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Temperature tests are performed on Roundabout PCB version 1.5 and version 2.0 (to be released with the second edition of Intermediate Robot Building). As previously discussed, PCB v2.0 has additional copper which we'll test to see if it helps the motor drivers run cooler.
Measuring motor driver chip temperature with thermistors embedded into DIP sockets on a Roundabout printed circuit board.
The motor drivers are IXYS IXDN404PI dual non-inverting power MOSFET driver ICs in a DIP package. They’re rated at 4 amps of peak current (to start a motor rotating), with a continuous output rating of 1 amp.
The motor drivers operate from 4.5 V to 35 V, with a typical voltage of 18 V. They’re driven by a 3 amp laboratory desktop power supply at 8.8 V (unloaded) to simulate a 9 V alkaline battery under load. A 33% reduction in resistance (and thus better performance) could have been achieved if the chips were run at their typical voltage (18 V), but that would not match most hobby robots.
The motor drivers are tested with a single chip, two chips in parallel (Dual), and two chips in parallel with a heat sink made of copper foil (Dual HS).
Three thermistors are monitored in parallel by the temperature testing board:
In all tests, the room temperature measured 73.2 to 74.2 degrees Fahrenheit, with 73.5 being the most common. For this reason, I believe that the slight changes in the room’s temperature did not affect the test results.
The testing process is:
This first graph shows five complete tests with a single motor driver chip on a v1.5 (no extra copper) PCB with loads:
Note: The actual current flowing through the circuit ends up lower than the maximum current stated above, due to the resistance of the motor driver chip.
Chart motor driver chip temperature by load.
Now let’s test variations of the circuit boards (v2.0 has more copper) and stacked motor drivers (dual, dual with heat sink) under the maximum load of 9.75 ohms.
Chart motor driver chip temperature based on single, dual, heatsink, and copper layer.
As you’ve seen, the filled copper areas do a good job of allowing heat to flow away from the chip under load. But, does this heated copper have a measureable negative influence on other components on the board?
During the previous tests, the second motor driver chip was powered on but not commanded to supply power to a motor. The following graph shows the temperature of this idle chip.
To be fair, note that the vertical scale of this chart is much smaller than the previous chart. There is up to a one degree of difference in starting temperatures that may fool your eyes into thinking some peaks are higher than others.
Chart temperature of an idle single motor driver chip (B) when various motor driver arrangements are tested under load.
I didn’t expect to see any change in temperature for the idle chip. Yet, there is a multiple degree increase in temperature where the heat had to travel through the board and into the idle chip itself to be measured by the thermistor. Or, the surrounding board had to be radiating that much heat into the thermistor that is buried inside the DIP socket.
So what have we learned?
All of that being said, these results are largely academic for MOST hobby robots. The voltage drop and power usage of most electronics has decreased to the point that voltage regulators run fairly cool. And, if your motor driver is getting hot, you should switch to a highly-efficient power MOSFET H-bridge (see Chapter 9 of Intermediate Robot Building) or even a power bi-polar H-bridge instead of hacking with dual chips or heat sinks.
However, if your competitive robot needs a slight edge, or your existing design almost meets your needs, you now know how to get a little more out of your chips without too much extra effort.