Transient voltage spikes and surges have varying degrees of impact on electronic circuits, ranging from annoying minor faults to catastrophic consequences that cause damage to circuit components. The causes of these transients are diverse, including lightning, static electricity, and induced discharges (Figure 1).
Figure 1. Transients may be caused by lightning, static electricity, or induced discharges, etc., which can cause serious damage to unprotected electronic equipment. Image source: LitInc.)
Transients like this can produce impulses with peak voltages ranging from hundreds of volts to tens of thousands of volts and currents up to kiloamperes, lasting from hundreds of nanoseconds to milliseconds.
Miniaturization of IC and processor and reduction of supply voltage make them more sensitive to electrical transient phenomena. This is particularly true for vehicles that are controlled by multiple electronic systems, including the engine, steering, braking, air conditioning and entertainment.
To protect sensitive circuits, a variety of design strategies have been developed, including shielded wiring, filters, arc suppression, and clamping devices. Shielding and filtering are passive, while arc suppression and clamp protection are active. Arc suppression devices such as spark gaps, gas discharge tubes, and thyristors shunt transient currents to the ground to protect the circuit. When the arc suppression device is active, the protected equipment does not work, but once the transient disappears, the equipment can work normally.
Clamping devices include metal oxide varistor (MOV), zener diode, and transient voltage suppression (TVS) avalanche diodes that maintain a constant voltage across the protected equipment by varying impedance. These technologies can be used separately or simultaneously. TVS diode is widely used as clamping device due to its fast response and large power dissipation.
Transient voltage suppression diode
TVS diode is an avalanche diode used as a clamping device. When the applied voltage exceeds its avalanche breakdown voltage, it shunts off excessive current and holds or clamps the voltage at a constant potential. When the applied voltage is lower than the breakdown value, it will reset automatically.
TVS diodes can be used as uni-directional devices to prevent transients of unipolar or bi-directional devices to prevent transients of either polarity (Figure 2). Bi-directional devices can be symmetric, clamped to any polarity voltage with the same amplitude, or asymmetric, clamped to different voltage levels according to the polarity of the transient.
Figure 2: The current-voltage breakdown characteristics of the three TVS devices and their schematic symbols. Image source: LitInc.)
The working principle of unidirectional TVS diode is similar to that of simple diode. It is on when forward bias is applied and not on when reverse bias is applied until the breakdown voltage (VBR) of diode is exceeded. When the applied voltage exceeds VBR, the diode is switched on and the voltage at both ends is maintained at the clamp voltage (VC). The maximum power that this diode can dissipate is peak pulse current (IPP) x VC.
Two-way TVS diodes are equivalent to two diodes back-to-back. When the breakdown voltage (VBR) is not exceeded in either direction, only a small reverse leakage current (IR) flows. This operation is symmetric because the breakdown voltage amplitudes are the same for both bias conditions.
Asymmetric TVS diodes behave similarly to bidirectional devices, but the breakdown voltages (VBR1 and VBR2) are different.
Asymmetric TVS diode
You may want to know why asymmetric TVS diodes are required. These components are designed to protect gate drivers on silicon carbide (SiC) MOSFETs. Due to the fast switching speed of SiC, these drives are vulnerable to damage due to over-voltage transients. Let's look at the SiC MOSFET or traction inverter for on-board charging (Figure 3).