Based on Capacitive Coupling Technology, NOVOSENSE Capacitance Isolation Technology to Easily Solve Power Problems

PublishTime： 2023-08-15 Article Source：NOVOSENSE News

All electrical products involve power supply, and the common power supply includes voltage regulation power supply, switching power supply, inverter power supply, frequency conversion power supply, and uninterruptible power supply. MOSt power supplies require isolation devices to ensure equipment and personal safety. Because of the different isolation technology used, the isolation effect is also different. Therefore, the choice of isolation products should promote the advantages and avoid the disadvantages, so as to achieve the best system performance as far as possible.

I. Why does the power supply need to be isolated

This is a commonplace problem, and the purpose is to prevent the high voltage of the power supply from endangering the human body. New energy vehicles are a common high-voltage power supply scenario, and the battery voltage is 400V or even 800V introduced recently; such a high voltage will endanger the human body; the same is true of the charging pile, which converts alternating current into high-voltage direct current, and also needs to isolate high and low voltages, which requires the use of isolation devices.

In addition, power supplies in such applications as photovoltaic or data center server, industrial frequency conversion servo and some industrial power modules or energy storage devices require measures taken to avoid the harm of high voltage to the human body, in addition to meeting the corresponding safety requirements.

II. Isolation requirements and classification

The isolation is subject to strict safety certification, including such common ones as the US’s UL certification, Germany’s VDE certification and IEC certification of the International Electrotechnical Commission. Safety certification includes two types, one is the system level, such as IEC60065, IEC60950, etc., and the other is the device level, such as UL1577 and IEC60747 standards. The IEC standard defines three levels of energy sources from the perspective of safety, all of which are based on the size of voltage and current, and the corresponding protection measures are implemented. It is worth mentioning that all of NOVOSENSE's isolation products have passed UL, CUL, VDE and CQC safety certification.

The isolation chip is widely used in the power system, for example, in the vehicle charger OBC/DC-DC system, the high-voltage battery charging input side is 220V to 380V, and the output side is 400V or 800V; it is 12V to 48V for low-voltage battery charging, which includes PFC and LLC two-stage topologies. The whole system topology is complex, often using two MCUs as the main control, and the communication between the two MCUs needs to be isolated. In addition, the power valves in these topologies, whether Si MOSFETs or third-generation semiconductor devices, need to be driven and isolated.

In addition, according to the control accuracy requirements of the system, voltage sampling, current sampling and external communication of the system also need to be isolated.

III. Several common isolation techniques and requirements

At present, there are three isolation technologies used in the industry: traditional optocoupling, magnetic coupling and capacitive coupling technologies. Traditional optocoupling technology is the most widely used and has the longest history. It uses light as the medium to couple the input signal to the output end, but the volume is large, the transmission speed is slow, the light decay will occur with the growth of the use time, and the operating temperature range is narrow. Magnetic coupling and capacitive coupling are the mainstream isolation technologies. The magnetic coupling has high voltage resistance, fast transmission speed and wide temperature range, but the process is complex with EMI radiation. Capacitive coupling technology has high withstand voltage, fast transmission speed, transmission delay of only twenty or thirty nanoseconds, and a very wide operating temperature range, and the process is not complicated, with high reliability.

NOVOSENSE's isolation chip is based on capacitive coupling technology, using Adaptive OOK® adaptive coding technology, with low EMI radiation and low bit error rate, which can effectively improve the isolation device's ability of common‐mode transient immunity (CMTI).

Important indicators of isolation products include: isolation voltage rating, CMTI capability, EMC performance, transmission delay and operating temperature, isolation life, etc. The isolation voltage level of NOVOSENSE’s microisolation products is up to 10kV, with CMTI of at least 100kV/μs, anti-surge capability of over 10kV, and bilateral ESD capability of over 15kV.

IV. Power supply system trends and application difficulties

(1) Power system trends

-High integration: Power systems are moving towards higher integration, so more integrated ICs are also required, such as integrating power supplies and digital isolators together to reduce the complexity of designing isolated power supplies by engineers.

-High voltage and high frequency: The photovoltaic system has been transferred from 800V to 1500V, the third generation of semiconductors (gallium nitride or silicon carbide) is applied more and more widely, the system switching frequency is getting higher, and the speed is getting faster. Isolation products need to have higher CMTI capabilities, withstand higher voltages, and have better EMI performance.

-High reliability: Isolation chips need to pass strict safety certification.

(2) The application difficulties of the power supply system

The application difficulty of power system is that the third-generation power device puts forward higher requirements for the driver chip, such as CMTI of greater than 100kV/μs. Due to the higher drive voltage of silicon carbide, the driver output voltage range is required to swing wider. In addition, the product switching rate is faster, and the switching loss is reduced, the driver has the ability to output greater Source or Sink current, the internal rise or fall time is shorter, and the transmission delay is smaller.

The new function of the silicon carbide driver chip is "Miller clamp". with the SiC switching, the middle point of the bridge arm has a large dv/dt, the lower valve Cgd capacitor will generate a Miller current; even if the lower valve is in the OFF state, it will also generate a voltage drop through the lower valve OFF resistance. Considering that the on-threshold of the silicon carbide device is relatively low (about 2V), if the pressure drop is large, the lower valve will be mis-connected, and the system will have the risk of short circuit.

The "Miller clamp" function addresses this problem in silicon carbide applications. Adding a MOSFET to the chip can directly connect GND to the silicon carbide grid. After the lower valve is turned off, the lower valve driver resistance will be skipped and the grid will be directly short-circuited to GND to eliminate the voltage difference caused by Miller current, thus avoiding the risk of mis-connection of the lower valve caused by Miller effect.

V. NOVOSENSE isolation products

NOVOSENSE has a wide range of isolation products, including digital isolators, isolation drivers, isolation voltage/current samplers, isolation CAN transceivers. In terms of driving, whether it is MOS, IGBT or silicon carbide, it has corresponding isolation products. In terms of sampling, it has both analog output isolation operational amplifier and digital output isolation ADC, which can meet the requirements of sampling rate and sampling accuracy in different application scenarios. In terms of interfaces, it has a wealth of isolation I2C interfaces, and 485 or CAN interface products, which can provide a one-stop solution for customers' power supply design.

1. isolation single valve driver with Miller clamp function NSi6601M

2. high-voltage half-bridge GaN grid driver NSD2621

3. 5A dual-channel non-isolation driver NSD1624

4. Isolation voltage sampler NSi1312

5. integrated power stage product NSG65N15K

The products of NOVOSENSE adopt capacitive coupling technology, which conforms to the current application trend of integrated, high-voltage and reliable power supply, and meets the higher requirements of the third-generation power devices for driver chips. Its isolation product range is very complete, including digital isolators, isolation drivers, isolation voltage/current sampler, isolation CAN transceivers, providing engineers with a variety of options.

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