Michael Frisch, Technical Marketing Manager, Vincotech GmbH (Germany)
Temesi Ernö, Manager Application Engineering, Vincotech Kft. (Hungary)
With the race towards highest efficiency innovative topologies are more often considered for the development of new power conversion products. Old well known ideas revive, as the advanced power module technology in connection with elevated requirements into efficiency makes the utilization economical. This article describes power module topologies with paralleled MOSFET – IGBT switch, dedicated for new high efficient solar applications.
The combination addresses two basic improvements in efficiency:
The MOSFET turns on fast and is delayed at switch off so that the switching losses are assigned to the MOSFET. The MOSFET will also carry the current at low power so eliminating one pn junction voltage drop of the IGBT whereas the IGBT will take the majority at max load condition. At low load the IGBT is not conducting at all so residential tail current losses are eliminated for the whole power range. With this topology it is possible to improve both the overall and the maximum load efficiency.
At switch on the Gate of the MOSFET is direct paralleled with the IGBT gate because the MOSFET will be faster and take over the switch on losses. But at switch off, the MOSFET has to be delayed to release the IGBT from switch off losses. For this timing there are different possibilities available:
The gate resistance for the MOSFET is included in the power module. The dynamic behavior is only minimal worse compared with the separate gate circuit.
4. Direct connected:
It is also possible to connect the gates directly. Here are two remaining possibilities. With a standard gate driver circuit we will reduce only the switch on losses, which will reduce already the total dynamic losses significantly.
But it is also possible to generate a 3 level gate signal and to use an IGBT with
higher threshold voltage. The IGBT will already switch off at the 2nd level
voltage whereas the MOSFET will take over the current before switching of
with 0V gate voltage. The draw back of this idea is the relative high tolerances
of the threshold voltages.
The parallel idea could be used in all IGBT based topologies were the switching losses are significant high and in every MOSFET topologies with significant static losses. For solar applications especially the following topologies might be of interest:
Utilization of 1200V diodes will transport the reactive current because the intrinsic characteristics of the diodes of the MOSFET´s are disabled. The suppresser diode connected with a high voltage FRED protects the IGBT from reverse current when the parasitic capacity of the MOSFET is charged at switch off.
In the following the module P965-F - 45mΩ - CoolMOSTM (solid line) is compared with the P969-F using a 99mΩ CoolMOSTM in parallel with an PT-IGBT(dashed line). f=16kHz, PN = 10kW
With this circuit efficiency > 99% is reachable at higher power levels. Because of the neutral clamping, it is possible to use the MOSFET up to nearly the brake down voltage. The handling of reactive power is limited due to the limitation of the body diode of the MOSFET. To cover this approach, the following circuit is the recommended alternative:
With the paralleling of MOSFET and IGBT, it is possible to achieve a nearly constant efficiency at the highest level. The exploitation of this well known paralleling technology was hindered by the complexity of semiconductor technology selection criteria, gate drive technologies, and the dependency of the results on layout causing parasitic stray inductances of discrete devices. With an advanced power module technology, the utilization now more than economical. The power modules which are supporting this idea are available as a standard product or they can be defined as customized components. The future approach of reactive power capabilities is already incorporated.
CoolMOSTM is a trade mark of Infineon Technologies AG