 60V for detecting highvoltage desaturation.

The 1SD536F2 is particularly suited for multi-level topologies up to 3300V insulation voltage. A dedicated operating mode allows the error detection and monitoring stages to be incorporated into a hierarchical control regime. In this way, the external system controller can determine the optimum response to any reported error depending on the current operating status of the system. If a desaturation error is detected in an IGBT of a 3-level inverter, for instance, the affected driver must not turn off the power switch because a single IGBT cannot block the whole DC link voltage. The correct timing of the turn-off sequence has to be given by the system controller.

Adaptation to IGBT Modules

The 1SD536F2 Plug & Play driver can be optimized for all usual IGBTs in IHM-type modules. This module type is widely used by leading manufacturers of high-power components such as ABB, Infineon/Eupec and Mitsubishi. CONCEPT Plug & Play drivers are completely matched to the respective IGBT module. To achieve this goal, an elaborate test and optimization procedure is performed on every new module/driver pair.

When starting to adapt a Plug & Play driver to a new module type, the principal criterion is maximum switching performance in realworld application environments. Reducing turn-on and turn-off losses is naturally of equal importance because it directly enhances the performance of the power system and at the same time cuts costs because it promotes cooling. That is why switching performance is the number one concern of all power system designers. CONCEPT endeavours to deliver the highest attainable module utilization while always keeping a keen eye on the very complex aspects of usability. Finding the optimal balance is often a delicate task that requires both technical understanding and extensive experience.

In the following, a complete adaptation cycle will be outlined in the same order that the actual measurements are taken.

The first concrete step in the adaptation process is to test a basic configuration of the drivers to be optimized. Automated optical inspection is applied during PCB assembly, followed by an in-house optical inspection at CONCEPT. The electrical status is verified by in-circuit testing and functional tests using an automated test system.

Figure 3 shows a typical test set-up comprising the high-voltage DC link, the IGBT module, the driver and measurement equipment. A large DC link capacitor bank is used at 1700V and 3300V respectively. This ensures realistic worst-case testing in a half-bridge environment. The measurement equipment is made up exclusively of state-of-the-art instruments of high bandwidth. A high acquisition bandwidth must be maintained through the whole chain of probe heads, current transducers and oscilloscopes in order to guarantee test results capable of detecting possible high-frequency effects such as unwanted voltage and current oscillations. To ensure reliable operation of the IGBT module, these effects must be avoided by the driver design.

Dynamic high voltage characterization site

The DC link has been built for low stray inductance, thus allowing for aggressive, hard-switching test transients. These measurements define the maximum performance under rapid worst-case conditions typically encountered in highly optimized fast-switching applications such as induction heating or SMPS. At the opposite end of the spectrum, analogous tests are selectively performed with critically high stray inductances to satisfy the demands of large high-power systems such as traction applications. Voltages are taken from the auxiliary terminals of the module and the measurements take the relevant module parasitics into account.

Low-Side Turn Off

Low-side turn off is the first switching measurement. The turn-off gate resistor is adjusted for this series of measurements in order to minimise the switching losses. At the same time, several components of the active clamping circuitry are tuned to assure a safe maximum collector voltage, fully exploiting but not exceeding the voltage rating and SOA of the IGBT module. The RBSOA curve Figure 3: Dynamic high voltage characterization site is taken under rapid worst-case conditions, i.e. the test conditions are characterized by hard switching with a low stray inductance, maximum permissible DC link voltage and twice the rated current (Figure 4). This worstcase RBSOA curve is then checked against the specifications of the IGBT module manufacturer.

Collector current vs. collector emitter voltage (RBSOA)

It is not unknown for up-to-date high power modules to exhibit collector current snap-off under hard switching, especially in partial load regimes. This trend can be expected to continue in the future, because the improved carrier engineering inside the power devices (IGBTs, diodes) themselves leads to smaller excess carrier reservoirs. Whereas these decrease turn-off losses, under certain critical conditions too few carriers are available to sustain collector current flow and the current snaps off. This can be seen in an extremely steep current transient towards the end of the turn-off process. Especially the massive oscillations introduced into the system make this effect so particularly undesirable. Oscillations constitute a potential threat to the overall signal integrity due to EMI.

Apart from snap-off, a driver-integrated di/dt control is optionally available to safely eliminate phases of steep collector current transients from the switching behavior. In the comparative measurement of Figure 5, the effect of di/dt control can be seen. The fastvarying collector current during turn off without di/dt limiting gives rise to high-frequency oscillations that also affect the gate drive signal, leading to possible violation of the system’s EMI margins. No such effect is observed with activated di/dt limiting. The collector current changes smoothly from twice the rated current to zero.

Turn-off waveform without di-dt limiting (left) and with active di-dt limiting (right)

This situation is most critical under high-current stress. Figure 6 shows a comparative measurement of a power module turning on into a short circuit with and without di/dt limiting respectively. Here, the prime function of di/dt control is not only to combat spurious oscillations but also to limit excess voltage across the main terminals of the power device due to stray inductances and the high rate of change of the collector current during turn off. The favorable effect on the excess voltage of limiting di/dt is clearly seen in Figure 6.

Short circuit turn-off without di-dt limiting (left) and with active di-dt limiting (right)

High-Side Turn Off

All parameters determined by the low-side measurements described are subsequently checked for high-side operability. The measurements are taken up to the maximum DC link voltage and twice the rated IGBT current.

High-Side Turn On

The first test for all turn-on characterizations is to check whether the freewheeling diode remains within the safe operating area during reverse recovery. The device under test for the RRSOA measurement is the low-side diode. All critical constellations are sought by varying the current and temperature. The two effects most commonly encountered are reverse recovery current snap-off and excessive values of the maximum instantaneous power dissipation. Both situations can be avoided by careful adjustment of the IGBT turn-on speed. Once again, a low-inductance DC link in the measurement set-up ensures that the driver/module combination can be tested at its full power capacity.

Low-Side Turn On

With the freewheeling diodes in their safe operating area during the whole forward and reverse recovery process, the low-side turn on can be analyzed (Figure 7). All turn-on parameters are fine-tuned during the course of this series of measurements.

Low-side turn on

Short Circuit Behavior

Short circuit testing is generally performed with a modified high-voltage set-up. This is to ensure the lowest possible residual inductance and resistance of the short circuit path between the IGBT under test and the DC link. Such a configuration results in the most critical condition with respect to turn-off overvoltages. In regular two-level mode, the driver needs to detect the occurrence of a short circuit quickly. Subsequently, the IGBT will be turned off with the driver’s full strength, thus reducing turn-off switching losses.

Short circuit detection delay is crucial in multi-level topologies because of the additional communication delay when the error is first transmitted back to the primary side and a coordinated turn-off signal is then issued by the system controller. The timing of the 1SD536F2 ensures secure turn-off of the worst-case short circuit in less than 10μs. This time-span is matched to the sustainable duration of short circuits in modern IGBT modules. If the module manufacturer does specify a dedicated short circuit SOA (such as Mitsubishi), the CONCEPT Plug & Play driver will naturally maintain that safe operating area (Figure 8).

Short circuit SOA measurement at -40°C and 125°C

CONCEPT Plug & Play drivers relieve the system designer of numerous complicated and highly specialized development steps. Series of measurements at different temperatures are particularly laborious and timeconsuming. All combinations of adaptation parameters are measured at CONCEPT up to 125°C in an elaborate work-flow scheme. Having a thoroughly validated and optimized driver for the IGBT module of choice combines system design flexibility with the benefits of using a tried and tested building block. This is particularly true for the vital interface between the digital control and power components. Our series of Plug & Play drivers aims to achieve this combination of reliability, high performance and ease of use.

The 1SD536F2 has already been adapted to a wide variety of IGBT modules from ABB, Infineon/Eupec and Mitsubishi (see Table 1). Mitsubishi recently carried out an in-depth examination of the 1SD536F2 and has subsequently approved the driver for their highpower modules.

Currently adopted IGBT modules for the 1SD536F2

Currently, the 1SD536F2 is available for voltages between 1200V and 3300V with module current ratings between 800A and 3600A. If your favourite module is not yet supported, please do not hesitate to contact our customer support (support@igbt-driver.com). We would be glad to share our 20 years of experience in power component drivers and provide your industry-scale project with the driver solution that makes the most of your high-performance modules.

 

 

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