Devices are susceptible to high voltage transients. Let us assume a device operating at +5V. If due to transient, the voltage suddenly raises in terms of kV, then there is every chance that internal circuitry of the device may get damaged. A voltage transient is a short duration surge of electrical energy due to stored statics, lightening or due to heavy inductive loads. These transients are divided into repeatable and random. Repeatable transients are due to motors, reactive component switching in a circuit, generators. A random transient is always unpredictable and this is due to lightening or a static discharge. This is where we have to use some kind of suppressors of this high voltage. Electrostatic discharge diodes or transient voltage suppressor diodes (TVS) are used in this case. They protect the I/O, digital and analog signals lines from transients. Available in different packages and are a fit for form factor boards also.
Check the following graph which shows a sudden transient:
The spike exists for a duration of sub nano seconds with high current. With modern day circuits operating at low voltages, arresting this spike is of primary importance for circuirt to function properly in case of such high transients.
Why does a static gets built up?
Static is due to friction between two dissimilar materials. This static gets discharged when it comes in contact with a conducting surface. Static above certain kV is pretty dangerous to the circuit. Generally, they say if any static above 1kV, it is always better to have static protection. there are some certain specifications like IEC61000-4-2 which is further divided into several levels showing the extent of ESD voltages. The below table indicates such levels:
What does a ESD diode do?
The below figure shows a ESD diode connection:
When supply exceeds due to ESD, the diode breaks down and shunts down the current. In this state, the voltage is clamped to the reverse stand-off voltage of the diode. The diode then automatically resets to the original state once the transient is OFF>TVS diodes have the faster response than any other devices so mostly preferred. These TVS diodes can be unidirectional or bi-directional. An avalanche diode can be thought of as combo of zener and normal diode. As a normal diode it allows current to flow in and as a zener it clamps the voltage level to safe value. Mostly, avalanche diodes are preferred for TVS operation compared to zener diode. When breakdown occurs, avalanche settles at a little drop from the breakdown point where as in a zener diode the breakdown voltage is always less then the maintained voltage.
The problem with using the TVS diodes available in market is in the case of high current transient. In such a case, the clamping voltage is high and may damage the circuit. So, in case of a high frequency, high current transients, it is always better to use a TVS diode which gives specifications in that case.
Requirements for using a TVS diode:
Check the following graph which shows a sudden transient:
Why does a static gets built up?
Static is due to friction between two dissimilar materials. This static gets discharged when it comes in contact with a conducting surface. Static above certain kV is pretty dangerous to the circuit. Generally, they say if any static above 1kV, it is always better to have static protection. there are some certain specifications like IEC61000-4-2 which is further divided into several levels showing the extent of ESD voltages. The below table indicates such levels:
The below figure shows a ESD diode connection:
When supply exceeds due to ESD, the diode breaks down and shunts down the current. In this state, the voltage is clamped to the reverse stand-off voltage of the diode. The diode then automatically resets to the original state once the transient is OFF>TVS diodes have the faster response than any other devices so mostly preferred. These TVS diodes can be unidirectional or bi-directional. An avalanche diode can be thought of as combo of zener and normal diode. As a normal diode it allows current to flow in and as a zener it clamps the voltage level to safe value. Mostly, avalanche diodes are preferred for TVS operation compared to zener diode. When breakdown occurs, avalanche settles at a little drop from the breakdown point where as in a zener diode the breakdown voltage is always less then the maintained voltage.
The problem with using the TVS diodes available in market is in the case of high current transient. In such a case, the clamping voltage is high and may damage the circuit. So, in case of a high frequency, high current transients, it is always better to use a TVS diode which gives specifications in that case.
Requirements for using a TVS diode:
- Must have low input capacitance. Else, can't be used on a high frequency data line. with high capacitance it may degrade the rise and fall time of the signal.
- Must have low leakage in case of normal operation. This is very important in battery powered application.
- The clamp voltage must be lowest possible.
- Reverse stand off voltage of TVS diode used must be greater then the operating voltage of circuit
- Inductance is the main cause of slow response of TVS diode to transient, so, eliminate it as much as possible. this is why we tend it keep it close to the device for which ESD protection is desired.
- Exposed interfaces like HDMI, USB, Audio are very much effected by ESD and TVS diodes on such interfaces is a must
- Place the TVS diode as close as possible to the connectors.
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