Test Interpretation | How to measure CMTI, the key indicator of isolation chips?
CMTI overview and testing background
CMTI: Common mode transient immunity refers to the allowable rise/fall rate dVcm/dt for high-speed transient common-mode voltage applied between isolation circuits, usually expressed in kV/µs or V/ns . As shown below:
CMTI points out the isolation circuit's ability to suppress high-speed transient signals from passing through the isolation layer and destroying the output state. It also reflects the sensitivity of the isolation circuit to fast transient signal interference. A higher CMTI index means that the isolation circuit/device can guarantee output when the common-mode voltage hits the isolation barrier at a higher rise or fall rate under its limited test conditions (such as IN=VCCI or GNDI, Vcm=1200V) (OUT) No error occurred.
CMTI testing requirements and applications
With the popularity of new generation wide bandgap semiconductor devices such as SiC and GaN, devices and applications require higher switching frequencies compared to traditional MOSFETs and IGBTs, resulting in higher switching frequencies during turn-on/off transients. The edge rate of the transient voltage. CMTI ratings for high-performance isolators can easily reach 100V/ns, and many CMTI test results exceed 200V/ns. Signal integrity issues are expected when using low CMTI isolators in high dVcm/dt environments, which may cause any pulse jitter, distortion, unstable operation or pulses in certain environments such as motor drives or solar inverters. Any loss of information has a significant impact on data integrity and can lead to dangerous short-circuit events. CMTI testing has become very important in this type of isolation circuit, especially in the chip index testing with isolation function.
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Testing principles and methods
CMTI's testing is based on the IEC 60747-17:2020 standard. Divided into static testing and dynamic testing.
Static CMTI test
Static refers to connecting the input pin to a logic high level or low level, and then simulating the application of a common-mode transient CMT. Theoretically, no impact within the CMTI specification can change the output state.
Dynamic CMTI testing
Like the requirements of static CMTI, the output should remain normal under the impact of dynamic common mode transient CMT. If the CMTI capability is not strong enough, errors like missing pulse, excessive propagation delay, high or low error or output latch will occur. .
Static CMTI measurement
Dynamic CMTI measurement
In view of the high-speed transient and high common-mode voltage test environment, it is recommended to use an oscilloscope test system with high bandwidth and excellent full-band CMRR to measure the CMTI capability of the isolation device and the signal stability of the isolation circuit . The test connections are as follows:
Schematic diagram of CMTI capability test of isolation circuit
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test system |
Typical advantages |
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Tektronix IsoVu Optical Isolation Test System |
MSO5B/6B series oscilloscope |
Low noise, 12-bit ADC |
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TIVP series optical isolation probe |
Up to 1GHz bandwidth Common-mode voltage withstand capability up to 60kV Up to 80dB CMRR @ 1GHz |
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