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How to Simplify Isolated 24 V Digital Input Module Designs While Reducing Power Dissipation: A new. less complicated design approach to digital input modules features several advantages over conventional methods.

Programmable logic controllers (PLCs) and motor drives use digital input receivers to obtain 24 V digital inputs from field sensors, switches, position encoders, and other devices. The digital input receiver compares the incoming signal with a certain threshold, and provides logic high or low output to a host controller. Most receivers implement current limits to reduce draw from the 24 V inputs, reducing power dissipation, and allowing high-density, multi-channel designs.

The International Electrotechnical Commission (IEC) 61131-2 standard for PLCs specifies three types of digital input receivers: Types 1, 2, and 3 (see Figure 1). The types of receivers vary by the input voltage transition thresholds, along with minimum and maximum current draw in the on and off states. The voltage thresholds are determined by noise immunity, variation in the 24 V supply level, and resistive drop in the input cables. The current levels are determined by the bias current needed by the transmitting switch or sensor in the on and off states. The IEC 61131-2 standard allows a current draw up to 15 mA for Type 1 and 3 and 30 mA for Type 2 in the on state, but a current draw as close to the required minimum will reduce power dissipation.

Common Digital Input Implementations

Figure 2 shows the two most common implementations of digital input receivers. In Figure 2a, resistors R1 and R2, in combination with the optocoupler characteristics, set the voltage thresholds and input current draw. The digital inputs are designed so that the required minimum current is drawn somewhere in the middle of the transition region (the white area between the gray on and off regions in Figure 1). However, the input current continues to rise until the field input voltage has reached its nominal level of 24 V. Moreover, the nominal 24 V field supply can vary 20 to 30 percent, so the maximum input power consumption actually occurs at 32 V. In the absence of a current limit, Type 3 inputs can easily draw up to 12 mA at a 32 V input. Similarly, a Type 2 input receiver designed to draw at least 6 mA at 11 V using the circuit shown in Figure 2a can draw close to 30 mA at 32 V.

The second implementation, shown in Figure 2b, uses several discrete components to implement a better current limit and controlled voltage thresholds. In this case, the stability of the current limit across component and temperature variations depends on the complexity of the external circuits. While achieving lower power dissipation than the approach shown in Figure 2a, this implementation can take up a lot of space on the board.

For both approaches shown in Figure 2, a Schmitt trigger buffer placed after the optocoupler provides hysteresis for noise immunity.

New Solutions for Digital Input Modules

Figure 3 presents a new approach with an adjustable current limit, precise voltage comparator with hysteresis, reverse-polarity protection, and galvanic isolation are combined in a CMSOS semiconductor integrated circuit (IC). The power required for operation of the "field side" of the IC is derived from the field input. The current limit is adjustable through an external RSENSE and the input voltage transition thresholds are adjustable by controlling the voltage drop across resistor RTHR. The independent current limit and voltage-threshold adjustment enable compliance to IEC 61131-2 Type 1, 2, and 3 characteristics, or any other custom specification.

Compared to traditional approaches using optocouplers, designing with a CMOS-based integrated solution as shown in Figure 3 greatly eases system design while bringing many performance advantages, including:

Ease of design and guaranteed switching characteristics. With a CMOS integrated solution, the voltage thresholds and current limits are well controlled, and guaranteed in the IC data sheet.

Lower power dissipation and thermal management. Implementation of a precise limit can reduce the current drawn from digital inputs up to a factor of five, reducing power dissipation. As shown in Figure 4, a current limit can draw just the minimum current required for the field sensors to operate correctly, avoiding a linear increase in current at higher input voltages. The lower power dissipation directly results in lower board temperatures, as shown in Figure 5.

Smaller boards and modules. Integrated current-limit solutions reduce component counts, leading to more compact boards. More importantly, the lower power dissipation and distribution of heat on the board enable designers to pack more channels into a smaller space.

Controlled thresholds. CMOS-based integrated solutions have very well-controlled transition thresholds, and sufficient margin to low and high levels, leading to better noise immunity (see Figure 6).

Greater reliability. Optocouplers have higher statistical failure-in-time (FIT) rates than CMOS ICs. Using several additional components (for a discrete current limit, for example) increases system FIT rates further. CMOS integrated semiconductor solutions have very low FIT rates--typically two orders of magnitude lower--and do not show significant change in properties during their operational lifetime. Integrated solutions are thus preferable for systems that have a greater need for reliability.

Other Digital Input Receivers with an Integrated Current Limit. Digital input receivers that offer an integrated current limit and higher channel count in a single package are also used in industrial control.

This approach is simpler, and has these advantages over existing serializer solutions:

* No need for field-side power supply. Serializers use a field-side power supply to power the integrated current-limit circuits.

* Integrated isolation. Serializers with an integrated current limit are typically not isolated, and need an additional digital isolator.

* Channel independence. In the approach presented here, damage to one channel on the field side (say, due to a short circuit) does not impact any of the other channels. This is not the case with serializers.

* Higher speeds. Serialization in multichannel devices limits speeds to <20 kHz, whereas independent receivers can support up to 2 MHz of clocking.

By Anant S. Kamath, Systems Engineer, Texas Instruments
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Title Annotation:TECH FOCUS: POWER
Author:Kamath, Anant S.
Publication:ECN-Electronic Component News
Date:Mar 1, 2018
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