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Transient power capability of zener diodes

Posted: 12 Dec 2000     Print Version  Bookmark and Share

Keywords:Transient power capability 

to voltage transients in excess of their ratings, circuits are

often designed to inhibit voltage surges in order to protect

equipment from catastrophic failure. External voltage

transients are imposed on power lines as a result of lightning

strikes, motors, solenoids, relays or SCR switching circuits,

which share the same ac source with other equipment.

/PDF document

Motorola TVS/Zener Device Data

11-1

Application Notes

Chapter 11

AN784 -- TRANSIENT POWER CAPABILITY

OF ZENER DIODES

Prepared by

Applications Engineering and

Jerry Wilhardt, Product Engineer -- Industrial and Hi-Rel Zener Diodes

INTRODUCTION

Because of the sensitivity of semiconductor components

to voltage transients in excess of their ratings, circuits are

often designed to inhibit voltage surges in order to protect

equipment from catastrophic failure. External voltage

transients are imposed on power lines as a result of lightning

strikes, motors, solenoids, relays or SCR switching circuits,

which share the same ac source with other equipment.

Internal transients can be generated within a piece of

equipment by rectifier reverse recovery transients, switching

of loads or transformer primaries, fuse blowing, solenoids,

etc. The basic relation, v = L di/dt, describes most equipment

developed transients.

ZENER DIODE CHARACTERISTICS

Zener diodes, being nearly ideal clippers (that is, they

exhibit close to an infinite impedance below the clipping level

and close to a short circuit above the clipping level), are often

used to suppress transients. In this type of application, it is

important to know the power capability of the zener for short

pulse durations, since they are intolerant of excessive stress.

Some Motorola data sheets such as the ones for devices

shown in Table 1 contain short pulse surge capability.

However, there are many data sheets that do not contain

this data and Figure 1 is presented here to supplement this

information.

Table 1. Transient Suppressor Diodes

Series

Numbers

Steady

State Power Package Description

1N4728 1 W DO-41 Double Slug Glass

1N6267 5 W Case 41A Axial Lead Plastic

1N5333A 5 W Case 17 Surmetic 40

1N746/957

A/4371

400 mW DO-35 Double Slug Glass

1N5221A 500 mW DO-35 Double Slug Glass

Some data sheets have surge information which differs

slightly from the data shown in Figure 1. A variety of reasons

exist for this:

1. The surge data may be presented in terms of actual

surge power instead of nominal power.

2. Product improvements have occurred since the data

sheet was published.

Figure 1. Peak Power Ratings of Zener Diodes

Power is defined as VZ(NOM) x IZ(PK) where VZ(NOM) is the nominal

zener voltage measured at the low test current used for voltage

classification.

1N6267 SERIES

GLASS DO-35 & GLASS DO-41

250 mW TO 1 W TYPES

5 WATT TYPES

PULSE WIDTH (ms)

0.1

100

0.01 0.02

PPK(NOM),NOMINALPEAKPOWER(kW)

50

20

10

5

2

1

0.5

0.2

0.1

0.05

0.02

0.01

0.05 0.2 0.5 1 2 5 10

1 TO 3 W TYPES

PLASTIC DO-41

3. Larger dice are used, or special tests are imposed on

the product to guarantee higher ratings than those shown

on Figure 1.

4. The specifications may be based on a JEDEC

registration or part number of another manufacturer.

The data of Figure 1 applies for non-repetitive conditions

and at a lead temperature of 250C. If the duty cycle

increases, the peak power must be reduced as indicated by

the curves of Figure 2. Average power must be derated as

the lead or ambient temperature rises above 250C. The

average power derating curve normally given on data sheets

may be normalized and used for this purpose.

At first glance the derating curves of Figure 2 appear to

be in error as the 10 ms pulse has a higher derating factor

than the 10 5s pulse. However, when the derating factor for

a given pulse of Figure 2 is multiplied by the peak power

value of Figure 1 for the same pulse, the results follow the

expected trend.

When it is necessary to use a zener close to surge ratings,

and a standard part having guaranteed surge limits is not

suitable, a special part number may be created having a

surge limit as part of the specification. Contact your nearest

Motorola OEM sales office for capability, price, delivery, and

minimum order criteria.

REV 1

CHAPTER 11

Motorola TVS/Zener Device Data

11-2

Application Notes

Figure 2. Typical Derating Factor for Duty Cycle

0.1

1

0.7

0.5

0.3

0.2

0.02

0.1

0.07

0.05

0.03

0.01

0.2 0.5 1 52 10 20 50 100

PULSE WIDTH

10 ms

1 ms

100 5s

10 5s

D, DUTY CYCLE (%)

DERATINGFACTOR

MATHEMATICAL MODEL

Since the power shown on the curves is not the actual

transient power measured, but is the product of the peak

current measured and the nominal zener voltage measured

at the current used for voltage classification, the peak current

can be calculated from:

IZ(PK) =

P(PK)

VZ(NOM)

(1)

The peak voltage at peak current can be calculated from:

(2)VZ(PK) = FC W VZ(NOM)

where FC is the clamping factor. The clamping factor is

approximately 1.20 for all zener diodes when operated at

their pulse power limits. For example, a 5 watt, 20 volt zener

can be expected to show a peak voltage of 24 volts

regardless of whether it is handling 450 watts for 0.1 ms or

50 watts for 10 ms. This occurs because the voltage is a

function of junction temperature and IR drop. Heating of the

junction is more severe at the longer pulse width, causing

a higher voltage component due to temperature which is

roughly offset by the smaller IR voltage component.

For modeling purposes, an approximation of the zener

resistance is needed. It is obtained from:

RZ(NOM) =

VZ(NOM)(FC-1)

PPK(NOM)/VZ(NOM)

(3)

The value is approximate because both the clamping

factor and the actual resistance are a function of

temperature.

CIRCUIT CONSIDERATIONS

It is important that as much impedance as circuit

constraints allow be placed in series with the zener diode

and the components to be protected. The result will be a

lower clipping voltage and less zener stress. A capacitor in

parallel with the zener is also effective in reducing the stress

imposed by very short duration transients.

To illustrate use of the data, a common application will be

analyzed. The transistor in Figure 3 drives a 50 mH solenoid

which requires 5 amperes of current. Without some means

of clamping the voltage from the inductor when the transistor

turns off, it could be destroyed.

Figure 3. Circuit Example

Used to select a zener diode having the proper voltage and power

capability to protect the transistor.

10 ms

2 s

26 Vdc

50 mH, 5

The means most often used to solve the problem is to

connect an ordinary rectifier diode across the coil; however,

this technique may keep the current circulating through the

coil for too long a time. Faster switching is achieved by

allowing the voltage to rise to a level above the supply before

being clamped. The voltage rating of the transistor is 60 V,

indicating that approximately a 50 volt zener will be required.

The peak current will equal the on-state transistor current

(5 amperes) and will decay exponentially as determined by

the coil L/R time constant (neglecting the zener impedance).

A rectangular pulse of width L/R (0.01 sec) and amplitude

of IPK (5 A) contains the same energy and may be used to

select a zener diode. The nominal zener power rating

therefore must exceed (5 A W 50) = 250 watts at 10 ms and

a duty cycle of 0.01/2 = 0.5%. From Figure 2, the duty cycle

factor is 0.62 making the single pulse power rating required

equal to 250/0.62 = 403 watts. From Figure 1, one of the

1N6267 series zeners has the required capability. The

1N6287 is specified nominally at 47 volts and should prove

satisfactory.

Although this series has specified maximum voltage limits,

equation 3 will be used to determine the maximum zener

voltage in order to demonstrate its use.

RZ =

47(1.20 - 1)

500/47

9.4

10.64

= = 0.9

At 5 amperes, the peak voltage will be 4.5 volts above

nominal or 51.5 volts total which is safely below the 60 volt

transistor rating.





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