What you can do with a high-CMTI isolator


In my previous blog post, I discussed the use of isolators for protection against high voltage. In this post, I discuss a key performance parameter of isolators: Common mode transient immunity, or CMTI.

CMTI describes an isolator’s ability to tolerate high-slew-rate voltage transients between its two grounds without corrupting signals passing through it. In general, higher CMTI indicates robustness to noise, and is an advantage in any isolation application. However, there are some particular applications where high-CMTI isolators can enable significant differentiation for the end product.

Solar, wind energy and grid storage applications use inverters to convert generated or stored DC power to AC power that is then fed into the electric grid. The inverter comprises insulated-gate bipolar transistors (IGBTs) or other power semiconductor devices controlled by a microcontroller. Isolated gate drivers or digital isolators combined with nonisolated gate drivers translate signals from the microcontroller to gate controls to the power devices. This provides the correct voltage and current drive to the power devices, and isolates the microcontroller from the inverter’s high-voltage parts. Figure 1 shows a simplified block diagram of a central solar inverter.

Figure 1: Simplified block diagram of a central solar inverter

The isolators used in these applications face high ground transients (see Figure 2), since one of the two grounds is connected to rapidly switching inverter outputs. For high-power inverter designs, the inverter outputs could switch up to 1500V in a few tens of nanoseconds. CMTI is a critical parameter here, since any bit errors caused in the isolators by these ground transients can result in dangerous short circuits.

Figure 2: Inverter output switching profile translates to high CMTI for isolators

Inverters in solar, wind energy and stationary storage are starting to use higher DC bus voltages, smaller switching times and higher switching frequencies. Each of these factors increases the CMTI requirement from the isolators.

Use of higher DC bus voltage increases output power while keeping current levels and copper costs the same, hence reducing per-unit cost of energy generated. Efficiency also improves since the total power output increases (higher V), but the conduction losses remain the same (same I). Inverter efficiency improves, given reduced switching transition times. A higher switching frequency results in less costly and less bulky magnetics such as inductors and transformers. The availability of reliable silicon carbide (SiC)-based power transistors, which can switch faster and tolerate higher voltages compared to traditional IGBTs, also contributes to an inverter’s ability to switch faster and more efficiently.

The availability and use of high CMTI isolators can thus bring several benefits to high power solar, wind energy and stationary storage applications.

TI has recently released the ISO5851 and ISO5451 isolated gate drivers, offering a 100kV/µs minimum CMTI. TI also offers the ISO78xx family of digital isolators, with a 70kV/µs minimum CMTI. These isolators enable less costly and more efficient high-power inverter designs.

What else would you like to know about isolation and CMTI? Leave a comment below and stay tuned to the Industrial Strength blog in the coming months to read more about this topic.

Additional resources

  • In Case of ISO5500 for Vce, It is 600V in the data sheet.  What about the Vce of 5852S? Is regarded VCM as Vce?

    This is because, the data sheet of ISO5852S does not have Vce parameter. Therefore I would like to know Which parameter in ISO5852S can be regarded as Vce ?

    Thanks

  • Hi Jin,

    ISO5852S can support a working voltage of 1.5 kVrms or 2121 Vpk.  As a first cut answer, VCE upto 2121 V can be supported. But the correct way to check this is to estimate the voltage across the isolation barrier (depends on driver architecture and design) and ensure that it is < 1.5 kVrms and 2121 Vpk.

    Rgds,

    Anant

  • Anant

    First of all, thank you for your answer.

    This Vce is one of important parameters for selecting Gate Driver because it give us Drive IGBTs up to VCE voltage.

    Currently I compare TI’s Gate drivers with HCPL-316J of Avago.

    Avago provides Vce and VCM repectively but TI does not offer Vce.

    Please let me clear whether Vcm on the data sheet of ISO545x can be regarded as Vce.

    Thanks

  • Hi Jin,

    The peak value of working voltage, 2121 Vpk can be used as the value for VCE. Could you please send me an email at anantkamath@ti.com? We can discuss this further.

    Rgds,

    Anant