TVS Diode Selection Guide Tutorial· November 29, 2013
How to Select Transient Voltage Suppressors (TVS Diode) ?
TVS Diode Tutorial and Selection Procedure:
Transient voltage suppressors (TVS) are clamping devices that limit voltage spikes by low impedance avalanche breakdown of a rugged silicon PN junction.
They are used to protect sensitive components from electrical overstress.
When a transient appears in the circuit, the TVS becomes active, clamping it to a harmless level.
The symbol and characteristics of the TVS diode are given below:
Its electrical parameters such as breakdown voltage (VBR), leakage current (ID) and capacitance should be invisible to the circuit and have no effect on performance.
The reverse standoff voltage (VWM), which approximates the circuit operating voltage, is normally 10% below breakdown voltage. This assures minimal standby leakage current.
- The TVS clamps instantly when transients occur, limiting the spike voltage to a safe level while diverting damaging currents away from the protected part.
- Most devices are specified with a 10/1000µs surge waveform (10µs rise to peak and 1000µs exponential decay to one half peak).
- TVS families are normally specified in KW of Peak pulse power (PPP).
- Power rating is derived from the product of peak pulse current (IPP) and the clamping voltage (VC).
- TVS diodes are available for operating voltages ranging from 5V through 376V.
- TVS diode is primarily intended to serve as a shunt-voltage clamp across sensitive components in the circuit to prevent high voltage transients from damaging them.
- Until these transients occur, the TVS diode will be idling at very low standby current levels and appear “transparent” to the circuit.
- When a high voltage transient does occur, the device clamps the voltage by avalanche breakdown.
Definitions (or) Terminology:
Reverse Standoff voltage (VRWM):
This is the normal dc operating voltage of the device. At this point, the device will appear as high impedance to the protected circuit.
It is also known as working voltage. Devices are available ranging from 2.8V – 440V.
Reverse Breakdown Voltage (VBR):
This is the point where the device begins to conduct in avalanche mode and becomes a low impedance path for the transient.
Breakdown voltage is measured at a test current (IT) typically 1mA or 10mA. This is important parameter we have to consider in datasheets for selecting the TVS diode.
Peak Pulse Current (IPP):
This is Maximum permissible surge current which the device can withstand without damage.
TVS diode datasheets specify a peak pulse capability for a particular transient waveform. (Mostly diodes are rated using 8/20µs or 10/1000µs impulse waveform)
Clamping Voltage (VC):
It is the Maximum voltage drop across the TVS diode for a particular peak pulse current.
When using TVS diodes, the most important parameters are identified as
- Rated Working Peak Voltage or Rated Standoff Voltage (VWM),
- Peak Pulse Power Dissipation (PPP = IPP * VC ),
- Peak Impulse Current (IPP),
- Clamping Voltage (VC).
Steps to be followed while selecting TVS Diode:
- The first step in selecting a TVS diode is to determine What highest continuous peak normal operating voltage will be at the point of intended protection in the circuit.
- This should include continuous dc or repetitive ac peak voltages such as sinusoidal peaks intended for normal operation.
- This highest operating voltage will then determine the Rated Standoff Voltage (VWM) selection of the TVS component.
- This is also identified as the rated working peak voltage for the selected TVS diode.
- The next higher voltage characterized for TVS diode is the breakdown voltage VBR.
- It is typically 10 to 15% above VWM and is the voltage that TVS devices go into avalanche similar to a zener diode.
- The highest voltage parameter specified for a TVS diode is VC or clamping voltage under high-current pulse conditions.
- It is typically 35 to 40 % higher than VBR (or 60 % higher than VWM) and represents the maximum clamping voltage during the specified peak impulse current IPP.
What is the deference between Clamping device and Crow-bar device?
Clamping device vs Crow-bar device:
A clamp device begins conducting when its threshold voltage is exceeded, then restores to the nonconducting mode when voltage drops below threshold level. Voltage spikes are clipped off to safe levels by clamp devices. Ex: TVS Diode & MOVs.
Crowbar devices conduct when threshold voltages are exceeded and then trigger to an on-state voltage drop or only a few volts, hence the name crowbar. These devices restore to nonconduction when the driving voltage and/or current is reduced with the passing of the transient.
What is the deference between MOV and TVS Diode?
Comparison of TVS Diode with MOV:
MOVs are most often used on the power mains to protect downstream electronics and electrical equipment from direct and nearby lightning hits.
Silicon TVS diodes are used extensively for protection across more sensitive data lines on telecommunication and microprocessor based monitoring systems.
The ratio of VC/VBR is known as the Clamping Factor.
The ratio of Clamping Voltage to Working Peak Voltage (VC/VWM) is known as the Voltage Clamping Ratio.
Neither the Clamping Factor nor the Voltage Clamping Ratio is directly shown on TVS diode data sheets.
Clamping ratios for TVS diodes are significantly lower than Metal Oxide Varistors MOV.
What is the deference between TVS diode and Zener Diode?
Zener vs TVS Diode (Avalanche Breakdown Diode):
Zener diodes were the first offered for those applications requiring a shunt (parallel) voltage regulator.
The TVS diodes specifically designed for transient voltage protection. The TVS diodes were located in front of the load to be protected and primarily served as a shunt path to ground for high surge currents that were generated by high-voltage transients such as inductive switching, induced lightning, and electro-static discharge (ESD) events.
Both of the Zener and TVS semiconductor diodes primarily use the avalanche breakdown voltage properties of a silicon p-n junction for their important features.
In that respect, the TVS diode is also sometimes called an “ABD” or Avalanche Breakdown Diode.
- Zener Diode Regulating the voltage
- TVS Diode Clamping the voltage
- Structure for both is same. Made up of Si based PN diode.
- Principle of working is same. Both are driven into reverse breakdown for either regulating or clamping voltage features
Some of the TVS Diode manufactures are Bourns, Littelfuse, Vishay.