Selecting Devices with Rated Peak Pulse Power Waveform Energy Consider

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Selecting Devices with Rated Peak Pulse Power Waveform Energy Consider

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Selecting Devices with Rated Peak Pulse Power Waveform Energy Considerations
Transient Voltage Suppressors (TVSs) primarily rated Peak Pulse Power (PPP) when selecting what type given environment. Most rated 10/1000 double exponential waveform originating telecommunications. However, other pulse widths waveforms also better describe transient threats. This also affects device selection rating shown Figure Each waveform shown behaves different manner primarily because energy effective power integral over specified pulsed time interval (tW) will differ severity. ratings, given device determined Peak Pulse Current (IPP) multiplied Peak Clamping Voltage (VC) TVS. This peak current only briefly occur initial portion 10/1000 pulse shown Figure unusual cases effective peak during entire specified pulse width worst case square wave condition energy. These different transient impulse waveform energy conditions will also result different capabilities shown Figure from three examples shown. well behaved relationship between capabilities evident Figure declining slope region three waveforms shown until very long pulse widths experienced. declining slope region closely approximated following expression K(tW)-0.5 where different constant each waveform. This expression (also historically known Wunsch-Bell relation) applicable nonrepetitive pulse widths (tW) significantly shorter than thermal equilibrium time constant package. Pulse widths less than this relationship very well, particularly when active junction element (die) bonded adjoining package materials immediate heatsink. very long pulse widths approaching conditions, continues decline level approaches power rating package design. Repetitive pulses less than greater than 0.01% duty factor will require temperature derating based average power cumulative heating effects. designs, size effective silicon junction (zener area) larger permit comparatively high short transients. factor EQ.1 dictated pulse waveform shape inversely
proportional integrated area energy) under current pulse-time waveform illustrated smaller insets Figure From various wave shapes shown Figure shown that double exponential waveform factor times greater than square wave. onehalf sinusoidal waveform with tW=0.7tP factor 1.33 times greater than square wave. various design capabilities, broadened application including another design factor that proportional size effective silicon junction area. This added design feature would then further provide capability expression: CK(tW)-0.5
Large small devices with their corresponding rating primarily designed with this relationship where larger active element junction areas increase factor higher rating visa versa lower rating sizes.
(PPP) Peak Pulse Power, (kW)
IMPULSE EXPONENTIAL DECAY
HALF SINE
SQUARE CURRENT WAVE FORM
1000
FIGURE PEAK PULSE POWER PULSE (tw)
(For 1500 Rated 10/1000 Wave Form)
TEST WAVE FORM PARAMETERS 10µsec 1000µsec Peak Value:
PULSE CURRENT
Series
seen this relationship, same negative slope behavior loglog plot Figure will result different higher lower designs versus various current impulse waveforms. This behavior provides similar identical negative slope lines seen Figure translated down ordinate axis higher lower ratings when designing with larger smaller package elements. energy which absorbs derived integrating power with time follows: where power P(t) function time primarily dictated pulse wave shape surge current multiplied times clamping voltage clamp characteristic, this latter often considered relatively constant whereas current waveshape function time i(t) shown Figure example double exponential 10/1000µs. should noted that P(t) EQ.2 except special case square wave. various current impulse wave shapes, then becomes:
Half-Value: 1000 Wave Form defined R.E.A.
Time FIGURE PULSE WAVE FORM
pulse width This constant P(t) also equates value described previously Substituting this value into square wave integrating solution will result
seen from energy (joules) that absorbed increases square root pulse width (tW). plotted similar manner Figure would instead have positive slope shown Figure with same magnitude negative slope transient peak pulse power increased. Although different slope direction, rating energy also still dependent pulse width this reason, simply rating power energy adequate without defining pulse width well.
gain insight energy capability device behaves with time simplified example, will square wave impulse where current i(t) constant during pulse width this case, power P(t) also remains constant over
various transient waveforms highest magnitude surge currents over short period time that most critical. example exponentially decaying wave damped sinusoidal wave, initial highest magnitude region impulse that greatest concern selecting TVS. This best understood from since energy rating decreases with pulse width while peak pulse power increases. This illustrated Figure Transient Voltage Suppressors characterized rated with this feature mind since high short time intervals, whereas energy absorbing capabilities minimal improves with longer pulse widths. example, region beyond half-value decay point double exponential waveform becomes negligible comparative added energy safely absorb prolonged tail region impulse. Double exponential waveforms such shown Figure therefore simply described
Peak Pulse Power
JOULES
PEAK PULSE POWER
Energy square wave pulse width
pulse their decay point. also said that such waveforms decay faster than capability curve pulse width figure thus making prolonged pulse widths negligible rating capability device. Similarly damped sinusoidal transient voltage surges (not shown), only initial worst case half-sine wave needs considered selection each direction transient decays half initial peak within eight periods oscillation. After considering source impedance with these described transient voltage waveforms, peak sinusoidal transient currents (IPP) used determine when multiplied their rated clamping voltages (VC). These transient currents shown damped sinusoidal waveforms with formula: i(t) e(-ft/Q) sin(2ft) where frequency, damping factor, approximates initial half sinewave peak surge current. sinusoidal peaks will subsequently decay less than initial peak within cycles less than Typical values range from many applications. actual factor dependent resonant frequency system where such transients observed length electrical lines other factors. These type waveforms often experienced from lightning induced transient bursts such with onboard systems aircraft ship applications. summary, transient power capability (PPP) inversely
0.01
1000
FIGURE PEAK PULSE POWER ENERGY TIME (tw) (Example shown square wave)
proportional square root short transient pulse widths contrast energy proportional square root these same short pulse widths. Since TVSs primarily rated terms PPP, defining pulse width shape both vital overall selection process seen Figure When transient current pulse widths shapes well defined (magnitude shape), wise design conservatively select TVSs with higher rating. Even needed, this also results improved (lower) clamping voltage performance added margin reliability during surges described Micro Note 108.
ENERGY CAPABILITY
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