Applications of transient voltage suppressors

Abstract: This paper introduces the working principle, characteristic parameters and usage of transient voltage suppressors. It also gives examples of their application in surge elimination of switching power supplies.
Keywords: surge transient voltage suppressor switching power supply

Eliminating noise interference and preventing surge damage has always been a headache for electronic equipment designers. Transient voltage suppressors (TVS: transient voltage suppressor) can easily solve these problems.

When the two poles of TVS are subjected to reverse high-energy impact, it can change the impedance between the two poles from high to low at a speed of 10-12s, absorb up to several kilowatts of surge power, clamp the potential between the two poles to a predetermined value, and effectively protect the components in the electronic equipment from damage by surge pulses. TVS has the advantages of fast response time, high transient power, low leakage current, small breakdown voltage deviation, easy control of clamping voltage, and small size. It has been widely used in various fields such as household appliances, electronic instruments, communication equipment, power supplies, and computer systems.

1 Characteristics and main parameters of TVS

Figure 1 TVS voltage-current characteristics

1.1 Characteristics of TVS

The circuit symbol of TVS is the same as that of ordinary voltage regulator tube. Its voltage-current characteristic curve is shown in Figure 1.

Its forward characteristics are the same as those of ordinary diodes, and its reverse characteristics are typical PN junction avalanche devices. Figure 2 is the current-time and voltage-time curves of TVS. Under the action of surge voltage, the voltage between the two poles of TVS rises from the rated reverse shutdown 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, and the voltage at both ends of it will be clamped to below the predetermined maximum clamping voltage VC. Afterwards, 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. This is the process of TVS suppressing possible surge pulse power and protecting electronic components.

1.2 Main parameters of TVS

(1) Maximum reverse leakage current ID and rated reverse standoff voltage VWM

VWM is the maximum continuous working DC or pulse voltage of TVS. When this reverse voltage is applied between the two poles of TVS, it is in the reverse off state, and the current flowing through it should be less than or equal to its maximum reverse leakage current ID.

(2) Minimum breakdown voltage VBR and breakdown current IR

VBR is the minimum breakdown voltage of TVS. At 25℃, TVS will not avalanche below this voltage. When the specified 1mA current (IR) flows through TVS, the voltage applied to the two electrodes of TVS is its minimum breakdown voltage.

Figure 2 TVS voltage (current) time characteristics

Breakdown voltage VBR. According to the degree of dispersion between the TVS VBR and the standard value, VBR can be divided into 5% and 10%. For 5% VBR, VWM = 0.85VBR; for 10% VBR, VWM = 0.81VBR.

(3) Maximum clamping voltage VC and maximum peak pulse current IPP

When a pulse peak current IPP with a duration of 20μs flows through the TVS, the maximum peak voltage appearing at both ends is VC. VC and IPP reflect the surge suppression capability of the TVS. The ratio of VC to VBR is called the clamping factor, which is generally between 1.2 and 1.4.

(4) Capacitance C

Capacitance C is determined by the cross section of the TVS avalanche junction and is measured at a specific frequency of 1MHz. The size of C is proportional to the current carrying capacity of the TVS. If C is too large, the signal will be attenuated. Therefore, C is an important parameter for selecting TVS in data interface circuits.

(5) Maximum peak pulse power consumption PM

PM is the maximum peak pulse power dissipation value that TVS can withstand. Under a given maximum clamping voltage, the greater the power consumption PM, the greater its surge current tolerance; under a given power consumption PM, the lower the clamping voltage VC, the greater its surge current tolerance. In addition, the peak pulse power consumption is also related to the pulse waveform, duration and ambient temperature. Moreover, the transient pulses that TVS can withstand are non-repetitive, and the pulse repetition frequency (ratio of duration to intermittent time) specified by the device is 0.01%. If repetitive pulses appear in the circuit, the accumulation of pulse power should be considered, which may damage the TVS.

(6) Clamping time tc

tc is the time from zero to the minimum breakdown voltage VBR. For unipolar TVS, it is less than 1×10－12s; for bipolar TVS, it is less than 10×10－12s.

2TVS Classification

TVS devices can be divided into unipolar and bipolar types according to polarity; general-purpose and special-purpose types according to use; and axial lead diodes, dual in-line TVS arrays, surface mount and high-power modules according to packaging and internal structure. The peak power of axial lead products can reach 400W, 500W, 600W, 1500W and 5000W. Among them, high-power products are mainly used in power feeders, and low-power products are mainly used in high-density installation occasions. For high-density installation occasions, dual in-line and surface mount packaging can also be selected.

3TVS Selection Guide

(1) Determine the maximum DC or continuous operating voltage of the protected circuit.

Voltage, rated standard voltage and maximum withstand voltage of the circuit.

(2) The rated reverse shutdown voltage VWM of TVS should 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 avalanche or the reverse leakage current may be too large to affect the normal operation of the circuit.

(3) The maximum reverse clamping voltage VC of the TVS should be less than the protected

The damaging voltage of the circuit.

(4) The maximum peak value of TVS within the specified pulse duration

The pulse power PM must be greater than the peak pulse power that may occur in the protected circuit. After determining the maximum clamping voltage, its peak pulse current should be greater than the transient surge current. Generally, the maximum peak pulse power of TVS is given as a non-repetitive pulse of 10/1000μs, and the actual pulse width is determined by the pulse source. When the pulse width is different, the peak power is also different. For example, a 600WTVS has a maximum absorption power of 600W for a pulse width of 1000μs, but the absorption power can reach 2100W for a pulse width of 50μs, and the maximum absorption power for a pulse width of 10ms is only 200W. Moreover, the absorption power is also related to the pulse waveform: if it is a half-sine wave pulse, the absorption power will be reduced to 75%, and if it is a square wave pulse, the absorption power will be reduced to 66%.

(5) Matching of average steady-state power

For TVS that need to withstand regular, short-term pulse group impacts, such as those used in relays, power switches or motor control, it is necessary to introduce the concept of average steady-state power. For example, in a power switch circuit, a pulse group with a frequency of 120Hz, a width of 4μs and a peak current of 25A will be generated. The selected TVS can clamp the voltage of a single pulse to 11.2V. The calculation of the average steady-state power is: the pulse time interval is equal to the reciprocal of the frequency, 1/120=0.0083s, the peak absorption power is the product of the clamping voltage and the pulse current 11.2V×25A=280W, and the average power is the product of the peak power and the ratio of the pulse width to the pulse interval, that is, 280×(0.000004s/0.0083s)=0.134W. In other words, the average steady-state power of the selected TVS must be greater than 0.134W.

(6) For the protection of data interface circuits, attention must also be paid to the selection of

TVS device with suitable capacitance C.

(7) Select the polarity and packaging structure of TVS according to the application.

It is more reasonable to use bipolar TVS for circuits; it is more advantageous to use TVS arrays for multi-line protection.

(8) Temperature considerations

Transient voltage suppressors can work between -55℃ and +150℃. If TVS needs to work at a variable temperature, its reverse leakage current ID increases with increasing temperature; power consumption decreases with increasing TVS junction temperature, from +25℃ to +175℃, it decreases linearly by about 50%; breakdown voltage VBR increases with increasing temperature by a certain coefficient. Therefore, it is necessary to consult relevant product information and consider the impact of temperature changes on its characteristics.

4Comparison between TVS and varistor

At present, many devices in China that need surge protection still have

The varistor used is a metal oxide resistor. The performance of TVS is much better than that of varistor. The performance comparison is listed in Table 1.

Table 1 TVS and varistor performance comparison

5. TVS Naming

The following series of TVS tubes can be seen on the market:

SA Series—500W

P6KE, SMBJ Series—600W

1N5629～1N6389, 1.5KE, LC, LCE series—1500W

5KP Series—5000W

15KAP, 15KP Series—15000W

Among them, the P6KE and 1.5KE series are the most common, and their naming rules are as follows:

Part 1 + Part 2 + Part 3

Part 1...P6KE or 1.5KE

The second part...a number representing the minimum breakdown voltage

The third part...A or CA, A represents a unidirectional TVS tube; CA represents a bidirectional TVS tube

For example, P6KE200A is a 500W unidirectional TVS tube with a minimum breakdown voltage of about 200V.

6TVS Application Examples

As shown in Figure 3, this is a typical switching power supply drive circuit. When the power switch tube is turned off, due to the leakage inductance of the switching transformer coil, an extremely high back electromotive force will be generated, which may break down the power switch tube. When a TVS tube is connected to the primary side of the switching transformer, it can effectively absorb voltage spikes, protect the safety of the power switch tube, and reduce the voltage withstand requirements of the power switch tube. The selection of TVS tubes is as follows:

After rectification and filtering, 220V AC mains becomes high-voltage DC to supply the switching transformer. The range of this high-voltage DC is 240V~360V. However, due to the existence of transformer leakage inductance and lead inductance, the shutdown overvoltage can be as high as several thousand volts. It is difficult for the power switch tube to withstand these two voltages at the same time when it is turned off. After comprehensive consideration, a TVS with a VWM of about 200V can be selected to control the shutdown overvoltage within 300V. In addition to the power supply voltage, the power switch tube can be selected with a withstand voltage of 700V.

Let's estimate the PM value of the TVS tube:

The power dissipated by the TVS tube is the power stored in the circuit's stray inductance, so we can simply find the power stored in the stray inductance.

PM=PL=E/t=0.5LI2/t

The stray inductance is generally several μH, here we take 5μH;

The average primary current of a 100W switching power supply is I=P/U=100/240=0.417A. Assuming the duty cycle D is 0.5, the maximum current = I/D=0.417/0.5=0.833A, and I=1A.

The turn-off time of power switch devices is generally tens of nanoseconds, so t=10ns, we have:

PM=0.5×5×10－6×12/10×10－9=250W. Since the switching frequency of the switching power supply is very high, reaching more than 100kHz, considering the accumulation of pulse power, PM should be multiplied and 600W should be taken in combination with the nominal value of PM.

Therefore, a 200V/600W TVS tube can be selected. Since a unidirectional overvoltage is only generated when the switch tube is turned off, a unidirectional TVS can be selected, such as P6KE200A.