2-wire 4-20mA sensor transmitters: Increasing Sensor Power with Switching Power Supplies (Part 6)

In part 5 of this blog series, I discussed the construction of an input-isolated 2-wire sensor transmitter. Input-isolated 2-wire transmitters require isolating the sensor power and data signals from the 2-wire loop supply and 2-wire loop return (RTN). The main challenge when creating input-isolated or other complex 2-wire sensor transmitters is keeping the total current consumption from the loop to less than 4mA, a requirement I explained in part 3. One way to increase the available current in a 2-wire transmitter is to use a switching step-down DC/DC converter. The step-down converter must be optimized for low-output current applications so that it can operate at high-enough efficiencies to be useful in the system. Figure 1 is a simplified block diagram of such a system.

Figure 1: Simplified circuit of 2-wire sensor transmitter with step-down DC/DC converter to increase current available for the sensor

With a step-down power converter in the system, the maximum sensor current, I_SENSOR, can be increased based on the converter efficiency, η, the output voltage, V^­­_OUT, the input voltage, V_IN, and the maximum I_RET current of 3.8mA, as defined in Equation 1:

Assuming an efficiency of 80%, an output voltage of 3.3V and an input voltage of 8V, the maximum sensor current can be increased to 7.36mA, an increase of almost 100% compared to the original maximum current of 3.8mA.

You must take special care when designing 2-wire transmitters with switching converters because the high-impedance loop power supply can negatively interact with the incremental-input impedance of the switching converter, causing loop stability and compliance issues (as explained in this DC/DC converter reference design. Therefore, isolated and nonisolated step-down converters for 2-wire transmitters are commonly designed with unregulated fixed-frequency charge pumps (such as the design shown in part 5 and explained in detail in these two reference designs:

1)      Uniquely efficient isolated DC/DC converter for ultra-low power and low power applications reference design (TIDA-00349).

2)      Isolated ultra-low power reference design for 4 to 20 mA loop-powered transmitters (TIDA-00167).

So to summarize, you can add switching step-down power converters to 2-wire 4-20mA sensor transmitters to increase the available sensor current. As I mentioned, you should construct the design carefully to verify that the operation of the step-down converter does not negatively interact with the input protection circuitry or impact the stability of the 4-20mA control loop.

This is the final post in the 2-wire transmitter series, but in future posts I’ll discuss 3- and 4-wire transmitters, expanding on this series as well as the information in fellow blogger Kevin Duke’s posts.