SEMIKRON’s SKiiP 4 now features a new, high-performance pin fin cooler optimised for baseplate-less designs alongside variable diode/IGBT chip area distribution for different asymmetric distributions between diode and IGBT chips that offer huge benefits on the generator and grid-side of compact wind power converters, for example.
For over 20 years, SEMIKRON intelligent integrated power modules – also known as SKiiP modules – have proven to be a reliable solution for electric energy conversion, offering a high level of integration in compact dimensions. In addition to the actual power electronic components – the IGBTs and SEMIKRON CAL4F diodes in half bridge configuration – SKiiP modules also include a gate driver unit for safe signal insulation including comprehensive protection functions, ultra-precise current and voltage sensors, and an efficient heatsink.
The roots of these intelligent power modules can be traced back to the wind power converter market with its ever changing, ever increasing requirements, which have forever fuelled the development and improvement of SKiiP technology. Power electronics have to be reliable and robust to be able to operate in the ambient conditions that prevail in the applications they are used in. Thanks to innovative packaging technologies such as sintering and solder-free contacts, the new SKiiP 4 meets these demands.
Structure of a SKiiP 4 module
The SKiiP 4 module is based on a baseplate-less design and is pressed directly onto the top of the cooler. Between them is a pre-defined thin layer of highly effective thermally conductive material. Fig. 2 shows the key components in the SKiiP 4 half-bridge configuration.
The SKiiP 4 DCB shown in Fig. 2 is a two-part substrate, which the IGBTs and diodes are sintered to and which is pressed onto the heatsink by the overlying structures. The entire half-bridge design ensures that the forces from the pressure part are distributed evenly to the busbar sandwich construction from the top down. This is facilitated by a layer of pressure foam. The busbar sandwich construction comprises laminar busbars stacked one above the other with insulation sheets in between. This pressure is then transferred to the underlying DCB substrate, which in turn is evenly pressed onto the surface of the cooler.
Conventional module designs, where using the copper layer on the substrate and a bond connection to unbundle the signals, will involve a number of compromises. The SKiiP 4, in contrast, has three additional levels that ensure that signals such as the (+) and (-) DC link voltage as well as the (AC) output are transferred to the DCB substrate homogenously. The laminar design achieves ultra-low stray inductance in the power module, while the multi-finger contacts ensure that the signals are transferred “downwards” to defined points on the DCB substrate. Besides the desired ultra-low stray inductance from the power connections right down to the chips, another important benefit of the laminar multi-finger design is the very homogenous distribution of these signals across the DCB substrate. This results in a very even distribution of static and, more importantly, dynamic stresses on the individual chips.
Thanks to these design details, the SKiiP 4 half-bridge offers various advantages over conventional module designs in high-current applications, one being its superior robustness, a key parameter that is even is further improved thanks to the purpose-developed fully digital SKiiP 4 driver. The entire logic circuit of the gate driver is integrated into SEMIKRON-developed ASICs that work with no more than a few external components and are controlled by a high-speed processor. To detect dangerous situations, the driver uses DCB substrate temperature, driver temperature, information from the integrated fast and precise closed-loop current sensors as well as desaturation detection. Where necessary, the driver can then initiate soft IGBT turn-off in response and send fault condition feedback to the master control unit via the digital interface.
A host of parameters, protective functions and safety limits can be set via the CAN bus interface, ensuring a high level of flexibility and configurability. SKiiP 4 also has a Fault Ride Through (FRT) mode that can be activated if needed, as would often be the case in double fed induction generator (DFIG) applications. The programmable digital driver offers the customers a host of configuration options via the CAN interface and comes ex works with a wide variety of adjustable, flexible options, meaning that even the most unusual of customer-specific functions and interface design variations are possible.
Reliability and robustness
Besides the robustness of the SKiiP 4 half-bridge and the many different driver protection functions, other factors also hugely affect its reliability in the field – ambient conditions or how the power modules are operated in the field including the stresses acting on the components. In the wind converter market, humidity and temperature as well as load cycling capability play an important role. SKiiP 4, developed essentially on the basis of class 3K3 climate conditions, has been hugely expanded at key points as seen in the temperature/humidity graph (Fig. 3).
As seen from the higher and lower temperature regions, SKiiP 4 (red) is rated for environmental conditions that go far beyond the limits of the class 3K3 conditions (black), underlining the robustness of this complex component with its integrated power stage, driver and integrated current sensors, including in off-shore wind power applications. In fact, the conditions that wind power units operate in are not fully covered by the 3K3 operation class. The actual climate conditions that occur are one of the known cause of some of the premature breakdowns in the field after years of problem-free operation. The development and suitable choice of packaging can help reduce the failure rate significantly, as verified by statistics on SKiiP 4 failure rates in comparison to failure rates for power modules with fewer protective functions. This applies not only to thermal cycling capability or humidity sensitivity, but also to the load cycling capability, a key parameter in power module design, especially for generator-side power electronics.
With a few exceptions, the majority of modern wind turbines use back-to-back converters (Fig. 3a): a grid-side converter that operates at 50/60 Hz output frequency and a generator-side converter that usually works at an output frequency of not more than a few hertz (DFIG and Direct Drive) up to more than 100 Hz (permanent magnet and induction generators).
Here, operation at low frequencies can cause clear temperature swing in the power electronic components, because at this generator frequency the load current is controlled alternately by the diodes and the IGBTs. Owing to the limited thermal capacity that can be allocated to the diodes and IGBTs, the temperature increases are higher, the smaller the output frequency is. For the generator-side power module, this leads to a particularly high load cycling that impacts the mechanical connection between the chip itself and the connection to the DBC substrate as well the bond wire, resulting in ageing. What is more, the large-area DCB-to-chip connection is particularly sensitive to stresses caused by load cycling.
As early as in 2007, SEMIKRON developed a sintering process that would be used to connect chips, introducing it for the first time in a series produced power module in the SKiiP 4. Here, instead of a conventional solder connection between chip and DCB, silver sintering powder that has roughly four times the melting temperature of conventional solder is processed under pressure at clean room conditions, resulting in an extremely strong and durable connection between the metalized chip undersides and the metal surface of the DCB substrates.
Sintering is not without its challenges, however. This is owing to the fine structures in the silicon chips and the enormous pressure acting on the components. An examination of the number of load cycles the sintered connections can withstand shows that the load cycling capability of sintered chip connections is six to eight times higher than conventional soldered connections. Sintering is therefore crucial when it comes to achieving a high degree of reliability, an absolute must in wind converters, especially on the generator side. The sintering process used by SEMIKRON is continually being improved on with a view to further improving reliability.
In addition to high load cycling capability, the maximum junction temperature also plays an important role in ensuring that a converter works safely and reliably and calls for the effective dissipation of the thermal losses in the chips.
In today‘s market for industrial converters and solar inverters for PV systems, air-based cooling systems are the most widespread solution. When it comes to wind energy applications, water-based coolers have proved to be most effective owing to the high power density and performance requirements that wind converters have to fulfil. SKiiP modules come with water or air-based coolers.
High-performance cooler (HPC)
Since early 2021, SKiiP 4 has been available with a purpose developed high-performance pin fin water cooler as an alternative to the existing water-based cooler. The two different SKiiP 4 water coolers, which have been optimised for baseplate-less SKiiP 4, are shown below (conventional water cooler in Fig. 4a and the HPC in Fig. 4b).
The thermal resistance in the chips on the conventional shaped cooler (Fig. 4a) depends to a relatively high degree on the position of the chips. This is owing to the different distance between each chips and the concentrated water channels in this cooler. Related to manufacturing constraints, the path that the power losses produced in the IGBTs and diodes take through the aluminium to the water channel is relatively long.
The new pin fin high-performance cooler design offers far more favourable conditions. The cross-section of the cooler shows that the distance that the heat travels from the chips to the water is far shorter and distributed far more homogenously across the entire DCB substrate for all the chips. The geometry of the resulting pin fin design was optimised for the respective chip sizes and chip positions on the baseplate-less SKiiP 4 half-bridge. As a result, the thermal resistance of the chips mounted on the DCB on the high-performance cooler is roughly half that of the predecessor water cooler for a similar amount of coolant flow rate. The permissible loss in pressure, however, is only ever so slightly higher.
For the new SKiiP 4 mounted on the high-performance cooler, this means an increase in output current by some 25 %, i.e. a 25% increase in power for the same junction-to-water temperature increase. Fig. 5 shows the maximum junction temperature as a function of the output current for the conventional cooler (the NHC300) and the new HPC.
This data was generated using the new web-based simulation tool SEMIKRON SemiSel V5 and refers to the new SKiiP2414GB17E4-4DUHP on HPC based on the parameters VDC = 1150 V, m = 0.85, cos(j) = 0.85, fsw = 2.5 kHz, Tw = 55 °C (16 l/min 50 % ethylene glycol).
One of the main reasons why this huge increase in power was possible was the short paths from the point where the power losses occur in the chip to the water, i.e. because the use of an additional base plate in the design was avoided. This means that more compact converters with far greater power density can be built at far more favourable prices per unit of power.
For SKiiP modules, which are already in widespread use in wind converters built by leading manufacturers worldwide, this basically opens up the possibility of them being retrofitted as part of an upgrade. This means it is possible to replace a standard SKiiP in an existing wind converter by a new SKiiP variant that is compatible from the point of view of the mechanical design but that delivers 25 % more power. The result is a considerable increase in output power with a minimum of time and effort.
A double-sided cooler design with a SKiiP 4 module on each side (Fig. 6) brings about further improvements in power density. In the right arrangement, this can also make it easier to render the DC link.
To have a better picture of how a double-sided SKiiP 4 can fit on a DC link design (not shown here) – as an assembly for example – Fig. 6 shows some of the optional add-on parts, such as the laminated busbar construction for DC(+) and DC(-) connecting the double-sided SKiiP to the DC link, the two water supply lines, and the mounting bracket. The double-sided high-performance cooler (DHPC) has two separate internal pin fin fields connected in parallel, meaning it works the same way as two independent HPCs and can be easily simulated using the new SemiSel V5 tool.
This special design can make it easier to render the DC link, provided the SKiiP is positioned on one side of the DHPC, e.g. to one generator phase, and the SKiiP on the other side of the DHPC is assigned to a grid phase. This would ensure that part of the current takes the shortest path from the generator side through the DC link to the grid side. At the same time, this would result in lower losses and ripple currents in the DC link capacitor than in separate dedicated generator and grid-side converters located at some distance to one another.
Asymmetric chip distribution
Owing to the comparatively low frequencies on the generator side and the high frequencies on the grid side as well as the direction of the flow of current, the requirements that the power stages have to meet can vary substantially. Depending on the type of generator used, it may well be that far more powerful and thus bigger diodes are needed for the generator than standard modules can provide. The most obvious option would be to add more standard half-bridges and create the diode space required for the application. This, however, would increase the overall volume and would mean the purchased IGBT chip area was not being used, which is why this approach is not the ideal way to go.
Thanks to the unique multi-finger busbar design and the resultant, extremely homogenous signal distribution across the DCB substrate, the SKiiP 4 half-bridge has one very crucial advantage over conventional module designs. The proportion of the total chip area allocated to the diodes can be increased on the DCB substrate without causing severe undesirable current distribution effects on the DCB substrate. In practice this means that for the very first time in a power electronic module customer or application-specific variations of the SKiiP 4 standard half-bridge are possible and, thanks to the possibility of asymmetric chip distribution, IGBT performance can be successively replaced by diode performance with decent granularity. If needed, the designated diode space can even be increased to as much as 50% without an additional half-bridge being required.
Such adjustments make particular sense in generators that produce high output voltages at low rated frequency already. Applications have shown that even a 13 to roughly 20% increase in diode space in the SKiiP 4 half-bridge can easily achieve the necessary longevity thanks to the high load cycling capability resulting from the sintered connections.
The double-sided high-performance cooler combined with asymmetric chip layout are the key to making power modules more compact and more adaptive, cutting down on raw materials use and achieving huge cost savings.