2 Key Considerations to Select ISO Devices

1. Begin with Some Examples and Confusing Questions

   Figure 2-1 shows some TI examples of creepage and clearance values for reinforced, basic, and functional isolation, and different packages.

   Figure 2-1 Some TI Example of Creepage, Clearance and Packaging

   Engineers often get confused and there are some typical questions such as,

   • What factors determine whether a device adopts functional insulation, basic insulation or reinforced insulation?
   • Why do devices with the same insulation grade come in different packages to realize different clearance and creepage distance specifications?
   • How to select suitable isolators for customers' safety protection applications? Should we refer to isolation voltage, working voltage, certification or relevant standards? What are the most critical considerations?

1. Key Insulation Specifications

   Figure 2-2 shows an example of key insulation specifications of ISO device and Table 2-1 lists the interpretation.

   Figure 2-2 Key Insulation Specifications Example of ISO Device
   Table 2-1 Key Insulation Specifications Interpretation of ISO Device

   Parameter	Definition	Interpretation	Customer Key Concerns
   Coordination/support safety standard
   CLR	Clearance	The shortest distance between two conductive materials (pins) measured through air	Whether the CLR of the device meets the safety requirements of the customer's system
   CPG	Creepage	The shortest distance between two conductive materials (pins) measured along the insulating surface	Whether the CPG of the device meets the safety requirements of the customer's system
   CTI	Comparative Tracking Index	A parameter describing the difficulty of charge migration on the insulator surface, only related to the material itself. A higher value indicates the material surface is less prone to leakage.	The higher the CTI rating (higher voltage), the lower the creepage distance requirement
   OVC	Overvoltage Category	The operating environment of mains-powered electrical equipment is divided into four overvoltage categories according to surge protection levels	Whether the OVC of the device meets the safety requirements of the customer's system
   Component safety standard
   VIORM	Maximum Repetitive Peak Isolation Voltage (VPK)	The maximum repetitive voltage (peak) that an isolated device must withstand during its lifetime	Parameters that isolated devices must meet under normal operation
   VIOWM	Maximum Isolation Working Voltage (VRMS or VDC)	The maximum continuous working voltage (rms or DC) that an isolated device must withstand during its lifetime
   VIOTM	Maximum Transient Isolation Voltage (VPK)	The maximum transient voltage (usually non-repetitive) that an isolated device must withstand during its lifetime	Generally related to the actual required transient isolation voltage, operating bus voltage and corresponding OVC requirements
   VIOSM	Maximum Surge Isolation Voltage (VPK)	The maximum peak voltage that an isolated device can withstand based on 1.2us/50us standard surge waveform	Characterizes the lightning surge voltage immunity

2. Differentiate between Clearance and Creepage

   Figure 2-3 shows the differences between clearance and creepage. About clearance, it's easy to understand that the shortest distance between two conductive materials (pins) is measured through air. However, for creepage, note that the shortest distance between two conductive materials (pins) is measured along the insulating surface. The insulating surface can be the chip top surface, side face or lower surface.

   Figure 2-3 Clearance and Creepage Differences
3. Differentiate Component and System Level Insulation Requirements

   Get back to those confusing questions in section 2.1, Does it mean that a reinforced isolated device can also make customer products reinforced? The answer is not always. Engineers must always differentiate component and system level insulation requirements.

   • What factors determine whether a device adopts functional insulation, basic insulation or reinforced insulation?
     • Insulation grade is related to the insulation material, insulation layers and other properties of the chip. Different designs classify devices into functional, basic or reinforced insulation types, which further define the maximum isolation voltage the device can withstand.
     • There is a specific voltage limit for devices to comply with basic or reinforced insulation standards, indicating the voltage range within which the device meets corresponding insulation requirements.
   • Why do devices with the same insulation grade come in different packages to realize different clearance and creepage distance specifications?
     • As mentioned above, insulation grade is determined by chip insulation materials and layers and is irrelevant to the actual achievable electrical clearance and creepage distance.
   • How to select preferred isolators for customers' safety protection applications? Should we refer to isolation voltage, working voltage, certification or relevant standards? What are the most critical considerations?
     • Most importantly, do not confuse system-level safety specifications with the inherent insulation grade definition of isolated devices (components). A device with reinforced insulation does not mean the end application system can directly meet reinforced insulation requirements.
     • Customers focus on whether key parameters of isolators such as clearance and creepage distance can satisfy the safety requirements of their application systems. The compliance depends on operating voltage, overvoltage category, comparative tracking index, pollution degree, altitude and other practical factors.
     • It is quite common, especially in high-voltage applications, that customers adopt reinforced insulation devices to realize basic insulation or even functional insulation at the system level.

   Figure 2-4 shows a 1500V solar or ESS example for ISO-CAN design. Both ISO1050 and ISO1042 are reinforced ISO-CAN transceivers. However, the maximum isolation working voltage of ISO1050 is 1200V which can't satisfy isolation requirements in 1500V system.

   Figure 2-4 1500V Solar or ESS Example for ISO-CAN
4. Mandatory Asked Questions to Select Suitable ISO Devices

Table 2-2 lists mandatory asked questions to select the ISO devices. They will be discussed in detail in section 3 to determine clearance and creepage distance.