Sales, Distribution and Marketing
We sell our products to original equipment manufacturers, or OEMs, and merchant power-supply manufacturers through our direct sales staff and a worldwide network of independent sales representatives and distributors. We have sales offices in the United States, United Kingdom, Germany, Italy, India, China, Japan, South Korea, the Philippines, Singapore and Taiwan. Direct sales to OEMs and merchant power supply manufacturers represented approximately 25%, 25% and 28% of our net product revenues in 2021, 2020 and 2019, respectively, while sales to distributors accounted for the remainder in each of the corresponding years. Most of our distributors are entitled to return privileges based on revenues and are protected from price reductions affecting their inventories. Our distributors are not subject to minimum purchase requirements, and sales representatives and distributors can discontinue marketing our products at any time.
Our sales are primarily made pursuant to standard purchase orders. The quantity of products purchased by our customers as well as shipment schedules are subject to revisions that reflect changes in both the customers’ requirements and in manufacturing availability. Historically, our business has been characterized by short-lead-time orders and quick delivery schedules.
Our top ten customers, including distributors that resell to OEMs and merchant power supply manufacturers, accounted for approximately 78%, 62% and 54% of net revenues in 2021, 2020 and 2019, respectively. In 2021 and 2020, two customers, both distributors, each accounted for more than 10% of revenues. In 2019, one of these distributors accounted for more than 10% of revenues.
Research and Development
Our research and development efforts are focused on improving our technologies, introducing new products to expand our addressable markets, reducing the costs of existing products, and improving the cost-effectiveness and functionality of our customers’ power converters. We have assembled teams of highly skilled engineers to meet our research and development goals. These engineers have expertise in high-voltage device structure and process technology, analog and digital IC design, system architecture and packaging.
Intellectual Property and Other Proprietary Rights
We use a combination of patents, trademarks, copyrights, trade secrets and confidentiality procedures to protect our intellectual-property rights. In 2021 we received 28 U.S. and 55 foreign patents. As of December 31, 2021, we held 392 U.S. patents and 341 foreign patents. Both U.S. and foreign patents have expiration dates ranging from 2022 to 2040. While our patent portfolio as a whole is important to the success of our business, we are not materially dependent upon any single patent. We also hold trademarks in the U.S. and various other geographies including Taiwan, Korea, Hong Kong, China, United Kingdom, Europe, Japan, India, Brazil and Russia.
We regard as proprietary some equipment, processes, information and knowledge that we have developed and used in the design and manufacture of our products. Our trade secrets include a high-volume production process used in the manufacture of our high-voltage ICs. We attempt to protect our trade secrets and other proprietary information through non-disclosure agreements, proprietary-information agreements with employees and consultants, and other security measures.
Manufacturing
We contract with three foundries for the manufacture of the vast majority of our silicon wafers: (1) Lapis Semiconductor Co., Ltd., or Lapis, (formerly OKI Electric Industry), (2) Seiko Epson Corporation, or Epson and (3) X-FAB Semiconductor Foundries AG, or X-FAB. These contractors manufacture wafers using our proprietary high-voltage process technologies at fabrication facilities located in Japan, Germany and the United States.
Our ICs are assembled, packaged and tested by independent subcontractors in China, Malaysia, Thailand and the Philippines; a small percentage of our ICs are tested at our headquarters facility in California. Our gate-driver boards are assembled and tested by independent subcontractors in Sri Lanka and Thailand; some of the boards are tested at our facility in Switzerland.
Our fabless manufacturing model enables us to focus on our engineering and design strengths, minimize capital expenditures and still have access to high-volume manufacturing capacity. We utilize both proprietary and standard IC packages for assembly. Some of the materials used in our packages and certain aspects of the assembly process are specific