Apparatus, consisting of a probe with a temperature sensor and a transducer in contact with a living subject's tissue. A power supply delivers electrical power to the transducer for tissue ablation. A controller receives a signal from the temperature sensor and in response outputs a tissue temperature. During a first time period the power supply delivers no more than a first target power to the transducer, and reduces the power when a first maximum allowable temperature of the tissue is exceeded. During a transition time period the power supply delivers no more than a second target power, while reducing the delivered power when the first maximum allowable temperature of the tissue is exceeded. During a second time period the power supply delivers no more than the second target power, while reducing the delivered power when a second maximum allowable temperature, less than the first maximum allowable temperature, is exceeded.
