While our team was testing the performance of the power supply's output voltage, the 12 and 5V linear regulators became too hot and entered thermal shutdown. To note, the 3.3 and -5V linear regulators have no issues supplying their respective currents (~50mA, and -35mA).
These tests were conducted by attaching power resistors to each voltage rail, which provides all the outputs of the power supply their own current draw. Each of the output voltages are then measured across the load resistances.
This practice is used to reduce the possibility of damaging the power supply, or other parts of the subsystem, acting as a real-world simulation of how the power supply will behave when the other subsystems, having equivalent loads, are connected to the power supply.
Due to the thermal shutdown the team redesign the circuit, initially, using two 12V linear regulators in parallel, one driving the 3.3V and one for the 5V regulators. The thought behind this was to spread out the load, but unfortunately this also failed almost immediately. The video below shows the thermal shutdown from the 5V linear regulator, displayed on the oscilloscope and current drop value can be seen from the multimeter on the bottom left.
At this point it was obvious that the +-24V, 2.7A power supply, that is being modified to provide multiple DC outputs that are required to work for the design, is what's generating too much heat for the 12 and 5V, 1.5A linear regulators and need a higher rated regulator of at least 3 amps.
Using the switching regulators, that were originally bought for the battery power supply, can supply up to 3 ampere of current each and reach significantly lower temperatures under normal operation. The LM2576 buck regulators were used to replace the 12V and +5V linear regulators. This was a major lesson learned and may have been overlooked and possibly been avoided with more patients to detail, rather than, what seems to be a rush in production.
Switching Voltage Regulator:
The LM2576 switching regulator have a specified 3A output current. The data sheet states how this series is considered a high efficiency replacement for the typical 3-terminal linear regulators, just like the kind that we were originally using in the first power supply design. These are far more temperature efficient, and substantially reduce the size of the heat sink, with some cases not requiring a heat sink at all.
Replacing the 12 and +5V linear regulators, with the 12 and +5V switching regulators proved far more efficient. Test results show the outputs are now working as expected. When testing for over 45 minutes their temperature barely changed from room temperature.
The linear regulator's that were not changed, the 3.3V and -5V, do get slightly warm but this normal for linear regulators if they stay within their limits. After being used for 45 mins, they also stay well below their thermal limits so changing them was not necessary. [1]
Linear Voltage Regulator:
Shown below, is a typical linear regulator schematic, and the exact kind used for the 12 and +5V regulators used in the initial power supply design. All of the linear regulators that are used have an output current up to 1.5A and was initially overlooked when choosing the 12 and 5V regulators. [2]
Thorough analyzation of all component specifications and pupose should always be carefully considered when choosing components and their values, but this can still be overlooked and should always be doubled checked by both teammates to try and prevent situations like this.
Even when simulations work in Pspice of a particular component, such as the regulators shown above, they may not operate efficiently if it is not noticed that they will be used close their limit in the real-world. If this is not taken into careful consideration at the time and recognized to go up or down in rating, so that its more suitable for its purpose, will usually result in time lost from troubleshooting what the problem is.
[1] Texas Instruments, “LM2576xx Series Simple Switcher® 3-A Step-Down Voltage Regulator,” LM12576 datasheet, June 1999 [Revised May 2021].
[2] Texas Instrument, “Fixed Positive Voltage Regulators,” μA7800 datasheet, May 1976 [Revised August 2021].