Importance of IGBT drivers

To ensure that power electronic components are reliably protected from the effects of non-permissible operating conditions, fast and reliable error detection and effective protective measures are essential.

In power modules, error management can be provided either by the system controller or by IGBT drivers. The system controller is suitable for reacting to slow failure modes such as overheating caused by excessive temperatures. Driver electronics, in contrast, are needed to detect and respond to sudden errors. Various driver concepts are available on the market today and differ as regards their applicability, efficiency and reliability.

Fast errors in power converter systems include short circuits and circuit-induced overvoltages. Short circuits are the fastest errors.

When power electronic systems are commissioned, connection and isolation errors are often the cause of short circuits, while in field applications short circuits may be due to faulty components.

If a short circuit occurs in the load path or bridge branch, the collector current in the IGBT increases starkly, causing transistor desaturation. The IGBT modules available on the market today are short-circuit-proof for a brief time only. To prevent the IGBT from being destroyed by thermal loading, it is crucial that the short circuit be detected within this safe period and turned off reliably.

Detecting short circuits

Driver electronics can detect short circuits by way of the di/dt measurement or VCE monitoring.

Figure 1a: Circuit diagram.

In di/dt detection (Figure 1a), the driver electronics measure the rate of change in current in the IGBT. The voltage drop at the stray inductance between auxiliary and power emitter is proportionate to the rate of change (di/dt) of the collector current. By comparing the voltages with a reference voltage, a fast short circuit can be detected. To monitor slow short circuits, this method uses the resistive components in the wire bonds and internal busbars between power and auxiliary emitters. This method also depends, however, on the screw connections used for the power connections. These display a certain distribution in the contact resistance characteristic, and are to be taken into consideration in series connection with the other ohmic components. This calls for precise adaptation to the given system. In general, di/dt detection can only be used for IGBT modules with an auxiliary emitter output.

Figure 1b: Circuit diagram for VCE monitoring.

VCEsat monitoring (Figure 1b) uses the correlation between collector current and on-state voltage. To do so, the collector-emitter voltage is measured and compared with a dynamic reference voltage by a comparator. If the voltage reading exceeds the reference voltage, the driver electronics automatically turns the transistor off. Owing to the rapid increase in transistor voltage, VCE monitoring is a reliable short circuit detection method. The advantage of VCE monitoring is that short circuits are detected quickly and it is suitable for use with any standard IGBT.

If the short circuit occurs in combination with a high inductance, for example on the power side, the collector current rises more slowly. In this case, the VCE threshold has to be adapted accordingly. To be able to apply the VCE method to overcurrent detection, multistage VCE monitoring can be used. Here, several trip thresholds with given reference times are defined. The disadvantage of this method, however, is its temperature-dependence, as well as the complexity involved in adapting the individual stages to the given system. In general, a more effective and reliable way of detecting slow overcurrents is to use integrated current sensors.

Besides fast error detection, an effective and reliable response to a short circuit is also crucial. If drivers are used in multilevel applications or in drives for synchronous motors, the master controller should be responsible for system turn-off. In this case, the driver sends only the isolated error signal to the controller and waits for instructions. In multilevel applications, for example, if the driver turns off the power semiconductor directly and then sends the signal to the controller, the entire DC link voltage may be present across one IGBT for the entire signal transmission and response time. This would lead to the destruction of the module. In the majority of applications, however, it is safer to allow the power modules to be turned off directly by the driver. The driver can respond more quickly, since it does not have to wait until the signal transmission process is complete, but can independently turn off the module from the secondary side. The avoidance of voltage spikes when turning off short circuit currents is ensured by the driver by way of a soft-off or two-level turn-off function. Here, the driver turns off the IGBTs that have higher gate resistances more slowly, in doing so protecting the module from exceeding the safe operating area (SOA). 

Circuit-induced overvoltage

The second fast error mode results from circuit-induced overvoltages. Overvoltages that occur during turn-off have to be detected and reduced quickly in order to prevent the IGBT module from being damaged. The switching surges result from stray inductance in the power circuitry, for example as a result of busbars. Externally induced overvoltages are slow and can be controlled more effectively by way of DC link voltage monitoring.

Driver electronics can control overvoltage directly by way of active clamping, or by use of IntelliOff, an intelligent turn-off feature used to reduce critical voltage spikes. Active clamping turns the IGBT back on as soon as an overvoltage occurs. The gate recharging process is essentially controlled by a central element between collector and gate in order to reduce the overvoltage.

Figure 2: Active clamping circuit diagram.

Here, the overvoltage value corresponds at a maximum to the Zener voltage. The transistor operates once again in the safe operating area, but converts the energy stored in Lk to heat. During this process, substantial additional losses occur in the IGBT within a very short time. These losses accelerate the ageing process of the components and limit the reliability of the converter system.

One way of preventing the occurrence of overvoltages would be to use the IntelliOff turn-off feature. IntelliOff offers optimised turn-off, combining the advantages of virtually immediate switch response with soft turn-off. The turn-off process itself is optimised by IntelliOff thanks to different-speed gate discharging. To start with, the driver initiates the IGBT turn-off process as quickly as possible. As soon as the turn-off process enters the overvoltage phase, the driver slows down the turn-off process, in doing so working proactively against the overvoltages. Finally, the IGBT driver turns off the module safely and reliably.

Figure 3: IntelliOff, proactive overvoltage protection.

As soon as the turn-off signal comes, the driver generates the negative gate charge. The discharging process of the gate collector and emitter capacitances begins and the gate current reaches its negative peak (period 0). Owing to the Miller effect, which describes the process of capacitive feedback that opposes the turn-off process, the gate emitter voltage remains at a higher level for a certain time (period 1). IntelliOff reduces this discharging time thanks to a low-ohmic turn-off resistance and allows for the process to speed up. During period 2, a high-ohmic resistance slows down the turn-off process, in doing so avoiding circuit-induced voltage spikes (period 2). Without IntelliOff, an overvoltage may occur in this phase which, in the case of active clamping, will produce additional losses and, if suitable protective measures are not taken, might ultimately lead to the destruction of the module. Once the critical, voltage spike time frame has passed, the driver establishes - by way of the IntelliOff function - the parallel connection of the turn-off resistances, ensuring that the IGBTs are switched off safely and efficiently. The simple adjustment is possible thanks to an adjustable time constant between high and low turn-off resistances.

New IGBT generations, in particular, have very fast and hard switching characteristics. The IntelliOff function can ensure faster turn-off without the risk of critical voltage spikes and, consequently, help ensure optimum performance in new IGBT modules. Alternative protective concepts, in contrast, respond by limiting the performance of the IGBT module, in doing so producing additional losses.

Conclusion

The ideal protection concept for gate drivers depends on the given application. In general, however, it is advisable to investigate and analyse the error mechanisms during the system dimensioning stage. Using the gate driver to permanently compensate for non-permissible conditions is not an efficient solution and reduces reliability into the bargain. A more effective way of providing overvoltage protection is to use the IntelliOff function, which prevents voltage spikes from occurring in the first place. VCE monitoring is a reliable short circuit detection method and has a number of advantages over di/dt detection owing to its easy adaptability and applicability with any standard module.

Many different driver protection solutions are on the market today, ranging from standard protection functions to highly complex driver solutions. With simple driver solutions, however, users have to integrate protective functions themselves and provide driver protection for the overall system themselves. This can be rather costly, and driver protection is often underestimated. Highly complex driver solutions, by way of contrast, often have the disadvantage that system implementation is rather complex and service life is often limited. An optimum driver solution has to meet system reliability requirements but should also factor in the all-important price considerations of mass-production applications