The key component of every power electronic system is – besides the power modules themselves – the IGBT driver which forms the vital interface between the power transistor and the controller. For this reason, the choice of driver is closely linked with the degree of reliability of a converter solution. At the same time, the driver should guarantee maximum system flexibility and user-friendliness.
Figure 1. Half bridge IGBT driver comprising of all basic driver functions
In a power electronics converter, a microcontroller, for example, provides the digital control signals needed to control the overall behaviour. The function of the IGBT driver (Figure 1) is to convert the incoming digital signals into signals with sufficient power to ensure safe switching of the IGBTs. On the other hand, the IGBT driver has to provide electrical isolation between the voltage potentials of the microcontroller and the power transistors. The use of protection elements is also recommended in order to ensure that power modules are properly and efficiently protected in the event of system faults.
Thus, drivers used in IGBT power modules have to fulfil both gate-drive and protection functions. They must also provide potential isolation between the control system and the power semiconductor and be suitable for use in many different IGBT configurations. To fully meet these demands, optimised gate-drive electronics is required. This means finding the optimum balance between factors such as functionality, flexibility and costefficiency, a balance which has been achieved in the new driver core concept SKYPER.
Figure 2. Block diagram of an IGBT driver
Figure 2 shows the block diagram of the driver core. This driver core is a half-bridge driver comprising all the basic driver functions, potential isolation, protection functions such as VCE monitoring, short pulse suppression, under-voltage monitoring and dead time. By using Application Specific Integrated Circuits (ASICs) and a driver concept based on fundamental driver functions, fewer components than in conventional driver solutions are needed. The driver cores functions with a 15V regulated power supply and process 15V digital control signals.
The most important requirements for potential isolation are high isolation voltage and sufficient dv/dt ruggedness. High dv/dt ruggedness can be achieved using small coupling capacitances within the pF range from the primary to the secondary side. This will minimise signal transmission interference caused by displacement currents during switching. In the case of inverters, the fast switching of the IGBTs causes steep voltage steps (high dv/dt values). It is therefore important to take into account that noise signals may affect the control signals. These noise signals can reach the control system via capacitive coupling of the device used for electrical isolation.
In the driver core shown, the power switches (secondary side) are isolated from the signal processing component (primary side) by magnetic transformers which transfer the driver signals, driving energy and error signals. This means that the driver core is suitable for IGBTs up to 1700V.
Potential isolation using signal transformers (pulse transformers) provides high isolation and has the added advantage of high dv/dt ruggedness (50kV/μs) between the primary and the secondary side. Unlike solutions with opto couplers, signal transformers are less susceptible to faults and also allow bi-directional signal transfer.
An isolated ferrite transformer is used to supply the secondary side of the driver and provide the power needed to switch the IGBTs. Figure 3 shows the circuitry of the DC/DC converter.
Figure 3. Primary circuit of the DC/DC converter
The components used for signal generation are integrated into an Application Specific Integrated Circuit (ASIC). Via the complementary 500kHz clock outputs TRP and TRN a p-channel and n-channel MOSFET are driven. To
prevent short circuits occurring in the bridge-arms of the complementary MOSFETs the 15V signals are interlocked On the secondary side, rectification and voltage stabilization is carried out in order to generate a positive and a negative voltage. Due to the integrated DC/DC converter, an external, isolated supply voltage is not needed.
The switching behaviour of IGBTs can be controlled by recharging the gate capacitance. In the driver cores, gate capacitance recharging is controlled with resistors. Figure 4 shows an example of an output stage. The output buffer of the driver cores is supplied by the +15V/-7V from the DC/DC converter. If the operation proceeds normally (no fault), the signal is transmitted to the gate of the IGBT through the gate resistor RG. The gate resistor in Figure 4 has been divided up into two resistors RGon and RGoff for turn-on and turn-off respectively.
Figure 4. MOSFET output stage
The main advantage is that this configuration offers the possibility of separate optimisation of turn-on and turn-off with regard to turn-on overc-urrent and turn-off over-voltage and to short-circuit behaviour.
The gate-emitter resistor (RGE) prevents unintentional charging of the gate capacitance under driver operating conditions with highly resistive output levels (switching, off-state and driver supply voltage breakdown). Every output stage recharges the gates with up to 15A of peakcurrent. The high pulse output currents allow for short turn-on and turn-off times for the IGBTs, as the IGBT gate capacitances are recharged quickly, meaning that IGBT modules with higher currents can be switched or IGBTs connected in parallel.
The charge needed to switch the IGBTs essentially depends on the type of IGBT technology used, the chip size, the DC link voltage and the gate voltage. The output stages of the driver cores can provide an output charge per pulse of up to 6.3μC. With a view to the mean output current value of 50mA and the IGBTs used, switching frequencies of up to 50kHz can be achieved. This is enough to drive 1400A/1200V half-bridge IGBT modules.
On the secondary side, to protect the IGBTs from over-load situations such as short circuits, each output stage has an integrated dynamic monitoring function for the saturation voltage. This monitoring function compares the collector- emitter voltage of the IGBT with an external adjustable reference value (VCEstat) after the given blanking time (tbl), which can be set using external circuitry. If the reference value is exceeded, the given stage will be turned off. The short circuit is sent to the error memory on the primary side of the driver core in the form of an error signal, where it is recorded and all outputs subsequently turned off. To prevent the turn-on on a short-circuit, the signal path for subsequent turn-on signals remains blocked until resetting is carried out via a reset pulse. In IGBT desaturation monitoring, dynamic saturation voltage characteristics must also be taken into consideration. In the first microseconds of turn-on the collector-emitter voltage is far higher than the final value VCEsat. The response characteristics of the monitoring circuit therefore have to correspond with the VCEsat profile during the blanking time (Figure 5).
Figure 5. Dynamic saturation voltage characteristic
If the driver supply voltage drops substantially, the gate-drive and protection and functions may fail. Moreover, the power transistors can no longer be fully controlled or blocked and the IGBT will operate in the linear area due to too low a gate voltage. If the IGBT operates in the linear area, there will be higher losses and thermal overloading of the IGBT may occur. To ensure that dropping of the driver supply voltage is detected, the supply voltage is monitored and, in the event of an error, the integrated error memory set. As is the case with the detection of a short circuit, the error memory blocks the input pulse for both output stages and sets the error output of the driver core.
To avoid a bridge arm short circuit, IGBTs of the same bridge arm must not be switched on at the same time in voltage source circuits. Due to the dead time generation in the driver core both output stages are interlocked even in case of errors in the input signals.
On the basis of the driver core described above (standard version), a premium version has also been developed which incorporates additional functions such as error input on the secondary side and extended error management functions. A soft turn-off has also been added to improve short circuit protection. In case of short circuit, this function turns off the IGBT at a lower speed and hence reduces the DC voltage overshoot, enabling the use of higher DC-bus voltages. This means an increase in the final output power.
Solder or plug connection
The driver cores are suitable for direct PCB assembly by solder or plug connection and can be adapted to many different IGBT modules. In this way, solutions can be developed which boast the functions common in standard intelligent power modules plus the flexibility of conventional modules thanks to the possible optimisation of switching characteristics. The flexible assembly allows for the possibility to directly assemble the driver core on top of the IGBT module. As a result a short connection between driver core and IGBT module can be reached.
The driver cores offer the designer simple gate-driver electronics with potential isolation and protection electronics, completely qualified and 100% tested. The developer is saving cost and time for the development and qualification and can therefore focus on the major task at hand, the design of the inverter. Challenges such as time-to-market by reducing development time can be solved by using ready to use driver cores. Since an individual configuration of the driver core is still possible, the driver cores can be integrated in different application topologies. The same driver core can be used in topologies for AC/DC drives, UPS, power supply and welding.
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