Power Supplies for Testing and Measurement by Recom

September 17, 2022

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Testing and measurement (T+M) covers a wide range of applications but basically involves detecting a physical variable (current, voltage, temperature, etc.) under various operating conditions (normal, at the operating limits, or over-stressed operation) and recording and analysing the result.
Often, measurement isolation from the ground is required because the part being tested or measured is, or can be, at a different ground reference voltage from the recording or analysing equipment. When the testing and measurement equipment is line-powered, an AC/DC converter provides isolation and may be followed by a DC/DC converter to provide galvanic isolation between separate power rails for inputs of multi-channel or multi-parameter T+M equipment. This is to avoid cross-talk or interference between the signal inputs. DC/DCs may therefore be necessary even if the measurement unit itself is battery-powered.Analog input test and measurement sensors commonly require dual rail supplies to power the operational or precision signal amplifiers. The amplified and conditioned output is typically fed into Analog-to-Digital converters and the data stream is then isolated using Digital Isolators. ADCs and DIs require independent 3.3V or 5V rails dropped down from the main 12V or 24V supply rail.
Finally, digital bus isolators (USB, CAN, GPIB, etc.) are often essential to avoid ground loops, for safety, or to ensure single fault tolerance in the T+M system.

What kind of AC/DC converters are required for T&M systems?

Line-powered testing and measurement equipment can range from desktop products to rack installations with input voltage varying from nominal 90VAC to 277VAC, or even higher for industrial applications. At low power, from 3W to 20W, PCB-mounted parts are often used such as the RAC03-K, RAC04-K/277, RAC05-K/480, RAC10-K/277, and RAC20-K. At higher power, from 40W to 550W, chassis-mounted parts RACM40-K, RACM60-K, RACM90-K, RACM230-G, and RACM550-G might be chosen. Fan-less operation is often desirable in T+M environments and all these parts can give useful power to high temperatures without forced air, with the 230W and 550W products additionally able to operate with baseplate cooling. Depending on the product, safety certifications are available for different end-application environments, IEC/EN 61010, IEC/EN 62368, IEC/EN 60601, and EN 60335 as examples. EMC standards are met without additional components and all parts have low mains leakage current which can be critical in T+M applications, particularly in medical environments. Similarly, for sensitive measurements, the low output noise of the RECOM AC/DC power supplies can be a benefit.

What kind of DC/DC converters are required for T&M systems?

High isolation
Bus power rail isolators
Low noise
Specialist converters with very low input current
The high input voltage, low output voltage
Single supply, dual output voltage

High isolation converters

RECOM offers very high isolation DC/DC converters in compact case sizes. Our exceptional RHV2/RHV3 series offers 20kVDC isolation (12.5kVAC/1 minute) in a SIP16 case for very high isolation power supply measurement technology in applications such as hard vacuum ion pumps, X-ray equipment, and high voltage dust filters.

Our REM series offers up to 10kVDC (5kVAC/1 minute) isolation and is certified to withstand continuous 230VAC across the isolation barrier. Output powers from 1W up to 30W are available. An example application is isolation for temperature measurement thermocouples that are attached to transformers, transistors, diodes, or capacitors that operate at mains voltages.

For testing and measurement applications that require functional isolation to avoid ground loops or interference in low voltage systems, the RKE series offers exceptional value-for-money with 3kV, 4kV or 6kV isolation options.

Bus power rail isolators

Industrial CAN-bus, RS-485 (ModBus) or GPIB systems are often used to interconnect sensors, relays, oscilloscopes, and programmable power supplies/loads to create Automatic Test Equipment (ATE) or SCADA (Supervisory Control and Data Acquisition) systems. Such systems may have significant potential differences between grounding points, particularly with high voltage or high current applications or long cable runs. As shown below, an isolated serial interface, CAN-Bus transceiver or USB isolator eliminates the effect of potential differences between sensor units.

An isolated data bus transceiver requires isolated power for the serial communication bus, commonly provided by an isolated DC/DC converter placed across the isolation gap and powered from the input supply. Typical input and output voltage combinations are 3.3V to 3.3V, 5V to 5V or 3.3V to 5V.

As Shown, RECOM can supply a ready-built evaluation board (R-REF03-CAN1) to allow quick prototyping of CAN-bus systems.

Low noise converters

Many sensors deliver low-level outputs; a type K thermocouple has a thermoelectric voltage of just 1mV at 25°C for example. High-gain thermocouple amplifiers are susceptible to common mode noise, so need very quiet power supply rails. A typical unregulated isolated DC/DC converter will have 60-100mV p-p output ripple, which may interfere with some measurements. As a solution, RECOM offers post-regulated DC/DC converters in the same pinout as standard converters which combine the advantages of an isolated DC/DC converter with the voltage stability and low noise output of a linear regulator. For example, the RIZX-0505 offers a regulated 5V output with only 30mV p-p ripple. If this is still too high, a simple input and output filter can be used to reduce the output ripple to only 2mV p-p to create a very low noise (-68db) isolated supply.

The high input voltage, and low output voltage converters

While linear regulators are low-cost and reliable components for dropping a higher input voltage down to a lower supply suitable for TTL or CMOS components, the efficiency is very poor which limits the output current available before power dissipation becomes too high. Switched mode DC/DC converters are power converters, meaning that a 24V input, 5V/1A output converter will only draw a little more than 208mA from the primary supply – less than one-quarter of the current that would be drawn by a linear regulator. So, for multiple independent channel T+M equipment, it makes sense to use switching regulators or isolated DC/DC converters to efficiently power each input channel.

Single supply/dual output converters

A DC/DC converter is often the simplest, cheapest, and most efficient way to generate a bipolar rail power supply. RECOM offers a very wide range of dual output (±5V, ±12V or ±15V) converters suitable for the power rails of operational amplifiers. Full load output power rating is as low as 250mW for very low power consumption designs up to 40W to supply large multi-channel systems.

An application example is a high-side current sense circuit. As the shunt resistor is connected to a high AC or DC voltage, an isolated op-amp is used to measure the voltage drop across it. The isolated bipolar supply for the op-amp is floating so the common (0Viso) can be connected directly to the op-amp input.

Courtesy: Recom