Driver circuit in digital technology controls IGBTs in the high-voltage range
Power systems in the voltage range of 1200 V and higher require fully insulated signal transmission for pulse width modulation and IGBT status signals. With the aid of a driver circuit for controlling IGBT modules and high-voltage applications that is fully digitalised for the first time, all types of signals – even slow sensor signals – can be transmitted with no interference, independently of temperature, and reliably. The pulse sequence during switching is regulated by the signals of the primary control unit.
Electric motors consume 60 percent of the electric power. If motors are operated with variable-speed drives, it is possible to make double-figure percentage savings. What is required in order to find appropriate solutions are electric and electronic power components, such as, for example, those supplied for over fifty years by Semikron, manufacturers of power electronics. Specialising in the development and production of power chips, power modules and systems constructed on the basis of these, the company with Headquarters in Nuremberg has a range of products for diverse electrical applications with a power spectrum from 1 W to 10 MW. These include industrial automation systems, robots and welding machines, variable-speed drives in industrial systems, everything from white goods to lifts, systems for electrical vehicles, fork-lift trucks, trolley buses, trains and tractors, as well as power inverters for decentralized photovoltaic systems and wind turbines.
Wherever modern power modules are used, there also has to be electronic triggering, or driver circuits that control these modules and systems. With the Skyper driver family, in 2004, the Franconian professionals turned their many years of expertise in this field to their first own product line – driver cores. Initially designed to control the Semix family of IGBT modules manufactured by the company, the flexible driver electronics soon found their way into numerous IGBT solutions in general, both those produced in-house and those made by other manufacturers.
The latest driver core solution for controlling industrial IGBTs is the Skyper 52, presented by Semikron at the trade fair in Nuremberg (PCIM) in May. It is a driver with a peak gate current of 50 A and an output power of 9 W per channel. “With this driver circuit, we are taking a bold step forwards. We are moving away from analog circuits, that we are familiar with from standard electronic assemblies, towards fully digital technology”, says Markus Hermwille, Senior Product Manager at Semikron. A “fully digital solution” means that processing of all signals used both by the IGBT and by the microcontroller or digital signal processor (DSP) of the application is fully digitalised within the driver assembly.
Figure 1. Interfaces of a digital driver solution
Thus, all switching signals are transmitted digitally. With an analog circuit, the user must carry out edge-triggered signal transmission. In other words, there is a series resonant circuit that transfers a signal. On the secondary circuit of the transformer, there is an edge memory where this signal is recorded.
This technology is only suitable for alternating on and off signals, not for the transmission of two signals send in succession or periodic impulses. A permanent data signal flow with zero and one is transmitted. In the transfer it means that an integrated logic creates impulses of defined length and form. These impulses are transmitted through the transformer on the secondary side and the impuls is received and decoded in a switching signal. “Because from an electronic point of view zero and one give a very clear signal – there is no in between –, we receive absolutely unambiguous, pristine signals”, explains Hermwille.
Even slow sensor signals can be transmitted separately
Clear signals also mean that those typical problems we are familiar with from an analog circuit, such as parameter fluctuations, temperature drift or the long-term stability of electronic circuits, no longer play a role with digital driver circuits. “Imagine a complex circuit with two components. One component drifts to the maximum value, and the other to the minimum value. The circuit immediately becomes instable”, says Hermwille. With a digital driver, on the other hand, the user can integrate complex functions and react rapidly to changes at any time using a software update, without any need to develop a new circuit.
Figure 2. Simplified circuit diagram of a digital signal transmission: the power bridge generates time-variant voltages on the primary side which are then transmitted to the secondary side, isolated from one another, via the transformer.
The second plus point of digital technology: even a slow sensor signal can be transmitted, evaluated in isolation and made available to the user as a digital signal. “That wasn’t possible previously. Think about temperature monitoring. Each IGBT module has temperature sensors – a NTC or PTC sensor. Here it is difficult to record clear electrically insulated signals with an analog circuit”, says Hermwille, emphasising the advantage of digital technology.
Plus point number three: as a connecting element between microcontroller or DSP of the application and IGBT module, the digital driver makes using control electronics “more versatile”. In other words, the control system elements, the microcontroller and DSP, normally operate in a low-voltage range between 3.3 and 5 V. Consequently, the electrical isolation has low noise immunity. Noise signals can reach the control system via the internal coupling capacitances of the device used for electrical isolation and may interfere with the control system. “Now if you want to operate an IGBT module using 3.3 V switching signal voltage, there is interference in signal transmission as a result of the rapid switching of the IGBTs, precisely because the small 3.3 V signals prove to be particularly susceptible to interference here”, is how Hermwille explains why the manufacturer had to seek and find a solution that is compatible with 3.3 and 5 V signals on the one hand yet at the same time holds its own with applications with high operating voltage or high electrical isolation in a high-voltage range.
The manufacturer has also introduced a so-called differential signal transmission. “Differential signals compare two signals and subtract these from one another. The result is a joint signal, and the process is as follows: normally both outputs of the pulse transformer are affected equally by interference signals that arise. Through the subtraction of the two different output voltages from each other, the interference signal can be eliminated. “And so the signal information also remains unaffected if the amplitude of the interference signal is greater than the actual transmission signal. High transformer currents and a considerably lower terminating impedance of the receiver guarantee reliable transmission”, says Hermwille. Thus pulses can also be generated without applying a high voltage. Consequently, the voltage can be kept low at 3.3 V because in this case the system parameters have relatively little influence on transmission. “So it is possible to operate with a smaller signal and still have a stable system”, Hermwille sums up.
Error management can also be set individually by the user
Since it is possible to programme the software, a digital driver allows the user to set error management individually. “This is another new thing that cannot be done with analog circuits”, adds Thomas Grasshoff, Head of Product Management, Semikron International. Therefore, status and error messages of power transistors can be recorded at various points and corrected in different ways: “A driver has specific protective functions, which means it can switch off the IGBTs when there is a short-circuit current, for example”, says Grasshoff. So far it had always been the case that, as soon as the driver had recognised a short circuit, it switched off the IGBTs automatically. With individual error management, however, the user can determine how the driver behaves in the event of a fault. “Should all the IGBTs be switched off, and should they be switched off sequentially? Or maybe should an error message be issued and no IGBTs switched off?” Grasshoff asks and gives the example of a multi-level application where there are a number of IGBTs connected which should not all be switched off simultaneously in the event of a fault. “They have to be switched off sequentially, so the IGBT modules are not destroyed by high currents or high voltages”, says Grasshoff.
If there is a fault, the user receives information specifying what type of fault has occurred and which IGBT it has affected. “We can only provide this information because we have a digital signal flow. It is possible to overlay the signal with a type of pulse pattern comprising the information. On the basis of a CAN compatible protocol, the information is available at the interface. And so the user can utilise the driver directly with his or her microcontroller and evaluate this information”, adds Grasshoff. With digital signal transmission, a number of different parameters can generally be individually configured under normal and fault conditions, as can the operating conditions of the digital driver core.
The prerequisite for all types of driver circuits is fast components. “Analog technology is fast and digital technology has become considerably faster”, Hermwille continues, describing the design of the digital driver. “Because the right components, in our case FPGAs, are available now and we can work with them in the 1 GHz range, we have been able to mount our driver in digital technology”. However, the manufacturer had to think carefully about which FPGAs would be appropriate. Conventional FPGAs are often used with monitor components that cannot operate in an environment with high currents or high voltages. “So we had to find look for suitably robust FPGAs and we found some produced by Actel and Altera”, Hermwille explains.
Figure 3. Design of the Skyper 52 digital driver core by Semikron: FPGAs on the primary and secondary side aid signal processing
The general design of the digital driver is that there is an FPGA on the primary side of the driver – the connection to the microcontroller or DSP of the user – as well as two FPGAs on the secondary side, the driver side, which is connected to the IGBTs. The isolation barrier is formed by two pulse transformers for control signals and a DC/DC converter to transform the gate power, which are designed to control power semiconductors with a collector-emitter voltage between 1200 and 1700 V. Using small coupling capacitances in the PF range of the primary to secondary circuit allows a high dv/dt of approximately 100 kV/μs to be reached. The isolated DC/DC converter supplies the secondary circuits, so the driver can be operated with a supply voltage of 24 V (not galvanically isolated). For the user, this means that no additional isolated power supply is required. “Thanks to this design, the driver core is virtually completely unreliant on analog circuits”, Hermwille concludes.
The digital driver circuit Skyper 52 is designed specifically for wind power stations where up to eight IGBT moduleswith 400 A have to be connected in parallel. A second application is X-ray systems with operating frequencies of up to 50 KHz and a high power output. And last but not least, USV systems for 750 KW to 1 MW, used when several IGBTs are connected in parallel.
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