The Problems of GaN and SiC When Fast Switches Generate New Losses

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GaN and SiC transistors reduce the classic switching losses through previously unattainable switching frequencies. In real designs, however, the problem shifts from the semiconductor to the inductance, which becomes the new source of loss under the fast edges.

Power supply units, on-board chargers and server supplies are achieving ever higher levels of efficiency. The reason for this is wide-bandgap semiconductors such as GaN and SiC. The promise: higher switching frequencies, smaller inductances, better efficiencies. This is true in the data sheets. However, real measurements often show a different picture. Although the switching losses of semiconductors are falling dramatically, at the same time effects that were long considered secondary are coming to the fore. It is no longer the transistor that determines the efficiency, but the inductance.

The reason is not a new phenomenon, but a shift in dominance. Fast edges and rising frequencies shift the loss mechanisms from the semiconductor to the inductor. What used to be negligible can become a limiting factor.

What GaN and SiC Change Physically

Power switches made of GaN and SiC allow very high voltage and current change rates. The edge slopes dv/dt and di/dt reach values that were almost impossible to achieve with silicon. Typical switching frequencies increase from the kilohertz to the megahertz range.

The classic switching losses of a transistor can be described in simplified form using the following formula:

with P_sw (switching losses), V_DS (drain-source voltage), I_D (drain current), f (switching frequency), t_r (rise time), t_f (fall time).

As the rise and fall times t_r and t_f are very small for GaN and SiC, this proportion decreases significantly. The developer appears to gain leeway. However, this leeway is immediately eaten up by increased losses in the inductance.

The Inductance Under High-Frequency Current Ripple

In a buck converter, the current ripple of the inductance results from:

with V_in(max) (maximum input voltage), V_out (output voltage), D (maximum duty cycle), f_s (minimum switching frequency), L (selected inductance).

With increasing switching frequency f_s, the inductance L could be selected smaller. In practice, this advantage is relativized by the steeper current curves. The current contains significantly stronger high-frequency components. The inductance is no longer subjected to an almost triangular-shaped current, but to a signal whose spectral composition extends far into the high-frequency range. As a result, the frequency-dependent losses in the conductor and core increase disproportionately.

Skin Effect as the Dominant Loss Mechanism

The skin effect means that alternating current no longer uses the entire conductor cross-section. The current density is concentrated on an edge layer with the thickness

where ρ is the specific resistance, ω = 2 π f is the angular frequency and µ is the permeability. As the frequency increases, this penetration depth becomes smaller and smaller. The effective conductor cross-section shrinks and the effective resistance rises sharply.

The Ohmic Winding Loss

thus becomes a frequency-dependent variable. At high switching frequencies, R_eff is many times greater than the DC resistance. A thick round wire worsens the situation, as its inner cross-section is practically no longer used.

Proximity Effect and the Geometry of the Winding

In addition to the skin effect, the proximity effect also occurs. Magnetic fields from neighboring windings also displace the current into certain areas of the conductor. This current displacement further increases the losses, even if the skin effect alone could still be controlled. The geometry of the winding thus becomes decisive. Flat wire, stranded wire, copper foils or planar structures can become the optimum design.

Core Losses Increase with Frequency

The signs also change in the core material. The core losses can be described approximately using the Steinmetz equation. The Steinmetz equation was originally only developed for sinusoidal excitation and only applies to PWM applications in a modified form:

The exponents a and b are typically greater than one, depending on the material. Doubling the frequency therefore leads to a disproportionate increase in core losses. Classic ferrite materials quickly reach their thermal limits here.

At the same time, the larger current ripple increases the flux density change B, which further increases the losses. Powder cores, nanocrystalline materials and distributed air gaps are necessary to control saturation and heating.

EMC as a Direct Consequence

The steep edges of GaN and SiC switches generate strong high-frequency fields. The inductance becomes the source of stray fields that excite both differential and common mode interference. Many EMC problems of modern converters are not primarily due to the layout or the switch, but to the magnetic component.

The parasitic capacitances between windings and to the core couple these disturbances into the system. This makes inductance not only a thermal problem, but also an electromagnetic problem.

Inductances Become a Development Discipline

Under these conditions, inductors can no longer be selected solely on the basis of rated inductance and current carrying capacity. Frequency-dependent resistances, core material properties, winding geometry, thermal connection and parasitic elements must be taken into account. Simulation, measurement and characterization become necessary tools. The selection of inductance is therefore rarely a catalog decision, but an integral part of the circuit design.

The New Bottleneck in Efficiency

GaN and SiC reduce losses where developers have suspected them for decades: in the switch. At the same time, they are forcing us to focus our attention on a component that has long been considered uncritical. Today, inductance plays a key role in determining the efficiency, temperature behavior and EMC robustness of modern power converters. Anyone who works with these semiconductors and does not devote the same care to magnetics as to the active component is already wasting the potential of the technology in the initial design. (mr)