TVS tube performance and selection tips

Everyone knows the circuit, but do you know what a TVS transient voltage suppressor is? TVS transient voltage suppressor. When the two poles are subjected to a reverse transient high-energy impact, the high impedance between the two poles can be changed into a low impedance at a speed of the order of 10 to the power of negative 12 seconds, so that the voltage between the two poles is clamped at a predetermined value, effectively Protect precision components in electronic circuits. Under the action of surge voltage, the voltage between the two poles of TVS rises from the rated reverse turn-off voltage VWM to the breakdown voltage VBR, and is broken down. With the emergence of breakdown current, the current flowing through TVS will reach the peak pulse current. IPP, at the same time, the voltage at both ends of it is clamped below the predetermined maximum clamping voltage VC. Then, as the pulse current decays exponentially, the voltage between the two poles of TVS also continues to decrease, and finally returns to the initial state; the TVS tube has Divided into unidirectional and bidirectional, the characteristics of a unidirectional TVS tube are similar to those of a Zener diode, and the characteristics of a bidirectional TVS tube are equivalent to two Zener diodes connected in reverse series.

1. Its main characteristic parameters

1. Reverse cut-off voltage VRWM and reverse leakage current IR: Reverse cut-off voltage VRWM represents the highest voltage at which the TVS tube does not conduct. At this voltage, there is only a small reverse leakage current IR.

2. Breakdown voltage VBR: the voltage when the TVS tube passes the specified test current. This is the symbol voltage indicating that the TVS tube is conductive.

3. Pulse peak current IPP: The maximum peak current of the 10/1000μs wave that the TVS tube is allowed to pass through (the peak current of the 8/20μs wave is about 5 times). Exceeding this current value may cause permanent damage. In the same series, the higher the breakdown voltage of the tube, the smaller the peak current allowed to pass, which is usually several to tens of A.

4. Maximum clamping voltage VC: the voltage present at both ends of the TVS tube when the pulse peak current IPP flows through it.

5. Pulse peak power Pm: Pulse peak power Pm refers to the product of the pulse peak current IPP of the 10/1000μs wave and the maximum clamping voltage VC, that is, Pm=IPP*VC; at a given maximum clamping voltage, the power consumption The larger the PM, the greater its surge current tolerance. At a given power consumption PM, the lower the clamping voltage, the greater its surge current tolerance. In addition, the peak pulse power consumption is also related to the pulse waveform, continuous Time is related to the ambient temperature: the typical pulse waveform duration is 1ms. When the pulse waveform duration applied to the diode is less than TP, the pulse peak power increases as TP decreases; the transient pulse type that TVS can withstand is not Repeated, if repetitive pulses appear in the circuit, it should be considered that the accumulation of pulse power may damage the TVS.

6. Steady-state power P0: TVS tube can also be used as a Zener diode, and steady-state power must be used at this time.

7. Inter-electrode capacitance Cj: Like the varistor, the inter-electrode capacitance Cj of the TVS tube is also larger, and the one-way one is larger than the two-way one. The greater the power, the larger the capacitance. The inter-electrode capacitance will affect the response of the TVS. time.

8. Peak current waveform: A. Half sine wave B. Rectangular wave C. Standard wave (exponential waveform) D. Triangular wave TVS The test waveform of peak current adopts standard wave (exponential waveform) and is determined by TR/TP. Peak current rise time TR: The time for the current to reach 0.9 IPP from 0.1 IPP. Half-peak current time TP: The time for the current to drop to the value of 0.5IPP after passing through the maximum peak value from zero. The TR/TP values ​​of typical test waveforms are listed below: EMP wave: 10ns/1000ns Lightning wave: 8μs/20μs Standard wave: 10μs/1000μs

2. Advantages and Disadvantages

Advantages: fast response speed (ns level), large transient power, and low leakage current; its 10/1000μs wave pulse power ranges from 400W to 30KW, and the pulse peak current ranges from 0, 52A to 544A; breakdown voltage ranges from 6, 8V The series value of ~550V is convenient for use in circuits with different voltages.

Disadvantages: poorer surge resistance than discharge tubes and varistor; Zener diode: slow response; generally used in places with high voltage accuracy requirements (generally small), anti-surge, accurate breakdown voltage, each voltage Value levels are available; Zener breakdown; Varistor: similar to Zener diode, but non-recoverable.

3. Selection basis and precautions:

1. The maximum reverse clamping voltage VC of TVS should be less than the damage voltage of the protected circuit;

2. The rated reverse turn-off voltage VWM of the TVS must be greater than or equal to the maximum operating voltage of the protected circuit. If the selected VWM is too low, the device may enter an avalanche or affect the normal operation of the circuit due to too large a reverse leakage current;

3. Only two-way TVS can be used for AC voltage;

4. Within the specified pulse duration, the maximum peak pulse power PM of the TVS must be greater than the possible peak pulse power of the protected circuit. After the maximum clamping voltage is determined, its peak pulse current should be greater than the transient surge current;

5. Junction capacitance is a key factor affecting the use of TVS in high-speed lines. In this case, a TVS tube and a fast recovery diode are generally connected in a back-to-back manner. Since the fast recovery diode has a small junction capacitance, The small capacitors connected in series between the two are also smaller, which can meet the requirements of high-frequency use;

6. Applications that need to be considered for derating; Applications: power switching circuit; rectifier diode (in the same direction as it); power transformer; preventing the reverse connection of the DC power supply or the instantaneous pulse generated when the power supply is turned on and off; suppressing motors and breaker coils , the instantaneous pulse voltage generated by inductive loads such as solenoids; the input and output terminals of the control system.

Precautions for use:

1. Regarding the relationship between the peak absorbed power of transient voltage and the pulse width of transient voltage, the manual only gives the peak absorbed power under a specific pulse width. The pulse width in actual lines is unpredictable. It is necessary to estimate it in advance. Wide pulses should be used with derating;

2. To protect small current loads, you can consciously add a current-limiting resistor in the circuit. As long as the resistance of the current-limiting resistor is appropriate, it will not affect the normal operation of the line. However, the current generated by the interference caused by the current-limiting resistor will be greatly affected. decrease, it is possible to use TVS tubes with smaller peak power to protect small current load circuits;

3. To suppress recurring transient voltages, pay attention to whether the steady-state average power of the TVS tube is within a safe range;

4. When the ambient temperature rises, it must be derated. The length of the lead of the TVS tube and the relative distance between it and the protected line.

4. Application examples

Example of selection in DC circuit: The DC working voltage of the whole machine is 12V, the maximum allowable safe voltage is 25V (peak value), the impedance of the surge source is 50MΩ, the interference waveform is a square wave, TP=1ms, and the maximum peak current is 50A. choose:

1. First select the maximum reverse working voltage VRWM from the working voltage of 12V to 13V, then the breakdown voltage is V(BR)=VRWM/0.85=15.3V;

2. Select the maximum clamping voltage VC(MAX)=1.30×V(BR)=19.89V from the breakdown voltage value, and take VC=20V;

3. Calculate the square wave pulse peak power from the clamping voltage VC and the peak current IP: PPR=VC×IP=20×50=1000W

4. Calculate the peak power of the exponential wave converted to TP=1MS, the conversion coefficient K1=1.4, PPR=1000W÷1.4=715W. AC circuit selection example: DC lines use unidirectional transient voltage suppression diodes, and AC must use bidirectional transients. Voltage suppression diode. AC is the grid voltage, and the transient voltage generated here is random. Sometimes lightning strikes (transient voltages generated by lightning induction) are encountered, so it is difficult to quantitatively estimate the instantaneous pulse power PPR. However, the maximum reverse operating voltage must be correctly selected. The general principle is to multiply the AC voltage by 1.4 times to select the maximum reverse working voltage of the TVS tube. The DC voltage is 1.1-1.2 times to select the maximum reverse working voltage VRWM of the TVS tube. TVS protected DC regulated power supply example:

The picture shows a DC regulated power supply with a transistor to expand the current output. Adding a transient voltage suppression diode to its regulated output terminal can protect the equipment and equipment using the power supply, and at the same time, the picture shows a DC regulated power supply. The power supply has a transistor that expands the current output, and a transient voltage suppression diode is added to its regulated output end to protect the instrument and equipment using the power supply, and at the same time, it can absorb the peak voltage between the collector and the emitter in the circuit. , to protect the transistor, it is recommended to add a TVS tube at the output end of each regulated power supply, which can greatly improve the reliability of the entire machine application;

TVS is also used to protect transistors, integrated circuits, thyristors, power MOS tubes (a transient voltage suppression diode is added between the gate and source to prevent gate breakdown), relays, etc.; the contacts of relays often use large currents to remove Switching motors and other high-current inductive loads, while the inductor has a high back electromotive force when switching, and has a large energy, which often burns out or breaks down the contacts to produce arcs, etc. The contacts must be protected to suppress the arc. generated to protect the relay. However, the surge current generated by this arc is very large. In the past, suppression schemes such as capacitors or capacitors in series with resistors, diodes, and diode series resistors were used. Nowadays, the transient voltage suppression diode scheme is more effective. The above is an introduction to some knowledge about TVS transient voltage suppressors. I hope it can inspire you.