V BR is the voltage where TVS devices go into avalanche. The V WM is typically 10 – 15% below the breakdown voltage (V BR). A bi-directional TVS is the same, except applies to both directions. Littelfuse refers to V WM for a unidirectional TVS as “the maximum peak voltage that may be applied in the ‘blocking direction,’” if you expect to have no significant current flow.
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(Source: MicroSemi MicroNotes 103)Īs for how to choose a TVS, make sure you know the maximum operating voltages for the devices in the protected circuit (load), as this will determine V WM, which is known in various datasheets as the working peak voltage, the maximum working voltage, the rated standoff voltage, reverse standoff voltage or maximum off-state voltage. Here, it diverts the overvoltage on the input to ground. Figure 2: A TVS is invisible to the circuit it protects. The TVS takes the jolt and immediately clamps it, or cuts it off, before the maximum voltage that the USB chip can withstand is reached. A USB receptacle or port is an ideal case since ESD is often experienced at the input of a device: when someone walks across a carpet to plug in a USB cable, a charge can build up that causes electrostatic discharge at that input. A unidirectional TVS diode will only allow voltage either above or below ground (positive or negative voltage.) A bidirectional TVS is chosen when the protected signal is usually supposed to swing above and below ground, as in an AC voltage or a DC signal intended to operate at positive and negative voltages. A TVS can protect a signal chain either unidirectionally or bi-directionally. TVS diodes (and TVS diode arrays) protect circuits from Electrostatic Discharge (ESD). Figure 1: Voltage on the Y-axis exceeds the upper limit of 20V and is instantly clamped to an acceptable lever by the TVS in the circuit. TVS devices are typically specified to handle in the range of kilowatts of peak power flowing through the TVS. A spike event that lasts a long time will necessitate the installation of a heat sink for the TVS, however. In normal operation, the TVS is invisible to the circuit. The pn junction returns to normal when the spike has passed and will clamp again on the next spike. Another type of TVS diverts a voltage spike away from your circuit (also known as shunting, clamping, or bypassing.) TVSes protect circuits by utilizing the avalanche breakdown point of a silicon pn junction that’s created for this purpose. A silicon Transient Voltage Suppressor (TVS) is a circuit protection component that either attenuates (reduces) or filters a transient voltage spike (overvoltage), thus protecting your circuit. Transients are sudden high-frequency overvoltages that can destroy a chip.