Cashing in on rising drug prices often unleashes an outcry from consumers and politicians.

But a little-known private equity investor, Royalty Pharma, has built an unusual investment portfolio valued at $15 billion — it buys up the rights to royalties on future drug sales — while largely avoiding public controversy. By its own count, Royalty Pharma owns partial rights to seven of the 30 top-selling drugs in the United States, including giants like Humira, the arthritis treatment that is the single biggest-selling medication in America. And its deals have been getting larger.

Outrage over outsize drug-price increases has dogged some pharmaceutical companies. Turing Pharmaceuticals raised the price on one anti-infection drug by more than 5,000 percent in 2015, and Valeant Pharmaceuticals International adopted a strategy of buying up drug companies and increasing prices. Both companies’ chief executives have since resigned. Mylan executives have come under fire after Mylan raised prices on its lifesaving EpiPen injector by 400 percent over seven years.

While Royalty Pharma has benefited by riding the rising tide of drug prices, the drug companies play the largest role in setting prices. George Lloyd, Royalty Pharma’s executive vice president and general counsel, said: “Royalty Pharma has no influence over how drugs in which it invests are priced. These prices are set by the marketer of the product, and we play no role in that process.”

To make big money on promising drugs, investors like venture capital firms and biotechnology funds typically place long-shot bets on drugs that are still in the development stage. Royalty Pharma, based in New York, steps in later in the game: It generally buys rights to the royalties on drugs that have already been approved for sale by regulators. It buys them from patent holders, often hospitals or universities, that want to convert their future royalties into cash right now.