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APPLICATION NOTE
®
THE THERMAL RUNAWAY LAW IN SCHOTTKY
USED IN OR-ing APPLICATION
by Y.LAUSENAZ
INTRODUCTION
2. TYPICAL PREFERRED DEVICE
Nowadays, some critical applications require very
high available power supplies. Typically, these
applications are servers or telecommunication
base stations.
In such systems, the power supplies are built with
several power supplies connected in parallel in
order to be fault tolerant. Thanks to redundancy,
the total failure rate stays very low and the availability can exceed 99.99%.
The connection of several power supplies needs
the OR function, commonly built with diodes, to
tolerate faults in the SMPS.
In normal operation the diode is conducting in
forward mode. So, the first requirement of the
component, irrespective of the maximum repetitive reverse voltage (VRRM) and the current rating (I F(AV)), is the forward voltage drop (V F).
The lower the forward voltage drop, the lower the
forward losses in the diode, and the better the
SMPS efficiency.
For this reason, Power Schottky diodes are commonly used in OR-ing application. The L series
(for example STPS60L30CW) are optimized to
provide very low forward voltage drop:
VF typ = 0.33V (30A @125°C per diode).
The following graph presents the typical Schottky
used in OR-ing application on common voltage
outputs:
Fig. 1: Supplies connected in parallel
Or function
Fig. 2: Typical Schottky used as OR-ing function
on common voltage outputs
Output
voltage
SMPS 2
48V
24V
SMPS 1
Vout
12V
5V
3.3V
1. OR-ing FUNCTION PRESENTATION
The OR-ing function is commonly built with diodes.
The diode has to let the current pass through when
the associated SMPS is working in normal operation. When a SMPS fails in short circuit, the diode
has to block reverse voltage in order to maintain
The purpose of the OR function is to prevent fault
propagation between supplies connected in parallel.
May 2002
L15
L25
L30 L45 L60 H100
Schottky
voltage
Using Schottky diodes provides very low forward
losses. But the main important technology trade off
for Schottky is between forward voltage drop and
leakage current:
The optimization of forward voltage drop is inevitably made to the detriment of leakage current.
High leakage current gives rise to the thermal
runaway problem.
1/6
APPLICATION NOTE
3. THERMAL RUNAWAY RISK
3.2. Result in classical cases
The risk of thermal runaway comes from the fact
that leakage current increases quickly with the
junction temperature.
In the classical simple case where both the following
Constant thermal resistance system
OR-ing diode on its own heatsink
The reverse losses in the Schottky diode, due to
associated SMPS short circuit failure, is a monotonous function of the time. Consequently the
thermal runaway diagram of fig. 3 is covered in
only one rotation- sense.
To determine if the Power Schottky will goes into
thermal runaway mode consists of finding the elements that will determine the rotation sense of
fig. 3.
During the forward mode, the forward current (IF)
defines the junction temperature (Tj) (linked to
forward voltage (VF), device thermal resistance
Rth(j-a)) and ambient temperature (Tamb):
Tj = Tamb + Rth ( j − a ) (I F xV F@ I fwd )
■
■
3.1. Problems
Using a Schottky as OR-ing function provides a
very low forward voltage drop. But when the diode
is blocking because its associated supply has a
fault in short circuit mode, the diode has to operate
in reverse mode with high junction temperature
(due to preceding forward losses) and so with relatively high reverse current.
This high reverse current can generate high reverse losses, and so increase junction temperature, and so reverse current as well… This is the
thermal runaway phenomenon.
Fig. 3: Thermal runaway diagram
Reverse
current
Junction
temperature
Reverse
losses
The problem is to quantify the risk of thermal
runaway in order to prevent it.
During the fast mode change of the diode (from
the forward mode to the reverse one, the change
is fast in comparison to device thermal constant),
the junction temperature due to the preceding forward mode stay continuous (c.f. fig. 5) and will determine the leakage current (Irev) (linked to the
reverse voltage Vrev):
c ( T j − 100 ° C )
I rev (Tj ;V rev ) = I rev (100 °C ;V rev ) × e
c ≈ 0.055°C-1 (thermal constant)
This reverse current will determine the new junction temperature trend (linked to reverse voltage
and device thermal resistance). This variation
trend between the initial junction temperature (due
to forward mode) and the new one (due to reverse
mode) gives the Tj variation and the rotation-sense
in fig. 3.
In a constant thermal resistance system, the thermal stability can be determined by comparing forward losses (Pfwd) in the power Schottky just
before the SMPS failure (t0-δt) and the reverse
losses (Prev) occurring just after (t0+δt) the eventual SMPS short-circuited fault.
The stability can be guaranteed if Pfwd > Prev @t0
2/6
APPLICATION NOTE
Fig. 4: Typical loss variation in the OR-ing before
and after the SMPS failure
Fig. 6: Example of junction temperature variation
in non-constant thermal resistance system
Tj
losses
Reverse
mode
Forward
mode
Thermal
runaway
Pfwd
Forward
mode
Reverse
mode
Fan switch
OFF
continuous
variation
Prev
SMPS
break down
time
t0
SMPS
break down
Tamb
NON-monotonous variation
Fig. 5: Typical junction temperature variation in
the OR-ing before and after the SMPS failure
Tj
continuous
variation
SMPS
break down
time
t0
Monotonous variation
Forward
mode
Thermal
runaway
Reverse
mode
Thermal
runaway
In these more complicated cases, device thermal
behavior can be simulated with tools like PSPICE.
The following analogies have to be used:
Thermal:
Electrical:
Temperature
Voltage
Power
Current
Resistance
Resistance
Tamb
time
Fig. 7: Thermal / electric analogy for simulation
Monotonous variation
Junction
Rth(j-c)
3.3. Results in more complicated cases
More complicated cases, where the assumptions
of §3.2 do not exist, can be considered.
For example:
OR-ing diode not on its own heatsink.
The OR-ing diode can be mounted on common
heatsink with other dissipative devices. In this
case, the junction temperature of the OR-ing diode can be influenced by the other devices,
thanks to coupling thermal resistance.
Non constant thermal resistance system:
The convection can be forced by a fan connected to the Anode side of the OR-ing diode. In
case of SMPS failure, the fan will stop and the
Rth(j-a) will increase. In this case, the junction
temperature variation will not be monotonous.
Tjunction
Case
P (losses)
Rth(c-a)
Tcase
Ambiant
Tamb
■
■
This analogy can be use to analyze any complex
thermal problem.
3/6
APPLICATION NOTE
4. FROM THERMAL RUNAWAY TO PRODUCT
OPTIMIZATION
STMicroelectronics has developed a Schottky
family dedicated to the OR-ing function. This “L”
family demonstrates very low forward voltage in
order to reduce conduction losses. Consequently,
the leakage current is relatively high.
For example, the “L15" family (VRRM=15V) is optimized for 3.3V, 5V and eventually 12V output
as OR-ing diode.
Due to the specific thermal runaway law of the
Schottky in OR-ing application, we can optimize
the device choice in order to improve the SMPS
efficiency, while keeping the risk of thermal runaway under control.
For example, let’s take a 3.3V 35A output, with a
STPS40L15C as OR-ing diode. The two diodes
have to be considered like connected in parallel:
Prev
= 2 × V out ⋅ I rev (Tj ;3.3V ) ( 2 diodes in parallel )
= 2 × V out ⋅ I rev (100 °C ; 3.3V ) × e
(thermal constant )
3.3V ) (per diode , datasheet )
c ≈ 0.055 °C −1
I rev (100 °C ;
Note that it is very important to use maximum reverse current values to evaluate reverse losses.
Actually, the worst case must be considered to
evaluate junction temperature in order to be sure
to avoid thermal runaway.
The limit of the thermal runaway criteria being defined by Pfwd = Prev, the maximal junction temperature Tj max corresponds to Prev max = Pfwd:
Prev max = 2 × V out ⋅ I rev (100 °C ; 3.3V ) × e
Tj max = 100 °C +
Fig. 8: SMPS output synopsis
Iout = 35A (=2Ifwd)
Vout = 3.3V
Pout = 115.5W
In the forward mode, the forward losses can be
calculated as:
(
= 2 × ( 0.18I
fwd
)
−3 2
fwd
+ 8.010 I
c(T j
max
− 100 ° C )

1 
Pfwd

In

c  2 × V out ⋅ I rev (100 °C ; 3.3V )
= 137 °C
STPS40L15C
2
Pfwd = 2 × V T 0 ⋅ I fwd + Rd ⋅ I fwd
c ( Tj − 100 ° C )
)
(datasheet )
= 112
. W
Theses losses decrease the global SMPS efficiency about 9.7%.
The risk of thermal runaway can be evaluated by
calculating the maximum junction temperature
that must not be reached in forward mode to
avoid reverse losses being higher than forward
losses, thus avoiding thermal runaway.
The maximum junction temperature reachable in
forward mode before the risk of thermal runaway
occurs is so high, that we can consider a well
adapted device. This one will have a lower forward
voltage so a highest reverse current.
The same process can be applied to different
devices
STPS80L15C
Forward losses Pfwd = 9.0W
Maximal junction temperature before thermal runaway: Tjmax = 127°C
STPS120L15
Forward losses: Pfwd = 7.6W
The efficiency loss is about 6.6%
Maximal junction temperature before thermal runaway: Tjmax = 100.3°C
STPS20L15 (the 20A average current specified is
only indicative value)
Forward losses: Pfwd = 16.1W
The efficiency loss is about 13.9%
Maximal junction temperature before thermal runaway: Tjmax = 155.9°C
The comparison between the 4 parts considered
on the 3.3V 35A output can be summarized on the
following graph:
4/6
APPLICATION NOTE
Fig. 9: Comparison between 4 parts, forward
losses, efficiency loss and Tjmax.
Forward Efficiency
loss
losses
Tjmax = 156°C
16.1W
13.9%
Vout = 3.3V
Iout = 35A
Tjmax = 137°C
11.2W
9.7%
9.0W
7.8%
7.6W
6.6%
STPS20L15
Tjmax = 127°C
Tjmax = 100.3°C
STPS40L15C STPS80L15C STPS120L15
Using the specific thermal runaway law, the SMPS
designer can optimize the OR-ing diode choice in
order to improve the global efficiency.
The risk of thermal runaway is controlled by limiting
the junction temperature during the forward mode
below the maximum value evaluated.
ANNEXE: EVALUATION OF MAXIMUM REVERSE CURRENT FROM DATASHEET
To evaluate the limit before thermal runaway, the
maximum value of the reverse current has to be
considered. Actually, this parameter is critical for
thermal runaway and the worst case must be considered.
To evaluate the maximum reverse current of a
power Schottky, take the typical value given in
figure. Apply the ratio between typical and maximal value given in the table (in the adapted VR
and Tj field). Finally, use the adapted formula to
get the expected junction temperature.
Example: STPS80L15C (twin diode in parallel)
under 3.3V @125°C
Figure 5 of the STPS80L15C datasheet gives the
typical value of the reverse current @100°C for
3.3V (per diode):
Fig. 5: Reverse leakage current versus reverse
voltage applied (typical values, per diode).
IR(mA)
1E+3
CONCLUSION
STMicroelectronics is developing “L” family diodes
dedicated to the OR-ing application. This family
shows very low forward voltage in order to reduce
conduction losses and to improve efficiency:
STPSXXL15,
STPSXXL25,
STPSXXL30,
STPSXXL45, and STPSXXL60.
With the very simple law presented, it becomes
straightforward to optimize devices choice by
evaluating the risk of thermal runway in Schottky
used in OR-ing function in SMPS.
This reliable and accurate law allows the optimization of the devices used in order to improve
converter efficiency while controlling the risk of
thermal runaway risk.
Tj=100°C
Tj=75°C
1E+2
1E+1
Tj=25°C
220mA
1E+0
VR(V)
1E-1
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
3.3V
Irev typ (100°C ; 3.3V) = 220mA
The static electrical characteristics table gives the
ratio between typical and maximum values (per
diode):
STATIC ELECTRICAL CHARACTERISTICS (per diode).
Symbol
Parameter
IR *
Reverse leakage
current
Tests conditions
Tj = 25°C
Tj = 100°C
Tj = 25°C
Tj = 100°C
Typ.
Max.
Unit
4
mA
280
400
0.44
1.1
A
16
mA
1.3
A
VR = 12V
Tj = 100°C
Tj = 25°C
Min.
VR = 5V
11
VR = 15V
0.53
Pulse test :* tp = 380 µs, δ < 2%
5/6
APPLICATION NOTE
I rev max (100 °C ; 3.3V ) = 220 ×
400
= 314mA
280
The following formula allows the calculation of the
reverse current in a power Schottky for every
junction temperature from a reference value:
I rev (Tj ; 3.3V ) = I rev (100 °C ; 3.3V ) × e
(c ≈ 0.055 °C
−1
thermal cons tant
c ( Tj − 100 ° C )
)
I rev (125 °C ; 3.3V ) = I rev (100 °C ; 3.3V ) × e
c ( 125 − 100 )
= 12
. A
So, the global maximum reverse current value for
the two diodes of the STPS80L15C connected in
parallel under 3.3V @125°C is:
I rev max (125 °C ; 3.3V ) = 2 × 12
. A
≈ 2.4 A
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