Advancements and Challenges of High Frequency Converters 

Power conversion technology has advanced quickly over the past few decades. Many industries, including consumer electronics, electric vehicles, and light-emitting diode (LED) drivers, are expected to have high efficiency and high power density properties. Inductors and capacitors are some of the passive components that affect the size, weight, and price of power electronic converters. 

The switching frequency of power converters also plays an important role. As switching frequency increases, new challenges arise, which reduce system efficiency because switching frequency is proportional to switching loss and magnetic loss. To overcome this challenge, converter topology, drive circuits, and magnetic components should all be improved to resolve the discrepancy between efficiency and operating frequency. 

Ⅰ. Wide-Bandgap Devices

Power converters are now often made with wide-bandgap (WBG) devices like silicon carbide (SiC) and gallium nitride (GaN), which have lower output capacitance and ON resistance than their silicon counterparts. The commercialization of wide-bandgap devices encourages high frequency converter development. Both SiC and GaN are WBG devices, but their output power, voltage level, performance capabilities, material properties, and device architectures are different. 

Features

A power versus frequency diagram of various devices is shown in Figure 1. The switching speed, forward voltage drop, and voltage blocking ability of Si devices are all inferior to those of WBG devices. Therefore, the use of WBG devices can enhance high-frequency converter performance. To be more precise, GaN is moving towards high frequency, and SiC technology is being used in higher-power products.

Fig. 1. Power versus frequency diagram of different devices Source: IEEE Open Journal of the Industrial Electronics Society

Silicon Carbide

SiC is typically made for working voltages of 650 V, 1200 V, and more. Due to SiC's high maximum junction temperature, it is always effective in harsh conditions. SiC always functions in hot and hostile environments because of its maximum junction temperature.

Gallium Nitride

GaN switching losses at low to medium voltages (below 1200 V) are about three times lower than SiC switching losses at 650 V. The majority of silicon-based designs today operate at a frequency between 60 and 300 kHz. GaN is the only material that can be used to achieve a switching frequency of 500 kHz or higher. 

Gallium nitride, one of the WBG devices' representatives, commands a sizable market share in high frequency converter applications, aiding in the continued advancement of the semiconductor industry. By 2025, the market as a whole will have grown from 740 million to more than 2 billion, with a compound annual growth rate (CAGR) of 12%. The main applications are SATCOM, military applications, and telecom infrastructure. There are currently suitable products on the market as a result of extensive research into GaN by numerous large businesses and research institutions.

Ⅱ. Topology of High Frequency Converters

The advancement of WBG devices has increased the frequency of power electronic converters. Switching loss can be decreased in terms of topology selection. Even though conduction losses may correspondingly rise, the soft switching characteristic of high frequency converters is a crucial characteristic for system efficiency. Resonant converters and non-resonant converters are the two main categories into which soft switching converters can be broadly subdivided based on their operating principles. 

Resonant Converters

Based on the number of resonant elements, it can be classified as two elements (LC), three elements (LLC), or multi-elements (CLCL). The preferred converters for high frequency applications are resonant converters, which can also be classified as series, parallel, or series-parallel resonance converters. One of them, the series-parallel resonance circuit (SPRC, also known as LLC), has well-balanced resonant networks, which allows it to benefit from both series and parallel resonant topologies. 

The LLC resonant converter's most appealing feature is that all of its components can achieve zero voltage switching (ZVS) across the entire load range, and when the converter is operating at or below resonance, secondary-side synchronous rectification (SR) switches can also achieve zero current switching (ZCS). Hence, resonance topologies like LLC and Class-E have received a lot of attention.

Non Resonant Converters

Non-resonance converters are classified as quasi-resonance technology, active-clamp, and zero-voltage-switching (ZVT). Quasi-resonant converters are suggested for those converters that lack an innate soft switching characteristic and that require additional steps to be applied in high frequency converters.

Another method used in isolated flyback or forward converters to achieve zero voltage switching and high efficiency operation is active-clamp. This method has a number of advantages over conventional ones, including the ability to operate at duty cycles above 50%, soft switching characteristics, reduced electromagnetic interference (EMI), and lower voltage stress on the main switch.

In addition to an active clamp, zero voltage transition (ZVT) is a preferred active soft switching method for converters that use MOSFETs as active switches. During nearly the entire switching period, converters using this soft switching technique function as regular pulse width modulation (PWM) converters.

Resonant Gate Driver

On the other hand, system efficiency will be significantly decreased as the loss of traditional gate drivers increases with the increase in switching frequency. The resonant gate driver is therefore frequently used in high frequency applications. Switching speed in voltage source resonant gate drivers (RGDs) can be increased, while driving loss and switching loss can be decreased. 

A drive circuit known as a "current source driver" (CSD) produces a constant drive current to charge and discharge the power MOSFET gate capacitance. It performs better than resonant gate drivers in this regard and can also lower switching losses in hard switching converters with rapid switching speeds.

Magnetic Components

The losses in inductors and transformers present another difficulty. However, as the switching frequency rises, the weight and volume of the magnetic elements will also decrease proportionally, and the core and winding losses will also rise quickly. Additionally, the stress on magnetic components during soft-switching operation is significantly higher due to resonant voltage and current, and some losses are actually transferred from semiconductors to magnetics. 

Planar magnetics with low profiles are being used more and more frequently in high-frequency converters lately to reduce the height and volume of converters for low-power applications (below 5 kW). Under the same power, the volume of the planar magnetic component is only 20% of the volume of the conventional winding magnetic component, which can effectively increase the heat dissipation area. Nevertheless, in high frequency and high voltage (above 200 V) applications, their performance is limited by a significant planar winding capacitance.

Ⅲ. Summarizing the Key Points

l High frequency power converters are essential for high efficiency and high power density properties in various industries.

l Wide-bandgap devices like silicon carbide and gallium nitride have lower output capacitances and ON resistances than their silicon counterparts. The commercialization of wide-bandgap devices encourages high frequency converter development.

l The series-parallel resonant converter is a popular choice for high frequency applications because all of its components can achieve zero voltage switching across the entire load range.

l Non-resonant converters use other methods such as quasi-resonance technology, active-clamp, and zero-voltage-switching to reduce switching losses and improve system efficiency.

l Traditional gate drivers can cause significant loss at high switching frequencies, which can decrease system efficiency. Resonant gate drivers, on the other hand, can increase switching speed while reducing driving and switching losses.