1700V GaN­ for Auxillary Power Applications

The benefits that can be achieved from GaN in terms of efficiency usually top out at around the 650V level. At higher voltages, SiC or silicon are normally used for conversion. SiC can be pretty efficient, but is more complex and expensive than other options. To perform the same task, silicon has to go through several conversion stages, and solutions also need regulation, lowering the overall efficiency. Power Integrations has tried to overturn this orthodoxy with its InnoMux-2 range of GaN devices which feature voltage capabilities of 750V, 900V, 1250V and now 1700V. Although the devices have limited power handling (100W for the 1,700V device), there are still a large number of applications that can take advantage, for example, in auxiliary power systems, where there is an existing high voltage supply, but a need for much lower voltages for certain areas. Applications like these can be found in smart metering for three-phase supplies, or control systems for electrical motors.

The key to the higher efficiency of the InnoMux-2 devices is their ability to eliminate post regulation and their use of zero voltage switching. Andy Smith, director of training at Power Integrations explains, “the zero voltage switching is very important at high voltage because switching losses are proportional to the square of the input voltage. So a 1000V input design is going to have very significant switching losses. InnoMux technology is capable of controlling up to three outputs very accurately and also offers low standby power and good no load consumption.

It achieves this by monitoring each output independently to provide input to the secondary side controller. When the controller sees an output starting to move out of its regulation band, it sends a switching request to the primary side through the FluxLink communication link, which triggers the primary switching cycle. The energy for the switching cycle is delivered only to the output that requires it. This method provides accurately regulation on all outputs. The process is similar to time division, where each switching cycle is multiplexed between outputs. Off-time is then divided between the outputs to ensure that in each switching cycle, each output receives a little energy to eliminate some harmonics.

Smith expands, “the key thing is that energy is directed to where it is required. That allows us to eliminate bleed resistance on the output, which wastes a lot of energy at no load and standby. Typically, a standard fly back regulator would deliver energy directly to all the outputs, load or no load, and the only way that they can store the energy is by increasing their output voltage. To prevent that happening, you have to bleed off some energy. The InnoMux engine eliminates the need to bleed energy”.

Figure 1, A comparison of the efficiency of the 750V and 1700V InnoMux-2 devices, along with the standard StackFET solution

He continues, “figure 1 shows an efficiency comparison between our 1700V GaN and 750V devices. The internal electronics control the switching rate of the transistor, but each of the devices requires the same input. This gives a really good comparison of efficiency, as we can use the same layout, the magnetics, and the same PCB, only the IC needs changed. The results show that they are almost exactly the same in efficiency. To put this into context, the graph also shows our conventional StackFET technology, where we put silicon switches in series to get up to 1700V. It's an approximation of the same circuit and it shows the GaN devices have up to 8% higher efficiency”.

https://www.power.com/products/innomux/innomux2-ep