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Digital Isolators Offer Improved Performance and Lower Cost Options for Solar PV Inverter Designs

Many alternative energy applications, including solar photovoltaic (PV) systems, benefit from reliable digital isolation. Digital isolators are becoming increasingly popular in solar PV inverter designs because they offer many advantages in performance, BOM count and corresponding reliability. Digital isolators can achieve channel counts and functional integration that offset the higher piece price of alternative isolation technologies because digital isolators are fabricated from conventional semiconductor process technology. As a result, engineers can select from a wide variety of channel, functional and performance options available in digital isolator families from Analog Devices, Avago Technologies, Linear Technology, NVE, Silicon Labs, and Texas Instruments.

Digital isolators are utilized in solar PV inverter designs because engineers need to manage high voltage and extract maximum efficiency from solar arrays and individual panels (see Fig. 1). Solar PV inverters are also typically found in central inverters and microinverters to provide isolation of MOSFET gate drive signals, digital control signals for batteries and panels, installation communications channels and overall power output.

Figure 1: Digital isolators play many important roles in solar inverters, including MOSFET gate driver signal isolation to control and communications. (Source: Texas Instruments.)

Engineers can use digital isolated gate drivers, such as the Analog Devices (ADI) ADuM3220, to control the switches of an H-bridge output circuit in a solar inverter (see Fig. 2). Based on the Analog Devices iCoupler technology, the ADuM3220 is a 4-A isolated dual-channel gate driver that offers full galvanic isolation with output that is electrically and physically isolated from input. Digital isolators, such as the ADuM3220, typically exceed the performance of alternative isolation technologies, providing the speed necessary in feedback operations.

Figure 2: In a single-phase inverter, two ADuM3220’s can control the four switches of an H-bridge output circuit. (Source: Analog Devices.)

Digital isolators operate similarly to optocouplers because they are fabricated using similar conventional semiconductor process technologies, but manufacturers leverage different mechanisms to couple input and output signals in optocouplers. For example, the Analog Devices iCoupler technology uses coreless microtransformers built on top of the CMOS substrate. NVE and Avago use the giant magnetoresistive (GMR) effect. Texas Instruments’ devices use capacitive-isolation technology, and Silicon Labs’ (SiLabs) ISOpro technology uses RF.

SiLab’s ISOpro devices comprise an RF transmitter and receiver located on separate die but separated by a differential capacitive isolation barrier – all contained within a standard package. A ‘logic 1’ on the input causes the transmitter to turn on an RF carrier that propagates across the isolation barrier to the receiver to generate a ‘logic 1’ on the output. The absence of input voltage disables the transmitter so no carrier is present, helping ensure signal integrity during power-up and power-down.

Simplified design

In general, the coupling technologies used in digital isolators are more efficient than conventional LED-based methods of optocouplers. As a result, digital isolators typically offer better performance than traditional isolation technologies. For example, beyond a pair of small external VDD bypass capacitors, using digital isolators results in simpler designs (see Fig. 3) and eliminates the need for extensive external circuitry, such as the circuitry used to bias optocoupler LEDs or enhance optocoupler performance.

Figure 3: Digital isolators typically require only a pair of small external VDD bypass capacitors for most applications, making them as easy to use as a standard CMOS logic gate. (Source: Silicon Labs.)

The simplicity of this design translates into tighter performance parameters because the timing parameters become strictly a function of the device’s internal timing precision and propagation delay. For example, in the SiLabs Si84xx family, rise and fall times vary by only 1 ns versus temperature or supply voltage.

Engineers can use digital isolators from different vendors to simplify system design and achieve improved performance and efficiency. The Texas Instruments ISO724x series of digital isolators achieves a maximum pulse-width distortion of 2 ns and jitter down to 1 ns at 150 Mbits/s.

Most devices in this class exhibit leakage current in the microampere range. The NVE IL26x series has 2 µA typical leakage current. The Avago HCPL-90xx/-09xx series typically achieves 0.2 µA.

Digital isolators also require a relatively small amount of operating current – only milliamps per channel. For example, SiLabs Si844xx operates at less than 1.6 mA per channel at one Mbit/s. Analog Devices ADuM140x family of quad-channel digital isolators require 1.0 mA per channel (max) up to 2 Mbits/s in 5-V operation and only 0.7 mA per channel (max) up to 2 Mbits/s in 3-V operation.

High Immunity

Along with reduced part count and improved efficiency, digital isolators offer significantly improved immunity and reliability due to inherent advantages in their fabrication technology. Electric field immunity and high common-mode transient immunity (CMTI) are critical aspects of a solar PV inverter design to help ensure device health and data integrity.

Many devices in this class, including the Texas Instruments ISO724x and the SiLabs Si844xx digital isolators, rely on semiconductor oxide layers for their primary isolator. Semiconductor fabrication methods are well controlled and uniform, so manufacturers can deposit oxide to any depth to achieve a desired insulation level and set a desired maximum breakdown voltage. For TI isolators, this approach yields galvanic isolation of up to 4000 V and life expectancy of over 25 years at the TI ISO724x’s rated maximum working insulation voltage (VIORM) of 560 V (see Fig. 4). Analog Devices quotes a 50-year minimum lifetime at 565 V maximum working insulation voltage for its ADuM140x quad-channel digital isolators. Digital isolator lifetimes will likely surpass the rated lifetimes of other components in a solar PV installation.

Figure 4: At its rated maximum working insulation voltage (VIORM), the TI ISO724x series achieves a life expectancy of over 25 years. (Source: Texas Instruments.)

In any practical application, the electric field immunity of digital isolators is likely substantially larger than the circuits connected to the isolators. As a result, external circuits and shielding will generally play a predominant role over digital isolator components in determining overall electric field immunity.

High CMTI is particularly critical in ensuring data reliability in control paths. The SiLabs ISOpro isolator achieves high CMTI through its combination of a fully differential signal path and frequency selectivity of the receiver, which combine to help enhance noise immunity. In the equivalent circuit (see Fig. 5), the input-to-output capacitance (CCM) is substantially lower than the CCM found in optocouplers. As shown on the right side of Fig. 5, high CMTI means that transients passing through the device will not result in false highs or lows.

For the SiLabs Si844xx, the result is a typical CMTI of 25 kV/µs. In fact, other devices of this class demonstrate similar performance, including the ADI ADuM140x Quad-Channel Digital Isolators, which typically spec out at 35 kV/µs. The NVE IL26x five-channel digital isolators typically achieve 35 kV/µs, and TI ISO724x devices usually exhibit a CMIT of 50 kV/µs.

Figure 5: SiLabs ISOpro devices offer fully differential signal paths and minimum input-to-output capacitance (CCM) (left) resulting in high common-mode transient immunity (CMTI) for improved signal integrity in the face of common-mode transients (right). (Source: Silicon Labs.)

Signal integrity is important in a solar PV installation to achieve reliable voltage measurements necessary for battery management and maximum power-point tracking (MPPT). Devices such as the NVE multichannel IL261 isolate the sampling analog-to-digital converter (ADC) from digital noise sources and ground currents (see Fig. 6).

Figure 6: Devices such as the NVE IL261 multichannel digital isolator are vital in solar PV designs for uncoupling ADC voltage measurements from digital circuitry. (Source: NVE.)

Alternatively, engineers can take advantage of the high functional integration available with these parts. Digital isolators can be combined with other functions on the same die to yield single-chip isolated ADCs and isolated communications transceivers, among others. The result is the ability to create more functionally rich designs with lower part count and increased reliability. For example, the AD7401A is a second-order sigma-delta (Σ-Δ) modulator that converts an analog input signal into a high-speed one-bit data stream by combining an on-chip reference and on-chip digital isolation based on the ADI iCoupler technology (see Fig. 7).

Figure 7: The ADI AD7401A is a single device that integrates a sigma-delta ADC with an on-chip reference and isolation circuitry. (Source: Analog Devices.)