TVS Calculation and Selection (02)

TVS  Calculation and Selection (02)

TVS Key Parameters

To do a good job, one must first sharpen one's tools. To make good use of TVS, one must first understand its key parameters.

Cutoff voltage: The highest working voltage of TVS, which can be continuously applied without causing TVS degradation or damage. It is the highest working peak voltage or DC peak voltage. For AC voltage, represented by the highest working voltage effective value, TVS considers it to be non working, i.e. non conducting, under V RWM. In other words, the maximum working voltage of the circuit must be less than Vrwm, otherwise it will cause TVS action and cause circuit abnormalities.

Leakage current: Leakage current, also known as standby current. The maximum current flowing through TVS under specified temperature and maximum operating voltage conditions. The leakage current of TVS is generally measured at the cut-off voltage, and for a certain type of TVS, I R should be within the specified value range. Apply voltage value V RWM to both ends of TVS, and the current value read from the Ammeter is the leakage current I R of TVS. For TVS with the same power and voltage, when V RWM ≤ 10V, the leakage current of bidirectional TVS is twice that of unidirectional TVS. For some analog ports, leakage current can affect the sampling value of AD, so the smaller the leakage current of TVS, the better.

Breakdown voltage refers to the voltage at both ends of TVS measured on the V-I characteristic curve under specified pulsed DC current I T or near avalanche current conditions. The test current It is generally selected to be around 10mA, and the applied current should not exceed 400ms to avoid damage to the device. V BR MIN. and V BR MAX. are deviations of the breakdown voltage of TVS, with a deviation of ± 5% in general. When measuring, V BR falling between V BR MIN. and V BR MAX. is considered a qualified product.

Peak pulse current, V C clamp voltage peak pulse current, the peak value of a given pulse current waveform. TVS generally selects 10/1000 μ Current waveform. Clamp voltage, the peak voltage measured at both ends of TVS when applying the peak pulse current IPP of the specified waveform. I PP and V C are parameters that measure the ability of TVS to resist surge current and limit voltage in circuit protection, and these two parameters are interrelated. For the clamping characteristics of TVS in lightning protection circuits, the parameter V C can be referred to. For the same model of TVS, the smaller the V C under the same I PP, the better the clamping characteristics of TVS. The impulse current impact resistance of TVS can refer to IPP. For TVS of the same model, the larger the IPP, the stronger the impulse current impact resistance.

Junction capacitance CI, leakage current Ir; Junction capacitance is a parasitic capacitance in TVS, which requires special attention in high-speed IO port protection. Excessive junction capacitance may affect the quality of the signal. Leakage current mainly brings about power loss, or in analog signals, it can affect the sampling value of AD signals.