Passivity-based control ensures the implementation of a desired behavior in a controlled system, while preserving robust stability. In this letter, we propose a new method to guarantee passivity, based on energy-tank control and Control Barrier Functions. The goal is to accomplish a desired behavior with minimal modification, while ensuring passivity. The proposed method is suitable for a wide range of applications: whenever some desired control action may disrupt the passivity of the system, the designed Control Barrier Function will modify the behavior to enforce the preservation of passivity. Simulations and real experiments were carried out to prove the effectiveness and the flexibility of the proposed method: in particular, a simple case of variable stiffness in a mass-spring-damper system and a multi-robot controller based on a time-varying artificial potential field.