Why is the Isolated Driver NSi6801 Superior to the Optocoupler Driver?

PublishTime： 2023-06-25 Article Source：NOVOSENSE News

In recent years, with the spread of the IoT, big data, and cloud computing in the industry, the global industry is undergoing a new revolution. To achieve intelligent, low power consumption and high-efficiency output, more sensors and nodes are applied in industrial applications. The number of electrical components continues to increase, while the weight and volume of industrial electrical systems are decreasing. The applications represented by NEVs and power transmission are increasing the working voltage.

All these changes have posed new challenges to industrial-Isolated Driver technology. Miniaturization and lightweight require higher switching frequency and shorter dead time for power MOSFETs, while system complexity and high voltage applications put forward higher requirements for the reliability of isolated components.

Optocoupler isolation is a widely used technology at present. A photocoupler isolator is a device that transmits electrical signals through the medium of light. Its principle is to encapsulate the light-emitting diode and photosensitive diode together. When the input is loaded with a certain voltage, the light-emitting diode emits light, and the photosensitive diode receives the light and generates a current, which flows out of the output. In this way, the "electricity - light - electricity" conversion is realized. Through electricity-light conversion, the optocoupler isolator insulates the input and output, allowing one-way transmission of signals.

In industrial automation systems where strong and weak electricity coexists, isolation measures are essential. Isolation can protect sensitive circuits from high voltage disturbances and damage, thus ensuring the normal operation of the system, prolonging the service life of the system, and protecting the safety of operators.

With the advantages of simple use, low cost, small size, and no contact, an optocoupler isolated driver is widely applied. However, the optocoupler isolated driver also has some disadvantages, such as poor common mode interference suppression ability, low isolation performance and reliability, long transmission delay, high driver loss and temperature rise, and low maximum operating temperature of the driver, which is difficult to meet the development requirements of the current industrial isolated driver technology.

NOVOSENSE NSi6801 optocoupler-compatible isolated driver solution

The digital isolated gate driver NSi6801 launched by NOVOSENSE solves the problem of the engineers in the selection of isolation devices. The external circuit of the isolated driver is the same as that of the optocoupler-isolated driver, but it has great advantages over the optocoupler-isolated driver in terms of anti-common-mode interference suppression ability, isolation performance and reliability, transmission delay, and driving ability. Engineers do not have to choose between ease of use and performance.

NSi6801 has three types of packaging that are compatible with the main optocoupler isolated driver pins on the market. In terms of input characteristics, NSi6801 built-in circuit simulates diode characteristics, and the threshold current, threshold voltage, and input operating current are exactly the same as those of the mainstream optocoupler isolated driver on the market. Like optocoupler devices, NSi6801 supports the input interlock protection mechanism. NSi6801 has the same application settings as those of the mainstream optocoupler isolated drivers, except that its external resistance needs to be modified to match the input forward voltage of the optocoupler at 10mA operating current. So when selecting NSi6801, engineers can directly replace the original optocoupler isolator design.

In terms of performance, what are the advantages of NSi6801 compared with optocoupler isolators?

Take common mode transient immunity (CMTI) as an example. Due to the parasitic capacitance effect of the optocoupler isolator, the interference pulse may cause the output terminal to be switched off or switched on by mistake. When there is a pulse signal of 35kV/μs at the output of optocoupler isolated driver Axx-Wxxx of certain mainstream brands, the input will be triggered by mistake. NSi6801 adopts a capacitor for isolation. To improve the common mode transient immunity, the isolation circuit of NSi6801 consists of four capacitors to form a differential line to transmit signals, which ensures that the capacitance values of the four capacitors are close, so as to effectively reduce the common mode interference. The Adaptive OOK™ technology, patented by NOVOSENSE, further enhances its common mode transient immunity. In the test, the NSi6801 can still maintain proper output under 200kV/μs common-mode interference.

In isolation performance, the NSi6801 wins across the board. NSi6801 meets the enhanced isolation standard of the latest new DIN VDE V 0884-11. The maximum working DC voltage of NSi6801 is 2121V, nearly twice higher than the maximum working voltage of the mainstream optocoupler; Its one-minute peak voltage resistance reaches 8kV and RMS value reaches 5700V, both of which reach or exceed the highest levels of the optocoupler. In terms of creepage distance, NSi6801 can be applied to a DC bus system of more than 1000V, and the optocoupler isolator can only be applied to a bus system of less than 800V DC according to IEC60950.

In terms of working time, NSi6801 can work normally for more than 45 years at 2121V bus voltage, with a failure rate of one in a million during its life. By comparison, the Axx-Wxxx series optocouplers have a recommended service life of 11.4 years at 1414V DC.

Compared with the optocoupler isolated driver, NSi6801 has a smaller transmission delay and higher transmission delay matching between chips. The maximum transmission delay of NSi6801 is 100ns, and the part-to-part skew of the maximum transmission delay between chips is 35ns. It can be used in power systems that need as short as 70ns dead time. However, the maximum transmission delay of optocoupler isolated driver Axx-Wxxx is 200ns, and the part-to-part skew of maximum transmission delay between chips is 100ns, which can only be used in power systems that need larger than 200ns dead time. Therefore, compared with the optocoupler isolated driver, NSi6801 is more suitable for the high-frequency trend of isolation applications.

In addition, the NSi6801 has the high driving capability. It has a minimum output current of 3.5A and a typical value of 5A, which has obvious advantages over the optocoupler isolated driver. Different from mainstream optocoupler isolated drivers that only adopt PMOS on the transistor, the NSi6801 driver adopts NMOS and PMOS in parallel in the transistor to effectively reduce the internal resistance of the driver, thus enhancing the ability to drive under different output voltage and reduce the driver loss. The driver loss reduction can reduce the device's heating. The working junction temperature of NSi6801 is 150℃, which effectively ensures the reliability of the system in high-speed switching or high-temperature application.

Through multiple comparisons, it can be found that NSi6801 has obvious advantages over the corresponding mainstream optocoupler isolated driver in terms of common mode transient immunity, isolation performance, working life, transmission delay, driving ability, and temperature rise. And because of its pin compatibility, engineers can apply it directly without changing the design, improving the design performance in one step. Therefore, NSi6801 is a better choice to replace the optocoupler isolated driver.

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