  We study a model of isothermal steady-state work-to-work converter, where a
single quantum two-level system (TLS) driven by time-dependent periodic
external fields acts as the working medium and is permanently put in contact
with a thermal reservoir at fixed temperature $T$. By combining Short-Iterative
Lanczos (SIL) method and analytic approaches, we study the converter
performance in the linear response regime and in a wide range of driving
frequencies, from weak to strong dissipation. We show that for our ideal
quantum machine several parameter ranges exist where a violation of
Thermodynamics Uncertainty Relations (TUR) occurs. We find the violation to
depend on the driving frequency and on the dissipation strength, and we trace
it back to the degree of coherence of the quantum converter. We eventually
discuss the influence of other possible sources of violation, such as
non-Markovian effects during the converter dynamics.
