These superjunction devices are rated for 600V drain source voltage and target high performance switching applications.
My initial reaction to STMicro’s MDmesh DM6 family is a mixture of interest and concern: interest, because these FETs appear to offer an impressive array of features and capabilities; concern, because the complicated terminology in the press release makes me feel like I’m in a senior semiconductor class running a test for which I’m not adequately prepared.
MDmesh’s DM6 MmSFETs are fast recovery superjunction devices that have high dV / dt capacity and better endurance in state compared to “previous generation” devices (I think this refers to the M6 series, while the new devices are in the reM6 series).
Let’s see if we can unpack some of this information.
What is a MOSFET superjunction?
If you really want to dive into the world of superjoint technology, I highly recommend this application note from Vishay. If you are just looking for basic information, read on.
The typical lateral representation of a MOSFET that you have probably seen in textbooks corresponds to a “flat” MOSFET. The structure of a MOSFET superjunction is quite different. Instead of a single doped area below the source and drain terminals, a MOSFET superfunction has a doped “column.” The following diagram shows the difference between the two designs.
These diagrams were taken from an application note published by Vishay Siliconix. As you may have guessed, Vishay also sells superjunction FETs.
Despite the fact that the physical configuration of a MOSFET is usually represented according to the planar structure, superjunction devices are widely used in high power switching applications because they offer lower resistance in state and reduced gate load.
How can a MOSFET be “fast recovery”?
If you think that recovery characteristics are usually associated with diodes instead of FETs, you are correct, and in fact the DM6 series is described as fast recovery because the devices include a fast recovery diode.
Diagram taken from the data sheet of one of the DM6 devices.
My understanding is that as a general rule of thumb you cannot rely on the internal diode of a MOSFET to suppress inductive flyback. However, in the case of DM6 devices, STMicro expects the body diode to be used for this purpose.
In addition to meeting the typical requirements for a freewheel diode in inductive switching applications, the DM6 body diode is intended to improve overall circuit efficiency.
Diagram taken from DM6 product page. TRR is the reverse recovery time, and Qrr is the reverse recovery load.
Ideal diodes conduct current in only one direction, but real diodes have something called reverse recovery. You can read more about reverse recovery in this article. The bottom line is that when a switching event occurs, the current flowing through the diode doesn’t exactly drop to zero amps and then stay at zero amps. Rather, it is bypassed in the negative region before settling into a zero current condition.
This reverse current flows every time the circuit changes, and if the system requires frequent switching, all of this reverse recovery will add up to a non-trivial amount of wasted energy. Therefore, to increase the efficiency of the circuit, we can use a freewheel diode with a lower reverse recovery load (as shown in the diagram).
You may have noticed something unusual about the component diagram shown above. In addition to the MOSFET and the body diode, the device has consecutive Zener diodes between the gate and the source.
Actually, I’m not sure how unusual this is in the context of high-performance power MOSFETs, but in my low-voltage world I don’t expect to see Zener diodes included in a MOSFET package. These integrated Zener diodes provide protection against ESD bumps and voltage transients. You could achieve the same with external components, but this saves you a bit of design work.
The dangers of voltage change
STMicro describes that DM6 devices have “extremely high dV / dt resistance.” The term dV / dt is an abbreviation based on computational notation for the change of voltage with respect to time. More specifically, it refers to the change in the drain source voltage of a MOSFET with respect to time.
If the rate of change of the drain source voltage exceeds the capabilities of the device, the MOSFET may start conducting (that is, when it is not supposed to). This is bad in itself, and the situation can lead to serious failure. The two DM6 devices that I checked had a dV / dt resistance rating of 100V per nanosecond. This sounds pretty good, but I honestly don’t spend a lot of time looking at MOSFET datasheets so I’m not sure how this number compares to newer devices from other manufacturers.
If you have extensive experience with high power switching applications, it would be great to hear your thoughts on which MOSFET functions are particularly valuable. You can share your thoughts in the comment section below.