Designers of programmable logic controller (PLC) I/O modules or other industrial low-power applications have to find a reliable design for proper and safe operation of their designed circuits under the conditions of a harsh industrial environment. Isolated power converters are frequently found in such applications and help in the following:

• Avoiding or breaking potential ground loops
• Avoiding coupling between adjacent channels or modules
• Providing electrical safety

Common design requirements for such isolated power converters used specifically in PLC I/O module applications include the following, which are also similarly applicable to other industrial low-power applications:

• Input voltage: Field power voltages, often 24 V ±20% to 30% (a wide range of up to 17 V to 36 V, sometimes even wider) or lower voltages like 4.5 V to 6 V (for example, from the backplane or generated as an intermediate supply rail from the 24 V)
• Output voltages: typical voltages like 3.3 V, 5 V, 12 V, or even 24 V, but also split rails like ±5 V, ±15 V , ±18 V
• Output voltage accuracy: better than 3% to 5% desired, optocoupler-less designs preferred to reduce complexity and improve reliability
• Output power: up to 4 W to 5 W, sometimes up to 10 W or above
• Size: small size designs needed, height often limited to a range of 4 mm to 8 mm
• Type of isolation: in most cases, functional isolation for breaking ground loops (1 kV to 2.5 kV for a 1-second to 1-minute test), but also more stringent ones ranging from basic, up to reinforced insulation in cases when electrical safety is required
• Power efficiency: needs usually to be very high (80 to 90% or higher desired) to provide the lowest full-power losses due to the following reasons:
  • Small plastic housing, no forced air flow
  • Maximum ambient temperature of application in the range of 50°C to 70°C, expected ambient board temperature level in the range of 85°C to 105°C
  • Total power consumption per PLC module is often limited to 2 W to 4 W due to thermal restrictions based on the previously-listed items. The majority of this power is targeted for the payload. Additional power losses in the isolated power converters; therefore, need to be minimized.

Table 1-1 provides an overview of usable isolated power topologies and proposed TI devices addressing the aforementioned requirements.

All of the listed topologies are optocoupler-less approaches – although the underlying traditional topologies which are found in higher power designs are known to use optocoupler feedback.

The table groups the proposed topologies into the following categories:

• Non-regulated
• V_IN controlled
• Quasi-regulated
• Regulated

The provided minimum and maximum input voltage values (V_IN minimum, V_IN maximum) of the devices represent the best-case values of all the listed devices supporting a specific topology. The 2.95 V given as V_IN minimum for the fly-buck topology is related to the minimum V_IN of the TPS55010, whereas the 120 V given as V_IN maximum for this topology represents the maximum V_IN of the LM5168 and LM5169. Specific topologies like fly-buck-boost and primary side regulated flyback⁽³⁾ require an additional margin to be applied.

The given maximum output power (maximum P_OUT) is representing the capability of the most powerful device given for a specific topology and depends furthermore on the ratio V_OUT/V_IN and the turns ratio of the used transformer.

V_ISO stands for the isolation voltage of the used transformer and is often related to specific technical standards.