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Posted on 17 July 2019

Power Efficiency within Solid Isolation and Insulation Barrier

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Reduction in power requirements without compromising on noise isolation and high voltage insulation

Optocouplers are needed to provide high voltage insulation against transient voltage surges and to reject common mode transient noise from interfering with the input signals. Avago’s next-generation optocouplers, ACPL-1L/061L/064L/W61L/K64L(ACPL-x6xL) offer a significant reduction in power requirements without compromising on noise isolation and high voltage insulation performance. These new optocouplers provide up to 90% power saving compared to the standard optocouplers available today and 40% power saving compared to alternative isolators.

By Yeo Siok Been, Digital Optocoupler Product Manager, Avago Technologies

 

Power efficiency is one of the key design parameters that equipment designers are constantly striving to improve. There are four important reasons why the designers are continuously hunting for low power optocouplers:

a) Low LED forward current is needed so that it can be directly driven by most microcontrollers or ASICs (as shown in Figure 1), without needing external buffers.
b) To reduce power requirement especially in multi-channels parallel communication lines. An optocoupler consists of a LED at the primary side of the isolation barrier and a detector IC on the secondary side of the isolation barrier. By having both a power efficient LED and detector IC, the current consumption in each communication channel can be reduced. This reduction will be multiplied by the number of communication channels and this value will be significant as the number of channels increases. With lower power consumption, the heat generated will be reduced and thus the thermal management of the design can be simplified.
c) The efficiency of the isolated DC-DC converter often determines the available power in the isolated module. To meet this limited power budget, low power optocouplers are needed.
d) By driving at a lower forward current, the operating life of the LED in the optocoupler improves.

ACPL-M61L can be directly driven by the ASIC without needing external buffer

Avago’s new-generation optocouplers, ACPLM61L/ 061L/064L/W61L/K64L(ACPL-x6xL) offers significant improvement in power efficiency, along with excellent noise isolation and high voltage insulation performance. These new optocouplers consume 90% lesser power compared to the standard optocouplers available today and 40% lower power as compare to alternative isolators.

Functions of an Optocoupler:

The two primary reasons for using optocouplers are to provide high voltage insulation and to provide noise signal isolation. Electrical equipment, especially for industrial applications, need to operate for many decades. Hence, a high quality insulation barrier in the optocoupler is needed to be able to provide outstanding reliability of high voltage insulation. In addition, optocouplers serve to reject high common mode transient noise which appears simultaneously or common to either set of floating points, which otherwise may result in abnormal voltage transitions or excessive noise on the output signal. This is known as noise isolation capability.

Existing Isolation Solutions:

There are alternative isolation solutions that improve power consumption by trading off the thickness of the insulation barrier to improve the signal transfer efficiency. In addition, edge-triggered coding technique (as compare to level-triggered coding technique in optocouplers) is adopted to meet power efficiency requirements. Figure 2 illustrates these coding techniques. In the edge-triggered coding technique, the internal signal is transmitted with short pulse during the input transition time. This coding technique improves the power efficiency at low data rate signal but the power consumption will increase with frequency. Conversely, in the level-triggered coding technique, the LED detects the level of forward current set by the input signal and pulses the light output to the detector. As the total energy (defined as the total area under the curve) of the coded signal is higher for the level-triggered coding technique, any common mode leakage current is less likely to be corrupted by this signal. It is thus more robust against common mode noise.

Different decoding signaling techniques

Avago’s New-Generation Low Power Optocouplers (ACPL-x6xL):

To ensure the optocouplers are built with a high level of insulation voltage and noise isolation performance, the composition and thickness of the insulation layer are preserved. The new ACPL-x6xL provides as high as 1140Vpeak working voltage and 5000Vrms isolation voltage. The same level-triggered coding technique is employed to ensure good common noise rejection during static (without signal transmission) and dynamic (with signal transmission) environment. ACPL-x6xL is also built with a Faraday shield to channel the common mode transient noise to ground. In addition, connecting a limiting resistor in series to the input of the LED itself provides a good RC noise filter for noisy environments (as illustrated in Figure 3). All these enable ACPL-x6xL to have the best in class application level common mode noise immunity (35kV/µs).

Current limiting resistor (R1) and the LED parasitic input

To ensure the optocoupler can be driven by a low forward current, while maintaining consistent speed performance across a wide range of temperature, the new optocouplers are individually trimmed for precision control of its switching threshold. As a result, the forward current driving requirement has been reduced to as low as 1.6mA. With this trimming technology, ACPL-x6xL optocouplers are able to transmit 10MBd signal with a maximum propagation delay of 80ns across wide temperature range of -40°C to 105°C.

Having low LED driving current (>1.6mA) not only allows ACPL-x6xL to be directly driven by most microprocessors (shown in Figure 1) but helps to increase the operating life of LED. In general, the operating life of the LED has an inverse relationship to the LED driving current. As shown in Figure 4, with the advanced LED technology developed by Avago, the LED of the ACPL-x6xL optocoupler degrades at only 1% after 22 years when operating at 3mA at 85°C temperature on a 50% duty cycle signal!

LED’s LOP degradation graph driving at 3mA at 85°C with the LED being turned on 100% of the time.

The operation of an optocoupler consists of an input current signal that drives a LED to generate light signal. This light signal is then converted to a small electrical current signal through a detector chip. This small current signal is then amplified and processed through a transimpedence amplifier (TIA). To allow the TIA to work across a wider supply voltage range of 2.7 to 5.5 volts, an internal regulator is used. To reduce power consumption on the detector chip, Avago has developed a proprietary circuit. In this patented design, the current used for the internal regulator is also re-used for signal amplification resulting in very efficient utilization of the supply current. A maximum of 1.3mA is needed to allow 10MBd operation of the optocoupler detector.

ACPL-x6xL’s Intelligent Features

Two additional features have been designed into this new generation of optocouplers. First of all, a feature similar to “under-voltage-lockout” is designed in. In many cases, due to the power up and down sequence, some components within an electrical module do not receive a sufficient level of supply voltage to ensure proper operation of the device. At times, glitches occurring at the output may accidentally trigger the next stage. This feature is designed-in to ensure the output of the optocoupler can be set to a deterministic state during power up and power loss.

On the other hand, designers often face issues where the rise and fall time of different communication line varies due to the variation of the load. Output nodes that have higher load capacitance will have higher rise and fall time (as depicted in Figure 5). This results in a large variation of propagation delay, pulse width distortion and propagation delay skew performance. ACPL-x6xL is designed with a slew-rate controlled output feature. This feature allows the rise and fall time of the output signal to be well controlled across a wide load capacitance range. This is important in parallel communication where different communication lines (for example clock and signal lines) may have different fan-out. This unique feature can only be found in ACPL-x6xL and not in other standard optocouplers/isolators available today.

Rise and fall of the output varies with load capacitance

Ease of Use

ACPL-x6xL has LED input configuration, not only help to provide noise filtration, but also to provide great flexibility for designer. By changing the input polarity, one is able to change the output from inverting to non-inverting output without adding an additional inverter. For users who wish to fine tune speed, performance, etc, a peaking capacitor can be added in parallel to the limiting resistor. In addition, these new optocouplers (ACPL-x6xL) are characterized with voltage driving mode. In the past, the designers will take the lead to calculate the value of the limiting resistor to set the LED driving current. The new ACPL-x6xL can now be easily driven in voltage driving mode by following the recommended resistors value stated in the datasheet. However, for designers who wish to have better speed performance, but are able to compromise on power, the optocoupler can be driven by a higher LED current within the recommended driving condition.

By inversing the input polarity of optocoupler output logic can be programmed to inverting or non-inverting logic

Summary:

In summary, the new generation optocouplers provide the power efficiency that is required while providing excellent high voltage insulation and noise isolation (in both a static and dynamic environment) performance. These new optocouplers are built in with additional features (slew-rate controlled and glitch-free output during power up and down). They are extremely easy to use and now available in voltage driving mode. In addition, ACPL-x6xL (ACPLM61L/ 061L/064L/W61L/K64L) is a flexible optocoupler that allows special customization to suit each application’s need. These customizations include inversion of logic polarity and tuning of speed parameter.These new 10MBd digital optocouplers (ACPL-x6xL) are suitable for communication interfaces (RS485, CANBus, and I2C), microprocessor system interfaces, and digital isolation for A/D, D/A conversion applications.

 

 

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