1700V GaN­ for Auxillary Power Applications

In a flyback, the benefits that can be achieved from GaN in terms of efficiency are usually limited to applications where the MOSFET stress is below 650V. At higher voltages, SiC or silicon are normally used for conversion. SiC can be pretty efficient but is more expensive than GaN and much more expensive that Si options. To deliver multiple output power, silicon has to go through an isolated converter (typically a flyback stage) plus downstream DC-DC converters which provide accurate regulation but lower overall conversion efficiency.

Power Integrations has tried to overturn this orthodoxy with its InnoMux-2 range of GaN devices which feature voltage switches with 750V, 900V, 1250V and now 1700V capabilities and provide accurate multi-output regulation from a single conversion stage. 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 of this capability; for example, in auxiliary power systems, where there is an existing high voltage supply, but a need for much lower voltages for different control and sensor functions. 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 (ZVS). Andy Smith, director of training at Power Integrations explains, “ZVS – which eliminates turn-off losses in the power switch is very important when working with high voltage rails because switching losses are proportional to the square of the input voltage – a 1000V input design is going to have significant switching losses. InnoMux technology is capable of controlling up to three outputs accurately and also offers low standby and 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 a primary switching cycle. The energy from that switching cycle is delivered only to the output that requires it. This power steering approach provides accurate regulation on all outputs. The process is similar to time division, where each switching cycle is multiplexed between outputs. Flyback reset time (energy transfer) is divided between the outputs according to need and ensures that in each switching cycle, each output receives some energy which eliminates the generation of sub-harmonic switching frequencies.

Smith expands, “the key thing is that energy is directed to where it is required. That allows us to eliminate downstream converters and remove bleed resistance from the output, which wastes a lot of energy at no load and standby. This is because conventional flyback converter would deliver energyequally to all the outputs, load or no load, and the only way that an output capacitor can store the excess energy is by increasing voltage. To prevent that happening, you must 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 and 750V GaN devices. In the InnoMux-2 power conversion IC, the internal controller sets the switching frequency of the power switch, and the controllers for different voltage ICs are identical, making device substitution for performance comparison easy. This gives a really good view of efficiency, as we can use the same layout, magnetics, and other components, only the IC needs to be changed. The results show that they are almost the same in efficiency up to the voltage limit of the 750 V device. Efficiency is then shown for higher voltages for the 1700 V GaN switch device, To put this performance into context, the graph also shows efficiency for our conventional StackFET technology, where we put silicon switches in series to get a 1700V switch capability. It’s an approximation of the same circuit and it shows the 1700 V GaN devices have up to 8% higher efficiency”.

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