WASHINGTON (Reuters) - A federal appeals court on Wednesday handed Rambus Inc. RMBS.O a major legal victory, throwing out fraud charges leveled by Infineon Technologies AG IFXGn.DE and resuscitating Rambus' claim that Infineon infringed its computer chip patents.

A three-judge panel of the U.S. Court of Appeals for the Federal Circuit ruled 2-1 that a lower court had erred and that the evidence did not support part of a jury ruling for Infineon.

The judges sent the patent infringement case back to a lower court for reconsideration, saying it had failed to properly define five key technical terms.

Rambus makes memory chip acceleration technology that virtually all of the world's big computer chip makers license. The company's stock soared on the news, rising $54.75, or 57 percent, to close at $11.69 on Nasdaq.

Infineon, of Germany, and some other technology companies have accused Rambus of tricking computer-chip makers into adopting technologies for which it held or was seeking patents for chips in a wide range of electronic gear.

Antitrust enforcers at the U.S. Federal Trade Commission leveled similar accusations in a lawsuit filed last year.

At stake in all the litigation are Rambus patent claims worth over a billion dollars in royalties from memory-chip makers.

Rambus general counsel John Danforth applauded the ruling and said it could help bolster the company's position in the other cases.

"We've been confident in this all along, but it's good to see our views (confirmed) in substantial part by the premier patent court in the country," Danforth said.

Infineon spokeswoman Leslie Davis said the company was still reviewing the court ruling. "We expect to request a rehearing by the Federal Circuit Court of Appeals on this decision," she said.

A lower court judge threw out Rambus' patent infringement claims against Infineon, and in May 2001 a jury found that the company was guilty of fraud and awarded damages of $7 million to Infineon.

But on Wednesday the appeals court concluded otherwise.

"In sum, substantial evidence does not support the jury's verdict that Rambus breached its duties" to disclose patents, the judges wrote in the ruling. "No reasonable jury could find otherwise."

The Infineon lawsuit is the first of several cases in which Rambus is squaring off with memory chip makers, such as Hynix Semiconductor Inc. 00660.KS of Korea and Boise, Idaho-based Micron Technology Inc. MU.N , over alleged patent infringement.

Micron is the No. 2 ranked maker of memory chips. Infineon is No. 3, and Hynix is No. 4.

The cases have cost Rambus tens of millions of dollars to pursue, company officials said.

Wednesday's decision could help bolster Rambus's position in the other cases, including its defense against the FTC lawsuit, filed in August of last year.

The FTC has asked an administrative law judge to rule immediately against Rambus, saying the company engaged in a campaign of "massive" document destruction at least partly out of concern that some internal documents could be used to press antitrust charges against the company and invalidate its valuable patents.

Rambus maintains the document destruction was part of the company's regular document retention policy. The document shredding cited by the FTC, the company notes, took place two years before the agency even began investigating the company.

The company consists of 180 people, including 130 engineers, 45 corporate staff and four lawyers.