Feature: Power
Until recently, manufacturers of GaN
By adopting good magnetic design principles, and retaining the diode on the secondary side, it is possible to achieve efficiency of around 85% when supplying a full load
HEMTs have concentrated on supplying devices suitable for expensive, high-voltage systems, in which higher unit cost for a GaN HEMT compared to a silicon MOSFET is more easily afforded in return for the power saving and size reduction provided by GaN technology. Now, as shown by the emergence of compact USB Type-C chargers based on GaN, the technology has also become viable for lower-cost power systems supplying < 100W.
Popular converter topologies in low-voltage systems For AC-DC power supplies supplying loads over 100W, the default power topology is the flyback converter. Tis topology uses an isolated buck-boost which, in its simplest form, consists of two power switches either side of an inductor. Figure 2 shows a modified version of this
Figure 2: Evolution of the step-down flyback from non-isolated buck-boost
basic topology, the isolated flyback converter. Figure 2c shows the simple inductor in Figure 2a replaced by an isolated coupled inductor (the so-called flyback transformer). In this transformer, the secondary windings are arranged to produce a positive output voltage. For an AC-DC converter operating from mains AC, an initial bridge rectifier stage and bulk capacitor produce the DC input voltage, Vg
. Tis will be around 300V when operating
from a 220V AC input. Te turns ratio between the primary
differences in the polarisation of the two materials; see Figure 1. Other physical properties confine this charge to the third dimension. Hence a two-dimensional charge sheet is created at the interface. Electrons can move freely in it without any doping of the semiconductor material. Concentration of the electrons is
determined by the mismatch in polarisation, and can be controlled by adjusting the concentration and thickness of the AlGaN barrier to produce high-performance lateral transistors with very high electron mobility, but which have very low on-resistance as a function of the die area. Te layers of GaN and AlGaN are grown
epitaxially onto a suitable substrate material. Silicon wafers are the most commonly-used substrate. Some suppliers, however, grow on sapphire, which is used extensively in the LED industry. Either manufacturing method supports low-cost manufacturing, building on established processes and readily-available equipment. Different approaches are taken by
manufacturers to turn the transistors into normally-off devices. So called “enhancement mode” devices establish a gate-controlled barrier in the structure of the AlGaN/GaN material. Other devices achieve control through the addition of a cascode series low-voltage MOSFET.
and secondary windings (N) of the flyback transformer can be configured to provide the steep voltage drop required for output voltages of as low as 3.3V whilst maintaining a viable duty cycle and on-time for both the primary and secondary switches during a switching cycle. When optimising the efficiency of a flyback
converter, the designer needs to consider the many elements which contribute to losses in the AC-DC power stage, including: • Primary-side FET switching and conductive losses;
• Secondary-side diode losses; • Losses associated with leakage inductance in the flyback transformer and primary-side voltage snubber network;
• Copper losses in the flyback transformer, including losses associated with skin effect and winding proximity; • Magnetic core losses;
www.electronicsworld.co.uk November/December 2020 37
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