Slobodan Ćuk has been on social media asserting that “APEC and other Power Electronics conferences and journal publications have been for years making widely disseminated wrong assumption that an increase of switching frequency will proportionally reduce the size of the magnetic cores needed for transformer! Not only that this is NOT the case……”
I commented to Mr. Ćuk that we should take this discussion to our worldwide, vast technical audience of electronics experts on Planet Analog for a true and fair discussion. I have asked Efficient Power Conversion (EPC) to start us off with their view on the subject, since GaN has increased the switching speeds of the power element like never before and then I ask our audience to make their comments and observations from their experience regarding this issue.
Here is Efficient Power Conversion’s Michael de Rooij’s comment to Dr. Cuk’s commentary:
Ćuk converters are certainly an extraordinary development in power conversion. One of several advantages of the Ćuk topology is that it is the only one that can offer continuous current for both the input and output simultaneously when operating in continuous conduction mode (CCM). This reduces ripple in the input and output capacitors but comes at a price; a) it uses two inductors that contribute to losses, b) the output voltage is inverted, c) it induces high ripple current in the interim capacitor, and d) yields poor magnetic material utilization in both inductors.
One of the barriers to wide adoption of the Ćuk topology may be because it competes head on with the standard Buck/Boost topology which only uses one inductor. The Ćuk topology can be combined with the SEPIC topology to yield a single FET split voltage output (±). One commercial product where this technique has been successful in is a low power (1-3kW) wind turbine inverter, where it converts the rectified generator voltage to a high voltage split supply for single half-bridge AC conversion around the neutral. The voltage stress on the FET is input plus output voltage, and it is challenging to make it synchronously switched, given the inverted diode.
GaN FETs bring further advantages to the Ćuk topology in a way similar to that for soft switched topologies; wider duty cycle control. This allows the converter to operate with larger voltage ratios, similar to the 48 V to 1 V Buck converter. For this reason, there appears to be little reason to operate at higher frequency as it buys little in size reduction given the need for two inductors. If the Ćuk topology is operated at higher frequency one could increase control bandwidth, but due to the rectifier limitations, it cannot be fully realized as it cannot sink output current effectively when using a diode.
The adoption of any technology requires certain elements that include its ease of use, enabling something useful, demonstrates cost effectiveness and proves reliable. This is true for power electronic converters. In some market sectors there is a demand for specific characteristics from these power electronic converters; such as low profile, compact size, or new solutions to old problems. These are areas where the Ćuk converter may not be able to penetrate as well.
The characteristics of GaN FETs, such as lower capacitance, lower switching losses and no reverse recovery, yield benefits to the entire range of power electronic converters, including Ćuk converters. The effect of these benefits can be divided into two groups; 1) improvements to existing products and 2) enabling new products.
An example of existing products benefitting from GaN is the AC/DC “brick” power supplies for portable computers where most people would agree smaller and lighter is better. Another example is the trend to have thinner laptops. These trends drive the need for high performance devices that, in turn, push an increase in the operating frequency. This increase in operating frequency is one of the few adjustments engineers can make to their designs to address reducing size, and the Ćuk topology is less physically scalable and therefore cannot meet the challenging size requirements.
As technology evolves, so do the products that can be imagined. Innovation drives emerging markets. The advent of GaN FETs over the last few years has already lead to a variety of emerging products, such a lidar drivers for 3-D real-time mapping, efficient highly resonant wireless power transfer, and envelope tracking power supplies for efficient broadband communications just to mention a few specific examples. These emerging end-use products have no choice but to operate at higher frequencies and they rely on it to succeed. This is no different from when semiconductors first became available and suitable enough for use in power electronic conversion that have evolved over the last 40 years leading to the products we all rely on today. By the same argument back then we would still be using mercury arc rectifiers instead of semiconductors.
At the end of the day this is not a discussion about frequency so much as it is a discussion about market demand and how technologies advance to meet it.
Designers can learn more about simplifying power designs. You can register for the free webinar, "Simplify Power Designs with Micromodules Products" sponsored by Analog Devices