Transient Peak Power Effect on Diodes

www.ecnmag.com • ECN • August 2002
43
Discrete Semiconductors
Edited by Aimee Kalnoskas,
Editor-in-Chief
Transient Peak Power Effect on Diodes
by Sze Chin, Central Semiconductor Corp.
enerally, it is common knowledge
that when excessive power is dissipated within a diode junction, its
junction temperature (TJ) rises. As TJ rises
beyond the critical limit, the diode would suffer irreversible damage. Although all diodes are specified with
maximum operating temperature (TJ), maximum thermal
resistance (Θ), and maximum power dissipation (PD), some
diodes still fail during operation despite the appearance
that they operate within the maximum ratings. During
the transient period, abnormally high current and voltage
spikes may be present, thus resulting in high peak power
(Ppk) that the diodes must be capable of withstanding.
From the average standpoint, the operating average TJ
may be within the maximum rating. Presented here is a
discussion of thermal stability from the transient peak
power standpoint.
G
the value of the capacitors, typically at 0.01 ␮F, may be
small, however when hundreds or perhaps thousands of
these small capacitors are used, it becomes significant. As
DC power is applied to these modules, surge current
would result because all the filter capacitors act instantaneously as short circuits.
The turn-off transient usually involves inductive loads
as in solenoid drivers, horizontal deflection and ignition
systems. During turn-off, high voltage spikes develop
because of inductive kick-back. To absorb the turn-off
transient, Zener diodes usually are used to clamp the
spikes of regular diodes used as dampers. There are many
other applications wherein surge currents are present
with notable examples in light control and medical equipment. In light control systems, the lamp impedance is
always low before warming up so initially, the current is
always high. Whereas in medical equipment, such as a
cardiac defibrillator, its output is a wave form lasting a
few milliseconds with current over 100 amps and voltage
typically at several thousand volts.
Transient Examples
Practically all electrical systems experience transient
periods immediately after input power is turned on or
off. During this period, high current and voltage spikes
are present, thus resulting in peak power and thermal
stress on the electrical components. Semiconductors
such as transistors and diodes must be able to withstand such stress, otherwise their characteristics would
deteriorate and the device would eventually fail.
One popular application for diodes is in DC power supplies wherein the diodes are used to rectify the line input
voltage waveform. As soon as the line power is turned on,
the filter capacitors at the rectifier output instantaneously act as a short circuit. Hence, the initial surge current
would be extremely high, resulting in severe thermal
stress on the diodes. Even after the DC power reaches
steady state, surge current still may occur when the load
is suddenly changed to a low impedance condition. One
good example is that within any electronic system, the
electronic models such as PC boards are composed of
many IC chips and active components. It is standard practice that filter capacitors are used at the DC bias inputs to
the IC chips and other active components. Even though
Figure 1. Characteristic of RC CKT vs. Time.
Figure 2. Characteristics of Thermal CKT vs. Time.
Thermal Response of
Diode Junction
When a diode is in operation, heat is generated within its junction. The rate of heat generation is a function
of Power (P). Heat is transmitted from the junction to
the case, case to heat sink, and then heat sink to ambient. Or if no heat sink is used, heat is dissipated from
case to ambient. The junction temperature (TJ) will rise
until equilibrium is reached when the rate of heat generation is equal to the rate of heat dissipation. How fast
and how high TJ rises depends on the conducting medium.
Generally, the characteristics of the conducting medium are defined in terms of thermal resistance (Θ). Using
an electrical analogy where the temperature gradient is
equivalent to the voltage gradient, power to current, and
Θ to resistance, the junction temperature rise ∆TJ relative to the case would be the product of the power (P)
and thermal resistance junction to case (Θjc) (∆TJ = P ⫻
Θjc). If relative to ambient, ∆TJ = P ⫻ Θja.
There are times when a diode would fail during operation even though its TJ is seemingly below the maximum rating based
on the DC or average power dissipation. It is very possible that some time
during operation
when a transient
surge occurs, the
excess heat associated with the surge
may not dissipate
fast enough from the
junction to the case,
thus causing TJ to
rise beyond the critical limit. Another
possible scenario
would be if the diode
current is in the
Figure 3. Characteristics of
form of repetitive
Thermal CKT vs. Time.
pulses. There is a
44
www.ecnmag.com • ECN • August 2002
Semiconductor Monthly
significant difference when ∆TJ is analyzed from the
average current standpoint as opposed to the current in
pulse form.
Transient Peak Power Analysis
As an illustration for transient peak power (Ppk) analysis, Figure 1 illustrates an RC network fed by a current
pulse source. Assuming the current max. (Imax) is 10
amps, duty cycle 20 percent, resistance 10 Ω and the
capacitor value is such that the RC time constant is
greater than the off-pulse period. from the average point
of view, the average current is 2 amps. However, the
voltage across the resistor is a function of time, and
eventually, when the steady state is reached, it may be
near 100 V, but not 20 V.
An analogy may be drawn in analyzing the temperature and power relationship on semiconductor diodes.
Figure 2 shows a thermal equivalent circuit where power
is equivalent to current, temperature to voltage.
Assuming the power pulse max. (Pmax) is 100 W, duty
cycle 20 percent and a junction to case thermal resistance (Θjc) 2°C/W, from the average point of view, the
junction temperature rise ∆TJ is only 40°C (20W ⫻
2°C/W). However, TJ would rise more than 40°C if the
thermal time constant is larger than the pulse-off period.
Transient Peak Power/Current
Determination
The same basic TJ = Px Θ governing the temperature
gradient, power and thermal resistance may be applied
in determining the maximum allowable peak power Ppk
for a given maximum TJ rating. The only difference here
is that P is Ppk and Θ is Θjc (t), the effective transient
thermal resistance which is a function of time.
Θjc (t) is analogous to the transient electrical resistance, if, as shown in Figure 1, the current pulse is a step
function with an amplitude of 1 amp, and the resistor
value 10 Ω. The RC network’s response to the step function is shown on Figure 3. As shown on Figure 3, the
network’s voltage at the steady state is 10 V, and at the
RC time constant point the voltage is 6.3 V. At another
time instant, less than the time constant, the voltage
would be less, based on the formula
V(t) = Imax R (1- e-t/RC).
If the transient voltage response V(t) is divided by
Imax, it becomes the transient resistance
R(t) = R(1-e-t/RC) and R(t) becomes 10 Ω when the steady
condition is reached. Before the steady state, R(t) is less
than 10 Ω because of the transient effect caused by the
capacitor C. Hence, at the time constant point, R(t) is 6.3
Ω, and at some earlier point T1, it may be 3 Ω. Once the
transient resistance R(t) of an RC network is known, the
transient voltage may be calculated or predicted for any
current pulse with a known pulse width PW. As an
example, referring to Figure 3, at T1 , R(t) = 3 Ω and for
a current pulse with PW = T1 , the voltage V(t) is Imax
⫻ 3 Ω. If the current pulse amplitude is 10 amps, then
V(t) is 30 V at T1. Using the above analogy, the transient
thermal response of a diode junction may be analyzed or
predicted by applying the basic formula:
TJ = P ⫻ Θjc (t) +TC or ∆TJ = P ⫻ Θjc (t).
Discrete Semiconductors
of the quadratic equation. The derived peak current vs.
transient pulse width PW is plotted and superimposed on
Figure 5.
Zener/TVS Diodes
The procedure in determining the peak tolerable
power and current for Zener/TVS Diodes is the same as
regular diodes except that the forward threshold
Figure 7. Thermal Response of CMZ5340B
From the known Θjc (t), one can determine the maximum tolerable peak power Ppk and peak current Ipk for
a given transient pulse width and reference case temperature, or one can determine the peak operating TJ for
given peak power or current surge.
Single Pulse Ppk/Ipk
Determination: Regular Diode
Rewriting the above formula, P=(TJ-TC) / Θjc (t), to find
the maximum tolerable peak power Ppk, it is just a matter
of using this formula once the maximum tolerable TJ, the
reference case temperature TC and the effective thermal
resistance are known. For illustration, the CMR1U-10M
diode is used. Shown in Figure 4 is the device’s effective
thermal resistance as a function of time. Using 200°C as
the maximum TJ and TC of 50°C, the peak powers Ppk’s
for various pulse width PW are calculated on the following
Table 1:
Figure 8. Peak Power/ Peak Current vs. Pulse
Width of CMZ5349B at TC = 50°C.
Figure 4. Thermal Response of CMR1U-10M.
Figure 9. Repetitive Pulses
Illustration with CMR1U-10M.
Table 1.
TJmax = 200°C, TC = 50°C
Figure 5. Peak Power vs. Pulse Width of
CMR1U-10M @ TC = 50°C
Figure 5 is a plot of the peak power vs. pulse width.
From this graph, one can determine the maximum tolerable Ppk at TC = 50°C for various pulse widths.
To determine the peak forward current Ifpk, the formula
Ppk = Ifpk ⫻ Vfpk may be used.
As shown on Figure 6, Vfpk = Vt + Ifpk Zd, where Vt is
the diode threshold voltage and Zd is the dynamic impedance. Therefore,
Ppk = Ifpk (VT + Ipk Zd), or Ppk = Ifpk VT+ I2fpk Zd.
The forward peak current of the corresponding Ppk
may be determined graphically or analytically via the root
Figure 6.
PW
(mS)
Θjc (t)
(C/W)
Ppk
(W)
Ipk
(A)
0.1
0.3
1.0
3.0
8.0
20
70
100
300
1000
0.051
0.11
1.19
0.26
0.59
0.81
1.50
1.80
2.80
4.50
1960
909
526
385
169
123
67
56
36
22
35
23
16
11
7.7
5.4
3.9
3.6
2.4
1.6
Table 2
TJmax = 150°C, TC = 50°C.
46
www.ecnmag.com • ECN • August 2002
Semiconductor Monthly
voltage VT is replaced by VZ, the Zener or Transient
Voltage Suppressor (TVS) clamping voltage. Hence,
Ppk = Ipk VZ + I2pk Zd. For illustration, the
CMZ5349B Zener is used for Ppk and Ipk determination. Figure 7 shows the device’s effective transient
thermal resistance Θjc(t) VS transient pulse width PW.
Using TJ max. at 150°C, reference case temperature TC
at 50°C, VZ at 12.6 V, Zd at 1.25 Ω and Θjc(t) from
Figure 7; the maximum Ppk, (TJ max - TC)/Θjc (t) and
the corresponding Ipk for various pulse width is calculated and presented in Table 2.
Figure 8 is a plot of Ppk and Ipk VS PW based on the
above data. From this plot, the peak tolerable power or
current at TC of 50°C may be determined for various
pulse widths.
Repetitive Pulse Ppk/Ipk
Determination
When the peak power pulse is repetitive, the off period
between pulses is an important factor to consider. If the off
period Poff is long enough for the junction temperature TJ
to recover to the equilibrium case temperature TC before
the start of the next pulse, it should be treated as a nonrepetitive case. However, if Poff is not long enough for TJ to
recover to the original equilibrium with the case reference
temperature TC, then the repetitive case for Ppk/Ipk
Discrete Semiconductors
analysis would be valid. Normally, the repetitive case
applies if Poff is less than four time constants of the device.
One simple and practical way to analyze the repetitive pulse case is by considering that the repetitive peak
power pulses consist of a DC or average component and
a super-imposed AC component. Since the peak junction
temperature is proportionally related to the peak power,
the resultant TJ rise consists of a DC and also an AC
component. Using the CMR1U-10M diode as an example, and assuming as shown in Figure 9 a pulse train
with peak power at 100 W, pulse width PW at 0.5 ms,
and period at 10 ms, the duty cycle is 5 percent, hence
the Pavg is 5 W. Referring to Figure 4, the steady state
thermal resistance is 13°C/W; hence the
average TJ rise is 85°C. Also from Figure
4, the effective thermal resistance at 0.5
ms is 0.7°C/W and the AC component
due to 100 W peak is 70°C. Therefore, as
shown on Figure 9, one would expect TJ
to move between 120°C and 50°C with an
average at 85°C. If the case reference TC
is 25°C, the peak TJ would be 145°C.
From the above example and the simple formulas of Figure 9 relating Ppk, TJ
rise, duty cycle, and effective transient
thermal resistance, one can determine the
maximum tolerable Ppk for any given TJ
max. Once Ppk is determined, the corresponding Ipk may be readily determined
using the same procedures shown on previous examples.
Summary
The conventional maximum junction
temperature TJ and maximum power ratings are for DC or steady state operation.
For transient peak power operation, the
thermal response of the diode junction
should be analyzed from the transient
peak power standpoint using the effective
transient thermal resistance. When the
peak power pulses are repetitive, thermal
response may be analyzed by considering
that the power consists of two components,
DC and AC.
Sze Chin a Senior Applications
Engineer at Central Semiconductor
Corp. located at 145 Adams Ave.,
Hauppauge, NY 11788;
www.centralsemi.com.
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