How much does a driver heat up and how is the heat loss calculated?
Conduction loss is calculated with the relation P = I x I x RDS(on); for example at 15 A a 10 milliohm switch produces roughly 2.25 W of loss per switch. Since the loss grows with the square of the current, doubling the current quadruples the heat. RDS(on) and efficiency figures are not given in the product data for AXI driver modules.
In a MOSFET driver the heat comes from two sources: conduction loss and switching loss.
Conduction loss: P = I² x RDS(on). Example: for 15 A and 10 mΩ, 15² x 0,010 = 2,25 W (per switch). At the same resistance the loss rises to 9 W at 30 A: twice the current, four times the loss.
Switching loss: It grows with the switching frequency. That is why a high PWM frequency buys quietness at a cost in heat.
The most common mistake when calculating is to use the 25 °C RDS(on) figure from the datasheet. RDS(on) rises noticeably with temperature; the correct approach is to read the value at the operating temperature from the datasheet's normalized RDS(on) - junction temperature curve. Otherwise you will underestimate the real loss and the junction temperature.
Important note: the MOSFET type, RDS(on) and efficiency figures for the ST42, KS250 and PT500 are not found in the product data; for that reason no numerical loss figure can be attributed to these modules. The practical approach is to run the driver at 70-80% of its continuous current rating and to leave air circulation at the mounting location.
Full topic: DC Motor Driver Selection Guide