As market demands for data centers, electric vehicles (EVs), energy storage systems (ESS), uninterruptible power supplies (UPS), and portable power supplies increase, so do the demands for more efficient power source solutions.
The ENERGY STAR® program, backed by the federal government, provides a simple and unbiased way to rate the energy efficiency of many electrical products including computers, data center equipment, appliances, office equipment, heating and cooling, and many building products. Many Fortune 500 companies follow the Environmental Protection Agency (EPA) guidelines to produce energy-efficient products to improve the environment, including air quality. It’s estimated that in 2017 alone, ENERGY STAR-certified products saved over $30 billion.
Additionally, the original 80 PLUS® program promoted achieving at least 80% efficiency for computer power supplies. What happens to a power supply with low efficiency? The wasted energy turns into heat. To meet the demands of increasing number of data centers, 80 PLUS launched the 80 PLUS Titanium standard, which requires power supplies to attain efficiency up to 96%. Because of the high-efficiency requirements, 80 PLUS Titanium is also the standard for which it’s toughest to become certified.
Many power supply manufacturers are rushing to meet these requirements to stay competitive. There are many design challenges to overcome when the goals are to increase efficiency, reduce switching loss, and achieve small overall size.
What are the design tradeoffs, and how can optimization be achieved?
Design challenges
The goals in designing an energy-efficient AC/DC power supply in the 400V range include:
- • A clear path to meeting energy standards
- • Achieving compact size
- • Keeping overall cost down
- • Achieving efficient thermal management
- • Minimizing EMI
While these are noble goals, accomplishing them is easier said than done. It is even more challenging to meet energy standards like ENERGY STAR and 80 PLUS Titanium.
No power supply design can reach 100% efficiency. Today, most switching power supplies can achieve about 94% to 95% efficiency, with the other 5% turning into loss in the form of heat. It was estimated that an efficiency increase of 1% equates to 10% reduction in heat dissipation. In other words, an efficient power supply will be able to use smaller heat sinks and smaller components such as magnetic coils and capacitors. The result is smaller overall product size. Most importantly, the total system cost could also be reduced. So how can manufacturers break the efficiency barrier?
Breaking the efficiency barrier
Wolfspeed established its technology leadership in 650V Silicon Carbide with the introduction of the 6th generation Schottky diodes that enabled the highest levels of system efficiency. Wolfspeed continues that leadership with the introduction of the 3rd-generation 15-mΩ and 60-mΩ (RDS(on) at 25°C) 650V MOSFETs, which further capitalizes on silicon carbide’s advantages to push conduction and switching losses lower and power efficiency and power density higher.
The new devices — C3M0015065D, C3M0015065K, C3M0060065D, C3M0060065J, and C3M0060065K — are qualified for operation over a wide temperature range of –40°C to 175°C and are available in through-hole (TO-247-3, TO-247-4) and surface-mount (TO-263-7) packages.
A key parameter to look for in lowering losses is a low on-state resistance — the higher the resistance, the greater the conduction loss and power wasted as heat. Wolfspeed’s new MOSFETs offer the industry’s lowest on-state resistances in a discrete package over the entire operating temperature range, with the 60-mΩ MOSFETs specified for an RDS(on) of just 79 mΩ at 175°C to achieve the 99% power efficiency.
The ultra-low reverse-recovery charge (Qrr) of the devices, with the 60-mΩ MOSFET offering Qrr of 62 nC, drastically reduces switching losses compared to silicon in hard-switched applications. This in turn enables higher switching frequencies that can reduce the size and weight of the transformers, inductors, capacitors, and other passive components in the system.
To combat the concern of parasitic capacitances that also increase switching losses when increasing the switching frequency, Wolfspeed has achieved much lower device capacitances with a small-signal output capacitance Coss of just 80 pF for the 60-mΩ models and 289 pF for the 15-mΩ models.
The ripple effects
How exactly does the increase in efficiency that silicon carbide MOSFETs provide impact the overall system design and bill of materials (BOM) for the power supply unit (PSU)?
- • More efficient thermal management leads to smaller heat sinks, which reduces the weight and size of the PSU
- • Smaller and fewer components reduce PSU BOM cost
- • Higher power efficiency makes PSU industry standards (ENERGY STAR and 80 PLUS Titanium) compliance achievable
- • Reduced overall PSU system cost makes meeting profit goals easier
How does the Wolfspeed C3M SiC 650V MOSFET family stack up?
Compared with the industry’s 650V silicon MOSFETs, Wolfspeed enables up to 50% lower conduction losses, up to 75% lower switching losses, and nearly zero reverse recovery charge in the body diode.
Compared with GaN HEMTs on silicon, Wolfspeed is over 50% lower in conduction losses, and the SiC MOSFET technology has higher field-proven reliability.
Finally, compared with other SiC MOSFET solutions, Wolfspeed has industry-leading lowest on-state resistance in the 650V SiC MOSFET category with the lowest on-resistance change over temperature, further simplifying thermal management in the system.
Supported with reference designs
Wolfspeed provides extensive support for its devices with reference designs, and the new MOSFETs are no different in that respect. For the data center server power application discussed above, the company’s global applications engineering team created a 2.2kW AC/DC PFC reference design using the C3M0060065K 60mΩ MOSFET in a totem pole topology, one that Si-based implementations could not achieve.
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