This word uses the on'yomi readings 頻 (ヒン) and 繁 (ハン). The 'ハ' sound of 繁 undergoes rendaku (連濁) to 'パ' in this compound. So, ひんばん is incorrect. It means occurring often or frequently.

頻繁は頻(ヒン)と繁(ハン)の音読みで、連濁により「ハン」が「パン」となります。「ひんばん」は誤りです。物事が繰り返して、しばしば起こる様子を表します。