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Posted on 01 December 2019

High-Linearity Analog Optocouplers

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Extend Working Insulation Voltage to 1.4 kV

High linearity analog optocouplers provide the versatility required to meet a wide range of analog isolation needs. For designers of high voltage applications, high linearity analog optocouplers can reliably send analog signals across very high voltage area and low voltage area without distortion.

By Chen Hong Lei, Avago Technologies

 

This article examines the internal operation and servo control mechanism of high linearity analog optocouplers in detail. Application examples are also presented, ranging from motor control current sensing to traditional current loop communication in process control.

Selecting the Ideal Optocoupler for High Voltage Applications

Standard digital optocouplers have long been used to address the optoisolation needs in high voltage applications. Combining a digital optocoupler with signal processing circuitry meets the need of high voltage isolation, but this complicates the design and is not suitable for applications that require analog in and analog out. Some linear optocouplers available in the market do provide analog isolation, however they fail to deliver necessary performance such as linearity, gain accuracy, along with high enough working insulation voltage. Isolation amplifier optocouplers can also be considered, but designers must consider the trade off between cost and performance [1].

An analog optocoupler with high linearity is ideal to isolate analog signals in a wide variety of applications that require excellent stability, linearity and bandwidth. An optimally designed circuit is capable of handling different type of signals including unipolar/bipolar, AC/DC and inverting/noninverting. Certain applications require very high isolation voltage. For example, in motor drive high side current sensing and phase current sensing applications, the working voltage could be as high as 1 kV. An optocoupler needs to be specially constructed to work under such harsh conditions. The following examples use Avago Technologies’ HCNR200/201 optocouplers to illustrate the wide range of isolation applications that can benefit from high linearity and up to 1.4 kV working insulation voltage.

Current Sensing and Voltage Monitoring Applications

High linearity is critical for current sensing and voltage monitoring in various application areas, such as motor control drives, switching power supply feedback loop, and inverter systems. As part of the motor control drives, variable-speed motor drives are finding increasing applications not only in industrial applications but also home appliances. Among the key components such as IGBT/ MOSFET, gate drivers, and of course the microcontroller unit (MCU), analog current and voltage sensors are critical to feed back to the MCU for stable and protected system control. Because of the presence of high voltages, it is necessary, and often mandated by safety and regulatory agencies, that people operating the motors and low voltage digital electronics are protected through galvanic isolation. An optocoupler with very high insulation voltage (5 kVrms/1 min rating) is required to handle DC bus voltage monitoring, DC bus current sensing, and AC phase current sensing, as well temperature and positioning sensing.

Figure 1 shows these applications (framed in the box named Analog Isolation Block) in a typical motor drive block diagram [2]. From this figure, one can figure out resistors R2 and R5 are used to measure the HV DC bus voltage and DC bus current respectively, while resistors R3 and R4 are used to measure motor phase current. Parameters such as temperature and position can be sensed by appropriate sensors attached to the motor, whose output is fed to another Analog Isolation Block. All the parameters are then transferred across the isolation barrier and collected by MCU. Figure 2 A and B [3] show a simplified schematic of the Analog Isolation Block for unipolar input and bipolar input circuit respectively, which are discussed in next section.

A typical motor drive block diagram

Theory of Operation

The operation [3, p.15] of the circuit may not be immediately obvious just from inspecting Figure 2A, particularly the input part of the circuit. The op-amp always tries to maintain the same inputs voltages at its two inputs in a linear feedback close loop connection. Thus, the input side op-amp A1 always tries to place zero volts across the photodiode PD1. Now, if some positive voltage VIN+ is applied at the input, the op-amp output would tend to swing to the negative rail causing the LED current to flow. This VIN+ will cause a current flowing through R1, and the LED light output will be detected by PD1 and generates and a current IPD1 flowing from the “+” terminal to GND1. Assuming that A1 is a perfect op-amp, no current flows into the inputs of A1; therefore, all of the current flowing through R1 will flow through PD1. Since the “+” input of A1 is at 0 V, the current through R1, and therefore IPD1 as well, is equal to VIN+/R1, or IPD1 = VIN+/R1.

Simplified schematic of the Analog Isolation Block for (A) unipolar input, and (B) bipolar input

Notice that IPD1 depends ONLY on the input voltage and the value of R1 and is independent of the light output characteristics of the LED. As the light output of the LED changes with temperature, amplifier A1 adjusts IF to compensate and maintain a constant current in PD1. Also notice that IPD1 is exactly proportional to VIN+, giving a very linear relationship between the input voltage and the photodiode current. The relationship between the input optical power and the output current of a photodiode is very linear. Therefore, by stabilizing and linearizing IPD1, the light output of the LED is also stabilized and linearized. And since light from the LED falls on both of the photodiodes, IPD2 will be stabilized as well.

Since PD1 and PD2 are identical to each other, IPD2 shall be equal to IPD1 ideally, while being varied by a coefficient K3 in reality. So we have IPD2 = K3 x IPD1,

where K3 is the transfer gain defined in the data sheet (K3 = IPD2/IPD1 = 1). Amplifier A2 and resistor R2 form a trans-resistance amplifier that converts IPD2 back into a voltage, VOUT, where VOUT = IPD2 x R2.

Combining the above three equations yields an overall expression relating the output voltage to the input voltage, VOUT/VIN+ = K3 x (R2/R1).

Therefore the relationship between VIN+ and VOUT is constant, linear, and independent of the light output characteristics of the LED. The gain of the Analog Isolation Block circuit can be adjusted simply by adjusting the ratio of R2 to R1.

Figure 2A is in a unipolar configuration that accommodates only positive voltage input. Figure 2B is configured to accommodate bipolar input (a signal that swings both positive and negative). Two current sources, IOS1 and IOS2, are added to offset the signal so that it appears to be unipolar to the optocoupler. Current source IOS1 provides enough offset to ensure that IPD1 is always positive. The second current source, IOS2, provides and an offset to obtain a net circuit offset voltage of a desired value (e.g., a 0 V may be desired if both positive and negative power supplies are used, whereas a midway voltage could be more appropriate for the case of single positive power supply circuit). Current sources IOS1 and IOS2 can be implemented as simply as resistors connected to suitable voltage sources. A note is that the offset performance is dependent on the matching of IOS1 and IOS2 and is also dependent on the gain of the optocoupler.

Current Loop Communication Application

In the process control industry, current loops have become the standard method for sensor signal transmission [4]. This method is especially useful for long distance transmission (up to 10 km). Current loop is a very flexible communication interface. There are a couple of types of current loops: analog (linear current represents analog signal), logic (high and low logic levels represent MARK and SPACE states), and combined analog and digital current loop that uses HART® (Highway Addressable Remote Transducer) communication protocol. Comparing to voltage signals, current loops have the following benefits:

* Insensitive to noise and are immune to errors from line impedance
* Long-distance transmission without amplitude loss
* Inexpensive 2-wire cables
* Lower EMI sensitivity
* Detection of offline sensors, broken transmission lines, and other failures

Adding insulation to the 4-20mA current loop is important to protect system electronics from electrical noise and transients, which commonly present in the industrial processmonitoring applications. The insulation barrier allows transducers to be galvanically separated by hundreds or even thousands of volts. Avago’s HCNR200 and HCNR201 offer the highest level of safety and regulatory performance available today, which make them suitable for these applications. An example block diagram of a 4-20 mA analog current loop transmitter and receiver is shown in Figure 3 and 4 [3, Figure 21, 22], respectively. Avago also offers optically coupled 20 mA current loop transmitter and receiver (HCPL-4100 and HCPL-4200) for systems using the 20 mA logic current loop [5, 6].

Block diagram of a 4-20 mA analog current loop transmitter

Elements and package construction

The superior performance and the design flexibility of high linearity analog optocouplers make it increasingly adopted for a wide variety of applications. Many new high linearity optocouplers are on the market today that tout similar benefits, but can make it a daunting selection task for designers. Some optocouplers consist of LED and PIN photodiodes, while other products are built with LED and photo-transistors. All of them offer similar element arrangements to utilize the servo-feedback advantages for better linearity performance.

Block diagram of a 4-20 mA analog current loop receiver

Avago Technologies’ high-linearity analog optocoupler consists of a high-performance AlGaAs LED that illuminates two closely matched photodiodes PD1 and PD2, as shown in Figure 5A. The input photodiode PD1 can be used to monitor, and therefore stabilize, the light output of the LED. As a result, the nonlinearity and drift characteristics of the LED can be virtually eliminated. The output photodiode PD2 produces a photocurrent that is linearly related to the light output of the LED. The close matching of the photodiodes and advanced design of the package ensure the high linearity and stable gain characteristics of the optocoupler [3].

All these advanced elements are housed in a unique widebody package (see Figure 5B). Avago has designed the HCNR200/201 with 400 mil lead spacing, 1 mm internal clearance (through insulation distance), 10 mm external creepage, and 9.6 mm external clearance. It is able to satisfy demanding external creepage and clearance requirements. These parts come with worldwide safety approvals including CSA, UL 1577 recognition of 5 kVrms/1 min rating, and the IEC/EN/DIN EN 62019-5-2 working insulation voltage of 1414 Vpeak. These devices are suitable for not only applications that require reinforced insulation but also failsafe designs.

Schematic of HCNR200-201 (A) and the Widebody package (B)

Linear Input Range

In addition to linearity performance, a final point of consideration during component selection is the circuit’s linear input range (LIR). A circuit’s LIR determines the input signal dynamic range that can enjoy the linearity claimed on the sheet, which is in turn determined by a particular optocoupler’s linear response range specified in its data sheet. For example, on the HCNR200 and HCNR201 data sheet, it is specified that the HCNR200’s “DC NonLinearity (Best Fit)” has a typical value of 0.01% and a maximum value of 0.25% under “Test Conditions” of “5 nA < IPD < 50 mA, 0 V < VPD < 15 V” [3, p.7]. Test conditions of photodetector current or worked-out photodetector current (when LED current is specified) in respective data sheet are used to calculate LIR of the circuit.

Assumption of application circuit topology is made to reach a comparison of LIR for various linear analog optocouplers from different vendors. In this case, the application circuit shown in Figure 2A has been used to calculate the LIR of input voltage. From the comparison chart shown in Figure 6, it can be seen that the HCNR200/201 has a much wider linear response range, which means a circuit applying HCNR200/201 enjoys a much wider linear input voltage range than its counterparts (60dB wider than that of Comp A, and 66dB wider than that of Comp B).

Comparison of different optocoupler’s linear input range

Summary

In a typical high-linearity analog optocoupler application, an external feedback amplifier can be used with PD1 to monitor the light output of the LED and automatically adjust the LED current to compensate for any nonlinearities or changes in light output of the LED. The feedback amplifier acts to stabilize and linearize the light output of the LED. The output photodiode then converts the stable, linear light output of the LED into a current, which can then be converted back into a voltage by another amplifier. By appropriate design of the application circuit, these wellestablished and versatile analog optocouplers are capable of operating in many different modes to meet various analog isolation needs.

 

 

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