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The datasheet for the DS18B20 says:
To assure that the DS18B20 has sufficient supply current, it is necessary to provide a strong pullup on the 1-Wire bus whenever temperature conversions are taking place or data is being
copied from the scratchpad to EEPROM.
I use w1-gpio and w1-therm, how can I configure them in a way that I have an active pullup? Is it enough the output "high" on the GPIO pin or do I need to put a transistor onto my brealout board?
The DS18B20 needs at max 1.5mA for a conversion. Simultaneous conversions on several DS18B20 aren't allowed in parasite powered mode, so that's the maximum current you have to provide.
Use a 1kΩ pullup resistor to 3.3V, that provides up to 3.3mA and is sinkable by both the host and the DS18B20 without problems. You never run into the problem described in the datasheet.
The only reason you want to use the "strong pullup" (and the pullup option of the w1-gpio overlay) is minimizing power consumption, getting rid of these 3.3mA. If you wanted this, you had to connect the 1kΩ resistor to that specified pullup pin instead of +3.3V.
