Apple Pie and Power Converter Efficiency

兰博

Apple Pie and Power Converter Efficiency [Copy link]

Are you looking to improve the efficiency of your power converter design? Perhaps you should consider using SiC FETs. Learn how SiC FETs are a safe way to increase efficiency in all common conversion topologies, and all the benefits that come with them.

This blog post was originally published by United Silicon Carbide (UnitedSiC), which joined the Qorvo family in October 2021. UnitedSiC is a leading manufacturer of silicon carbide (SiC) power semiconductors, and its addition enables Qorvo to expand into fast-growing markets such as electric vehicles (EVs), industrial power supplies, circuit protection, renewable energy, and data center power.
When writing a blog post, many of us like to start with something like "mother and apple pie" (American slang for a statement that everyone agrees on), such as the benefits of improving power conversion efficiency. Of course, higher efficiency is undoubtedly an advantage, but sometimes the description of "net gain" is inappropriate: a small amount of heat generated by household appliances can make central heating systems run easily in cold conditions compared to an inefficient boiler, and may also improve the overall efficiency of energy use and reduce its overall cost. The same is true for incandescent light bulbs, which can be very effective heaters when you need to warm up.

Other users do see a major advantage: In warm places, the heat generated by air conditioning systems increases their power consumption, driving up costs exponentially. But in industrial areas, such as server farms, whose energy needs now exceed 1% of global energy demand, every fraction of an ounce of efficiency improvement represents a huge cost savings and a significant reduction in environmental impact. Sometimes, efficiency gains reach a "tipping point" after which the benefits soar exponentially. In the case of electric vehicles, improvements that create smaller, lighter power converters would reduce energy needs and extend driving range.

As a result, many engineers will strive to improve efficiency, even if it means only a small improvement. When designing with a new, unfamiliar topology that offers some improvements, they evaluate whether the design will produce a lower total cost of ownership over a certain time frame. Needless to say, as efficiency continues to increase, any improvement becomes more difficult, and when efficiency has reached around 99.5%, a measurement error of only ±0.1% on input and output power can result in calculated losses being 40% higher or lower than the actual value. The situation is even worse when the power input is an AC power supply with distortion and poor power factor, and the DC power supply output has residual noise components that confuse the DVM. Today, we often use calorimetry to measure heat output rather than inferring heat output from electrical measurements.

Figure 1. At high efficiency levels, even if the test equipment is accurate to ±0.1%, this can lead to large fluctuations in efficiency measurement accuracy.

Improving power converter efficiency by simply improving well-designed semiconductors is a relatively low-risk approach. MOSFET-based converters can be upgraded to newer devices with lower on-resistance and potentially lower switching energy requirements, with due consideration for changes in EMI emissions. However, taking advantage of newer wide-bandgap devices such as SiC MOSFETs or GaN HEMT cells requires significant changes to the circuitry, especially the gate drive. If the existing circuit is based on IGBTs, a complete redesign will need to be considered to use wide-bandgap devices.

The gate drive issue is related to the voltage level. So, to achieve full improvement, SiC MOSFETs need to be driven with a significantly higher voltage level than Si-MOSFETs, which should be very close to the absolute maximum rating of the device and should be strictly limited. The large voltage fluctuations between the on and off states also mean that a certain amount of drive power is required, because the gate capacitance is charged and discharged with each on and off cycle. In addition, the voltage threshold is not a fixed value and there is hysteresis, so it is difficult to achieve optimal drive. To some extent, GaN HEMT cells are the opposite of the above situation. Their gate threshold voltage and absolute maximum are very low, but their drive circuits must also be strictly controlled to avoid overstress and failure.

If the power converter circuit requires reverse or third quadrant conduction, the body diode performance in SiC MOSFETs is critical, otherwise excessive losses may occur due to its high recovery energy and forward voltage drop. The absence of a body diode in GaN devices allows reverse conduction through the channel, but with a higher voltage drop during the dead time before the channel is actively enhanced by the gate drive. If the gate is negative in the off state, the voltage drop during "commutation" will be even higher.

Using multiple SiC FETs (i.e., cascode combinations of Si-MOSFETs and SiC JFETs) is a more beneficial solution. Such devices use the simple, non-critical gate drive of Si-MOSFETs, but their performance figures of merit "on-resistance x area" and "on-resistance x EOSS" are better than those of SiC MOSFET and GaN HEMT units. In addition, such devices inherently have excellent avalanche withstand and self-limited short-circuit current performance, and their body diode effect is similar to that of low-voltage Si-MOSFETs with lower forward voltage drop and fast recovery performance. In other words, efficiency improvements can usually be achieved by simply inserting SiC FETs into Si-MOSFET or IGBT slots. Unlike other technologies, we cannot control the speed of SiC FETs to limit EMI and stress by simply adjusting the gate drive resistor, but using these ultra-fast devices, we can effectively limit overshoot and ringing through a small RC snubber circuit. In addition, this circuit can also be used to simplify the parallel operation of devices. Replacing IGBTs with such devices allows higher switching frequencies without excessive dynamic losses, resulting in smaller, lighter, and lower-cost magnetic components.

SiC FETs are a safe way to increase efficiency in all common conversion topologies, while offering a host of benefits. Some say that if you’re afraid of the heat, just don’t bake the apple pie. That’s not the way to go, but in converter designs, you can use SiC FETs instead.

led2015

The use of efficient components is the general trend