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 24V ±20% to 30% (a wide range of up to 17V to 36V, sometimes even wider) or lower voltages like 4.5V to 6V (for example, from the backplane or generated as an intermediate supply rail from the 24V)
• Output voltages: typical voltages like 3.3V, 5V, 12V, or even 24V, but also split rails like ±5V, ±15V , ±18V
• Output voltage accuracy: better than 3% to 5% desired, optocoupler-less designs preferred to reduce complexity and improve reliability
• Output power: up to 4W to 5W, sometimes up to 10W or above
• Size: small size designs needed, height often limited to a range of 4mm to 8mm
• Type of isolation: in most cases, functional isolation for breaking ground loops (1kV to 2.5kV 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 2W to 4W 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 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 2.95V given as V_IN minimum for the fly-buck topology is related to the minimum V_IN of the TPS55010, whereas the 120V 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 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.

Figure 1 Open-Loop LLC

Figure 3 Duty Cycle Controlled Push-Pull

Figure 5 Fly-Buck- Boost

Figure 7 Fully-Integrated Modules

Figure 2 Open-Loop Push-Pull

Figure 4 Fly-Buck

Figure 6 Primary Side Regulated Flyback