If the ambient temperature is high, do I have to reduce the current capacity?
Yes. Since the RDS(on) value of semiconductors rises with temperature, more loss occurs at the same current and the junction temperature climbs quickly. In hot environments the operating current has to be brought below the driver's continuous rating. No numerical derating curve or operating temperature range is given in the product data for AXI modules.
The mechanism works like this: when the ambient temperature rises, the board's starting temperature rises too. At high temperature the MOSFET's RDS(on) value increases, the loss grows at the same current, and as the loss grows the temperature climbs further still. This is a self-feeding loop and it consumes the safety margin quickly.
Practical measures:
- Reduce the current. In a hot environment keep the continuous operating current noticeably below the nominal.
- Leave air circulation. Do not squeeze the module into a closed, airless volume; provide ventilation inside the panel.
- Keep it away from heat sources. The motor body, the exhaust and metal surfaces in the sun are not suitable for close mounting.
- Thin out the duty cycle. In applications with back-to-back starts, pause so the module can cool.
- Raise the voltage. Using 24V for the same power halves the current and cuts the loss to a quarter.
Since the operating temperature range and the derating curve for the ST42, KS250 and PT500 are not found in the product data, no percentage derating figure can be attributed to these modules. If you have an application with demanding temperature conditions, describe the operating conditions and consult the support team.
Full topic: DC Motor Driver Selection Guide