Akshay Mehta

It’s not unusual to find industrial automation control equipment like field sensors (for proximity, pressure, flow or temperature, for example) housed in increasingly inconspicuous packages. Figure 1-1 shows an example of a proximity sensor housed in a tiny screw (which can be as small as 8 mm or in some instances even smaller). While the electronics that go into the housing have to be ultra-small, they still need to be rated for the right parameters to ensure long-term equipment reliability.

Addressing the challenges of unregulated input voltages

Electronics in smart sensors may include a low-power microprocessor, analog-to-digital signal-processing circuitry and a voltage regulator. Sometimes, designers might not consider power until the end of the design process; therefore, the space allotted to power can be really sparse.

A typical application for power-conversion circuitry in factory automation equipment might be down-converting a 24-V input voltage and regulating it to the required potential. The 24-V input voltage might be a standard industrial bus which could have an input voltage as low as 8 V or as high as 36 V.

Sensors in process automation need to withstand overvoltage transients. The severity of voltage transients depends on how the 24 V was derived. In some situations, where the lead lengths cover long distances in the field, the industrial 24-V bus may experience higher voltage transients. In addition, field applications could have clamp circuits that limit the voltage transient to a safe extra-low voltage limit. This means that in a short-circuit condition; the output of the industrial bus could be stuck at 60 V_DC.

Considering another example, a 24 V_AC could be an input source. The maximum root-mean-square voltage of such a supply can often swing to 28 V, leading to a peak voltage delivered that’s close to 40 V. The overvoltage protection setting on the input supply could be about 120% of the peak voltage, which takes an input voltage close to 48 V.

In these examples, it is important to use a voltage regulator rated for the maximum DC voltage at the input to ensure uninterrupted operation of equipment like field sensors.

Ensuring appropriate pin spacing for long-term reliability

The TPSM265R1 is a high-voltage embedded power module rated for 65 V that integrates the voltage regulator and inductor. This gives the device enough margins to cover potential overvoltage conditions. However, just because a power module has an integrated high-voltage regulator doesn’t mean that you can safely rate it for the high voltage. To properly rate the device for a higher input voltage requires appropriate design of the pad/pin spacing on the module. The Association Connecting Electronics Industries has a standard called IPC 2221-B which goes over this in detail.

The IPC-2221B standard lists the minimum spacing between the edges of conductors that is required to ensure proper operation at worst-case voltages. To ensure that the regulator can indeed sustain a peak voltage of 65 V the spacing between the edges of the conductors must be at least 0.5 mm.

Figure 2 shows the pin pitch of the TPSM265R1, which is 0.8 mm; the width of the pad is typically 0.3 mm. From edge to edge, the space between the pads is 0.5 mm, in compliance with the IPC-2221B requirement.

Meeting the requirement for a small solution size

IPC spacing requirements put system designers at odds with the requirement for miniaturization. After all, I began this technical article talking about how electronics need to fit into an 8-mm screw. To address both spacing requirements and miniaturization requirements, designers have to be smart about designing the power module. TI designed the layout of the TPSM265R1 package with a small-solution space in mind. Figure 1-3 shows an example layout, with the minimum required components for a fixed-voltage option of 3.3 V or 5 V.

The module package size is 2.8 mm by 3.7 mm. To complete the circuit, you only need two capacitors. With two 0805 capacitors, the overall solution size for a 5 V or 3.3 V output can be as small as 3 mm by 7 mm. The placement of the input capacitor so close to the module’s VIN and GND pins makes for a “quiet” switching operation, with less ringing on the switch node.

When designing the power supply for small industrial automation control equipment, it’s important to choose products with appropriate ratings for a reliable operation. Well-designed power modules will not only help engineers address size challenges and ensure long-term reliability but can also help simplify the design process.

Additional resources

• Learn more about the IPC organization and the standard for pin spacing.