The Broadcom ACNT optocouplers family offers 15 mm creepage and 14.2 mm clearance for high voltage isolation applications. These optocouplers provide 2,262 VPEAK working insulation voltage and 12,000 VPEAK transient overvoltage in a compact stretched surface mount SO-8 package. The ACNT optocouplers consist of comprehensive product types to meet the different galvanic isolation requirements and features. They are the ACNT-H343 gate drive optocoupler, ACNT-H61L low power 10 MBd digital optocoupler, ACNT-H790/H79A/H79B high linearity isolation amplifier for current sensing, ACNT-H870/H87A/H87B voltage sense, and ACNT-H50L/H511 low speed analog optocoupler. In the example of a 3-phase inverter that converts DC-AC power to a load (motor), Figure 1 highlights the various isolation locations and the purpose by each ACNT optocoupler product type. ACNT optocouplers are reliable and easy to use.
Figure 1: ACNT optocouplers for various isolation needs in an inverter. (Image source: Broadcom Limited)
High Voltage Trend and Requirements
In the market segments of renewable (solar or wind) power, traction, and healthcare systems, there is a trend for higher DC bus voltage or high transient voltage immunity requirement. The new solar/PV systems have been adopting 1500 VDC from 1000 VDC. This upgrade has the benefits of higher energy efficiency and lower costs.
A contributing factor to both of these benefits is that more PV blocks per string (a longer string of PV arrays) are able to be formed. Combiner boxes consolidate incoming power into one main feed. With longer and lesser numbers of PV strings, the need for combiner boxes declines. The higher input DC voltage also reduces wire (copper) power losses. As a whole, the 1500 V system has lesser connections between PV string arrays and inverter compared to 1000 V or lower DC voltage systems. The higher power density and less equipment also reduces labor maintenance costs. One challenge of the 1500 V system is the electrical safety and full range of certification standards for components. The isolation between the control module and the power products must be able to withstand high voltage (steady and transient) and the relevant creepage distance (Figure 2). Broadcom’s 15 mm wide ACNT optocouplers are able to meet these requirements.
Figure 2: PV power generation system – 1500 V. (Image source: Broadcom Limited)
In the wind solution of the renewable energy segment, ACNT optocouplers gain popularity in the European countries as part of a national-level program of distribution grid renewal to replace nuclear power plants with renewable resources.
Figure 3 shows the isolation of the frequency converter using the 15 mm wide ACNT-H61L 10 MBd low-power optocoupler. These components isolate the control and fault feedback signals between low voltage control and high voltage IGBTs. In another sub-segment of the electrical grid, metering for low or mid voltage distribution lines becomes more advanced as part of the smart grid adoption. Wider creepage is required in the 3-phase (>400 VAC metering) for safety and high voltage isolation of data communication. Security also has a higher importance in the smart grid. The low-speed analog optocoupler ACNT-H50L’s long term reliability helps secure isolated data communication.
Figure 3: Renewable energy – Frequency converter of wind solutions. (Image source: Broadcom Limited)
Another high voltage application example is the traction control of light rail or monorail systems. In these systems, the ACNT optocouplers are used in DC-DC conversion of 1500 V. Figure 4 illustrates a monorail power converter with a 1500 VDC bus voltage. There are four digital optocouplers for the isolated high voltage - low voltage control interface and two voltage sense isolation amplifiers for isolated voltage level control and the low speed analog optocouplers for isolated I/O communication to provide high voltage insulation for reliability and robustness in the high noise environment.
Figure 4: Power converter in traction applications. (Image source: Broadcom Limited)
In the healthcare system, the 4th edition medical standard IEC 60601-1-2 stipulates a higher ESD level for contact and air discharge types. This ESD immunity type test applies between the sensing circuitry (e.g., blood pressure, ECG), where the patient is in direct contact with the equipment and the control board of a patient monitoring device. The ACNT-H61L meets this revised requirement with a transient overvoltage of 12,000 VAC. Its wide creepage and clearance allow a wider isolating gap and minimize electric arcing.
Revision in the regulatory standards
More stringent creepage and clearance requirements resulted during the harmonization of international standard bodies UL and IEC. From 2016, UL 508C (Power Conversion Equipment) transits to IEC 61800-5-1 (Adjustable speed electrical power drive systems). Wider creepage and clearance for the new drive model are needed to maintain the same rating specification. For example, at least 13.8 mm creepage and clearance is required for reinforced insulation of a 690 VAC rating.
In medical systems, ACNT optocouplers have benefits in addressing the high ESD immunity discharge transient overvoltage needed across an isolation barrier. In the patient monitoring application, the ESD immunity type test applies between the sensing circuitry (e.g., blood pressure, ECG) where the patient directly contacts the equipment and the control board of the device. To address electromagnetic interference (EMI) threats of today for medical devices that are increasingly used outside hospitals, there is the new fourth edition of the medical electrical equipment standard, IEC 60601-1-2, that increases ESD immunity discharge levels during testing of medical devices (Figure 5). The ACNT-H61L used in patient monitoring devices has the benefits of a high transient overvoltage rating of 12,000 VPEAK, and its 15 mm wide creepage and clearance allow a wider isolating gap to minimize arcing.
Figure 5: Meet the high insulation voltage need in medical systems – ACNT-H61L. (Image source: Broadcom Limited)
Broadcom ACNT Optocouplers
Broadcom optocouplers have excellent performance and can withstand a high voltage surge (1.2 µs / 50 µs voltage waveform). As per component safety standard IEC 60747-5-5, Broadcom ACNT optocouplers pass more than 25 kV. The pass criterion is no puncture or partial breakdown of solid insulation at less than 5 pC partial discharge. As shown in Figure 6, the ACNT-H50L is tested under non-air conditions to remove electric arcing at with a high voltage surge (ref TUV test result report).
Figure 6: Broadcom optocouplers meet high voltage surge per IEC 60747-5-5. (Image source: Broadcom Limited)
The ACNT-H343 is a newly released 5 A gate drive optocoupler in a 15 mm SSO-8 package designed for high voltage, space-constrained industrial applications including 690 VAC motor drives and 1500 V solar inverters. The ACNT-H343 features common mode transient immunity (CMTI) greater than 100 kV/μs, preventing erroneous gate driver failures in noisy environments. The device has minimal propagation delay and is three times faster than the previous generation device which enables high-frequency switching to improve efficiency in driving IGBTs (insulated-gate bipolar transistors) and SiC/GaN MOSFETs.
The ACNT-H61L is a 10 MBd low-power digital optocoupler that requires only 4.5 mA minimum for LED driving current with the detector IC consuming 2 mA maximum IDD across the operating temperature range. The output of the detector IC is a CMOS output. The internal Faraday shield provides a guaranteed common-mode transient immunity specification of 20 kV/μs. The ACNT-H61L suits isolated logic interface communication and control in high voltage or transient power conversion systems.
The ACNT-H50L/H511 is a single-channel 1 MBd optocoupler with open collector transistor output. Separate connections for the photodiode bias and output transistor collector increase the speed up to a hundred times over that of a conventional photo-transistor by reducing the base-collector capacitance. The ACNT-H50L/H511 is suited for applications such as low speed analog, isolated fault or power control feedback.
The ACNT-H87B (±0.5% gain tolerance), ACNT-H87A (±1% gain tolerance), and ACNT-H870 (±3% gain tolerance) voltage sensors are optical isolation amplifiers designed specifically for voltage sensing. Its 2 V input range and high 1 GΩ input impedance meet the isolated voltage sensing requirements in electronic power converter applications. In a typical voltage sensing implementation, a resistive voltage divider is used to scale the DC-link voltage to suit the input range of the voltage sensor. A differential output voltage that is proportional to the input voltage is created on the other side of the optical isolation barrier.
The ACNT-H79B (±0.5% gain tolerance), ACNT-H79A (±1% gain tolerance), and ACNT-H790 (±3% gain tolerance) isolation amplifiers are designed for current and voltage sensing in electronic power converter applications. These optocouplers provide the precision and stability needed to accurately monitor motor current in high noise motor control environments, providing for smoother control (less “torque ripple”) in various types of motor control. Combined with superior optical coupling technology, the ACNT-H79B/H79A/H790 uses sigma-delta (∑-∆) analog-to-digital modulation, chopper stabilized amplifiers, and a fully differential circuit topology to provide unequaled isolation-mode noise rejection, low offset, high gain accuracy, and stability.