AN-60-110 Rev: A DCO-001385 (02/13/24) File: AN-60-110.docx
This document and its contents are the property of Mini-Circuits. Page 1 of 9
Introduction
Depending on the end system design or architecture, the ZHL-2425-250X+ power amplifier module can
be either water cooled or air cooled. Water cooled systems have superior cooling capabilities, but the
requirement of a water chiller can be a large and expensive proposition for the evaluation of a single
unit. Instead, a heat sink and attached fan can provide a more economical and less bulky cooling
solution for single unit evaluation.
Mini-Circuits provides the ZHL-2425-250X+ as an amplifier pallet “without a heat sink”leaving the end-
user to decide what type of cooling they want to use.
This application note describes how to mount the ZHL-2425-250X+ to the Mini-Circuits heatsink (HSK-
2425-250+). This is a heat sink that is designed specifically for the ZHL-2425-250X+ amplifier and includes
mounting holes for attaching a 60mm DC Fan to one end for forced-air cooling. Recommendations are
provided for all commercially available tools and accessories not currently provided by Mini-Circuits.
Importance of the Heat Sink
The ZHL-2425-250X+ can generate and dissipate a significant amount of power. At the nominal
operating mode of 250W output and assuming 50% efficiency, 250W is dissipated in the amplifier and
up to 200W (max reverse power limit) of reflected power is dissipated in the load for a max total of
450W dissipated power. The max operating temperature (65°C) allows for a 40°C temperature rise from
a room temperature of 25°C. By making a few simplifying assumptions, we can calculate that the
effective thermal resistance of the heat sinking solution from base plate to ambient must be less than
40°C/450W or 0.089°C/W. This thermal resistance cannot be achieved by any reasonably sized heat sink
by the passive effect of convection alone. Indeed, some powerful fans are needed in order to cool the
amplifier sufficiently in its max dissipation state. The thermal resistance requirement can be relaxed
slightly if the amplifier load is well matched or if the amplifier is not operating at full power. In any case,
it is critical that the amplifier is always mounted to a heat sink with a fan and its airflow adjusted to keep
the amplifier below 65°C at full RF power when operating. Otherwise, the amplifier will get too hot, the
built-in protection alarms will be activated, and the power amplifier will shut itself down.
