The impact of transient voltage spikes and surges on electronic circuits varies, ranging from annoying minor faults to catastrophic consequences that can cause damage to circuit components. The causes of such transient phenomena are diverse, including lightning, static electricity, and induced discharge (Figure 1).
Figure 1: Transient can be caused by lightning, static electricity, or induced discharge, which can cause serious damage to unprotected electronic devices. (Image source: Littelfuse Inc.)
Such transients can generate pulses with peak voltages ranging from hundreds of volts to tens of thousands of volts and currents reaching kiloampere levels, with durations ranging from hundreds of nanoseconds to milliseconds.
The miniaturization of ICs and processors, as well as the decrease in power supply voltage, make them more sensitive to electrical transients. This is particularly true for vehicles that are controlled by multiple electronic systems, including engine, steering, braking, air conditioning, and entertainment.
In order to protect sensitive circuits, various design strategies have been developed, including shielded wiring, filters, arc suppression, and clamping devices. Shielding and filtering adopt passive design, while arc suppression and clamp protection adopt active mechanisms. Spark gaps, gas discharge tubes, thyristors, and other arc extinguishing devices divert transient currents to the ground to protect the circuit. When the arc extinguishing device is in an active state, the protected equipment does not work, but once the transient disappears, the equipment can work normally.
Clamping devices include metal oxide varistors (MOVs), Zener diodes, and transient voltage suppression (TVS) avalanche diodes, which maintain a constant voltage across the protected device by changing impedance. These technologies can be used individually or simultaneously. TVS diodes are widely used as clamping devices due to their fast response speed and high 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 will divert excessive current and maintain or clamp the voltage at a constant potential. When the applied voltage is lower than the breakdown value, it will automatically reset.
TVS diodes can be used as unidirectional devices to prevent unipolar transients, as well as bidirectional devices to prevent transients of any polarity (Figure 2). Bidirectional devices can be symmetrical, capable of clamping any polarity voltage with the same amplitude, or asymmetrical, capable of clamping to different voltage levels based on the polarity of transients.
Figure 2: Current voltage breakdown characteristics and schematic symbols of three TVS devices. (Image source: Littelfuse Inc.)
The working principle of a unidirectional TVS diode is similar to that of a simple diode. It conducts when forward biased and does not conduct when reverse biased, until it exceeds the breakdown voltage (VBR) of the diode. When the applied voltage exceeds VBR, the diode conducts, maintaining the voltage across its terminals at the clamp voltage (VC). The maximum power that this diode can dissipate is the peak pulse current (IPP) x VC.
A bidirectional TVS diode is equivalent to two back-to-back diodes. When the breakdown voltage (VBR) is not exceeded in any direction, only a small reverse leakage current (IR) flows. This operation is symmetrical because the breakdown voltage amplitudes under the two bias conditions are the same.
The function of asymmetric TVS diodes is similar to that of bidirectional devices, but their breakdown voltages (VBR1 and VBR2) are different.
Asymmetric TVS diode
You may be wondering why asymmetric TVS diodes are needed. These components are designed to protect the gate drivers on silicon carbide (SiC) MOSFETs. Due to the fast switching speed of SiC, these drivers are prone to damage caused by overvoltage transients. Let's take a look at SiC MOSFET or traction inverter used for car charging (Figure 3).
Figure 3: Asymmetric TPSMB1505CA TVS used to protect SiC MOSFET switch gate drivers. (Image source: Littelfuse Inc.)
Asymmetric Littelfuse TPSMB1505CA TVS is used to protect the gate driver of MOSFETs. The gate driver has two states; The gate voltage in the on state is between -5 and 10 V, while in the off state it is below -10 V. The rated cathode (K) to anode (A) breakdown voltage of TPSMB1505CA is 16.7 to 18.5 V, and the maximum clamping voltage is 24.4 V. The Ipp in this direction is 24.6 A, and the transient pulse duration is 10 to 1000 ms.
The breakdown voltage of TVS from A to K is 6.8 to 7.4 V, and the maximum clamping voltage is 11.5 V. The peak pulse current in this direction is 60 A, and the transient pulse duration is also 10 to 1000 ms. It is worth noting that this can be achieved only through a single component. If independent components are used to achieve this asymmetric working mode, multiple components need to work together.
The Littelfuse TPSMB asymmetric series TVS diode (Figure 4) includes two additional components with different K to A breakdown voltages. TPSMB1805CA provides a K to A breakdown voltage range of 20.0 to 21.1 V, with a maximum clamp voltage of 29.2 V. The rated Ipp is 20.6 A, and the pulse duration is 10 to 1000 ms. The A to K breakdown voltage range is the same as TPSMB1505CA (6.8 to 7.4 V).