Industrial Power Supplies
In today’s industrial sector, power supplies which have high-frequency capabilities, and lower switching losses, are a power trend contributing to lowering costs and increasing power density. Advanced topologies and new control strategies, as well as innovative high frequency magnetic designs, are offering creative solutions in this sector. Wide bandgap semiconductors provide optimum performance at the center of these designs.
Some key power architectures that fall under industrial power supplies are Grid-Tied converters, solar inverters, induction heating, traction/motor drive and UPS.
Some other Key Power Architectures for Industrial Applications
Industrial Converter Efficiency – Power Factor Correction (PFC)
Power Factor (PF) is a dimensionless number ranging from -1 to 1 and is defined as the ratio of real power to apparent power absorbed by the load. A PF of 1 means that 100% of the power is being absorbed by the load. PFC is key to greatly reducing wasted power by increasing the power factor of a power supply. Without PFC, power supplies draw current in short, high magnitude pulses. With PFC, these pulses are smoothed out to reduce the input root mean square (RMS) current and apparent input power. This will effectively shape the input current to maximize the power realized from the supply.
PFC front ends and more efficient semiconductor devices are needed to meet ever increasing efficiency standards. The table below shows the Energy Star 80 Plus efficiency specifications. In order to accomplish this, the trend is to move from simple bridge rectifiers with a bulky capacitor to smooth out the ripples on the DC output, to the newer technique of using a Totem pole technology which will bring the supply into the 90+ percent efficiency realm. See Table 1, at right.
How do we do this? Remove the standard diode bridge and replace it with either a Dual-boost semi-bridgeless topology or a Full-bridge Totem pole architecture with high frequency, enabled by SiC or GaN, that enables the use of smaller and more affordable surrounding components. See Figure 1, at right.
Higher frequency capability in SiC power ICs enable smaller and lower cost external components in the power supply.
Let’s look at a GaN-based Totem pole PFC solution as well. The example in Figure 2 uses GaN devices in the higher frequency leg for improved efficiency and Si MOSFETs in the slower switching second leg. Depending on the efficiency requirement, either diodes or MOSFETs can be used in the second leg to meet cost-efficiency needs.
We will also look at a good EMI filter design between the Totem pole and the AC source so we can meet EMI standards. The filter will attenuate noise from the high-speed switching of the Bridgeless Totem Pole-Power Factor Correction (BTP-PFC) circuits. See Figure 2, at right.
GaN HEMT power devices will give system benefits of high power density with a compact EMI filter and inductor while improving efficiency. GaN circuits can be designed to operate at high frequency, meet EMI requirements, and achieve high power density and high efficiency.
Grid-Connected Converters
Grid-connected converters control the flow of power between the electric power utility three-phase outputs to the various loads along with energy storage and power generation devices. Grid-connected, three-phase AC/DC (or DC/AC) power conversion is necessary in a wide range of industrial applications including power electronic interfaces of renewable energy systems like wind, solar and battery storage.
For bidirectional power applications, two-level topologies using SiC MOSFETs have a marked improvement over Si IGBTs. The SiC power devices greatly reduce switching losses compared to 1200V IGBTs. SiC devices also extend the switching frequency range of an architecture using a two-level, six-switch converter which will maintain a higher full-load and part-load efficiency. See Figure 3, at right.
Another example of how SiC MOSFETs improve this design over Si IGBTs is a significant increase in the power density of the system due to smaller magnetics and a smaller or no heatsink. Another advantage of a SiC MOSFET design is that the device’s body diode (Shown in Figure 1) may be used as the anti-parallel diode which reduces circuit cost and complexity.
Note: The SiC MOSFET can only carry positive current (n channel MOSFET, from drain to source). If the load is inductive, there are times when the switch (MOSFET) must be on, but current flows in the opposite direction. The diode gives this current a path to flow. If the diode is not used, the inductive current ceases instantly, generating high voltage peaks.
Grid-Connected Converters
The benefits of this type of topology using WBG devices here are:
- Reduced overall energy consumption
- Higher efficiency
- Improved thermal performance
- Reduction in size and weight of the power supply
DC-AC Inverters
Common implementations of DC-AC inverters include Single and 3-Phase Inverter stages for AC Motor Drives (Figure 5, at right) and Solar Inverters (Figure 6, at right).
Applications like solar inverters and AC motor drives greatly benefit from the use of WBG semiconductors.
WBG power benefits in this design are:
- Lower circuit complexity
- High efficiency and power density
- Improved thermal performance
- Bi-directional power flow capability (battery-to-grid)
IT Power Supplies Use SiC
Data centers are consuming between 2% and 4% of all electrical energy in the United States. This means that IT power supplies (or server power supplies) will make a huge difference in efficiency and operational costs.
Data centers consume a large amount of power and small % increases in efficiency result in big savings. Cooling a data center can consume up to 40% of the electric bill. Power supply designers are integrating SiC diodes and MOSFETs, with excellent results. SiC components are enabling a peak efficiency greater than 98.5% which drastically reduces heat generation.
When data centers employ SiC MOSFETs and diodes or GaN devices in their power supply designs with PFC, server thermal performance improves enough to create a 40% savings in energy costs on cooling alone. Meanwhile, operational costs will drop and switching frequency rises to a peak efficiency of greater than 98.5% to attain an 80+ Titanium Standard. In the period from 2010 to 2020, servers running SiC devices will have contributed 620 billion kWh in energy savings.
Appliances Need to Meet Tough Energy Star Ratings
Let’s take a refrigerator and replace that older technology appliance to meet the Energy Star rating. When doing this, consumers can save over $200 in the 12-year lifetime of that appliance.
Designing with GaN and SiC power devices in a BTP-PFC power supply will enable better than 95% efficiency with a 5% loss in dissipated and wasted heat. GaN and SiC can affect efficiency increases of better than 1% improvement over that 5% loss. 1% improvement in efficiency equates to a 10% reduction in heat dissipation. This leads to smaller heat sinks, magnetics and capacitors in a reduced footprint on the PC board.
Wolfspeed has the industry’s lowest on-state resistances in a discrete package over the -40 degrees C to +175 degrees C operating temperature range, with the 60mΩ SiC MOSFETs specified for an RDS(on) of 79 mΩ at 175°C, just 1.3x the rating at 25 degrees C.
Table 1: 80 PLUS Efficiency Specifications
Figure 1: The figure on the left is a dual-boost semi-bridgeless PFC using Si power ICs; The figure on the right is a hybrid totem pole PFC featuring SiC power devices (Image from Wolfspeed)
Figure 2: A typical BTP-PFC circuit with EMI Filter (Image from GaN Systems)
Figure 3: A two-level converter power stage using SiC MOSFETs (Image from Wolfspeed)
Figure 4: An industrial AC/DC with PFC followed by a DC/DC power converter (Image from Wolfspeed)
Figure 5: 3-Phase Inverter stage for AC Motor Drive
Figure 6: Solar inverter
Benefits of WBG Power Devices over Si
Traditional power electronic converters using Si-based power devices will not be able to satisfy the increasing demands for higher efficiency, control bandwidth, power density, and switching frequency.
Higher switching frequencies, leading to smaller magnetics and higher operating temperatures, are useful especially in harsh industrial applications.
When we compare Si to GaN and SiC, we find some distinctive properties that allow these devices to operate at lower leakage currents and higher voltages. It is a fact that a higher operational frequency can be achieved through an increase in the electron mobility and electron saturation velocity. Compared to Si devices, WBG semiconductors have a lower intrinsic carrier concentration (10–35 orders of magnitude), higher thermal conductivity (3–13 times), higher electric breakdown field (4–20 times), and larger saturation velocity (2–2.5 times).
SiC also has higher electron mobility than Si and GaN exhibits a higher electron mobility than both Si and SiC. This means that GaN delivers the best performance at very high frequencies. It is predicted that the total price gap between SiC/GaN and Si will continue to steadily decrease. It is also projected that GaN will offer a better result over SiC and even Si in terms of the added cost in the highest switching frequency applications
Let’s look at thermal conductivity which represents another crucial factor: the higher the thermal conductivity, the more efficient the heat conduction properties. SiC exhibits a higher thermal conductivity than either GaN or Si; so SiC devices are theoretically able to operate at higher power densities than both GaN and Si devices. In summary, the higher thermal conductivity, wider bandgap, and higher breakdown field give SiC semiconductors the edge over other semiconductor devices in high-power applications.
In conclusion, SiC and GaN discrete devices are moving into higher content modules which lower cost, reduce size, and improve performance. WBG devices have made a significant strides in replacing Si, offering the benefits of reduced switching losses, lower cooling volume, and reduced thermal solutions cost.
