AN4209
Application note
Design methodology for repetitive voltage suppressors (RVS)
in repetitive mode: STRVS
Introduction
The silicon transient voltage suppressor (TVS) device such as the Transil™ was initially
specified with a power surge capability to respond to industrial standard test conditions,
especially against high-energy single transient voltages. Today, many components in
switched mode power supplies are continuously subjected to very short transient voltages.
Little data is given in TVS specifications regarding the repetitive mode operation. Therefore,
it is not easy for the designer to accurately assess the clamping voltage and power losses
under these conditions.
This application note introduces the new repetitive voltage suppressor STRVSX features,
specifically adapted to the repetitive mode operation. A design guideline is presented and
selection processes are described.
September 2013
DocID023949 Rev 2
1/19
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Contents
AN4209
Contents
1
STRVS parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
1.2
Electrical parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1
Simplified electrical model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Thermal parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.1
1.2.2
Steady state parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Transient parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2
General design procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.1
2.2
Key Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Transil selection process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
2.2.1
2.2.2
2.2.3
2.2.4
Step 1: STRVS pre-selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Step 2: Clamping voltage assessment . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Step 3: Dissipated power calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Step 4: In-situ test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3
Transil selection flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3
Design example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.1
3.2
3.3
STRVS preselection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Clamping voltage assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Dissipated power calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.3.1
3.3.2
Power losses calculation: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Peak and average junction temperature calculation: . . . . . . . . . . . . . . . 16
3.4
In-situ verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4
5
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2/19
DocID023949 Rev 2
AN4209
STRVS parameters
1
STRVS parameters
This section defines the electrical and thermal characteristics of the STRVS.
1.1
Electrical parameters
Figure 1. Typical reverse characteristic V
R
= (I
R
) of an STRVS device
V(V)
V RM (25°C )
V CL (25°C )
V BR (25°C )
0
IRM(25°C)- 1µA
IBR(25°C)- 1mA
IPP(25°C)
TJ2 > TJ1
TJ = 25°C
1
I(A)
Figure 1
shows the typical reverse characteristic given at two temperatures of an STRVS
device.
Stand-off voltage (V
RM
)
The stand-off voltage is specified for I = I
RM
= 1 µA and T
j
= 25 °C. Under these conditions,
the device is still acting as an open circuit. This parameter is one of the key parameters in
circuit protection.
Breakdown voltage (V
BR
)
The breakdown voltage corresponds to the voltage from which the STRVS starts to go into
the avalanche region. This parameter is specified for I = I
BR
= 1 mA and T
j
= 25 °C. The V
BR
parameter follows a linear variation with junction temperature as shown in
Equation 1.
Equation 1
V
BR
(T
j
) = V
BR
(T
REF
)·
(
+
α
T
· (T
j
- T
REF
)
)
1
Where T
REF
= reference temperature expressed in °C, generally given at 25 °C and
T
= temperature coefficient in 1/°C.
DocID023949 Rev 2
3/19
STRVS parameters
AN4209
Clamping voltage (V
CL
)
The clamping voltage is the total voltage across the STRVS over the peak pulse current I
PP
at a given temperature. A range of typical values provides the characteristics of the
clamping voltage for given values of peak current (I
PP
), and three controlled junction
temperatures: 25 °C, 85 °C and 125 °C. All curves start from V
CL
(1 mA) = V
BRmax
.
Figure 2
illustrates V
CL
curves of an STRVS185X02B. These curves are useful for verifying the
suitability of the allowable clamping voltage in application.
Figure 2. Clamping voltage characteristic over temperatures of an STRVS185X02B
I (A)
5.0
T
j
=25°C
T
j
=85°C
T
j
=125°C
4.0
3.0
2.0
1.0
V
BRmax
25°C
V
BRmax
85°C
V
BRmax
125°C
1m
0.0
155
160
165
170
175
180
VCL (V)
185
190
195
1.1.1
Simplified electrical model
A linear model can be employed to approximate the V
CL
= f (I
PP
) characteristics of the
STRVS. A straight line is used to approximate the actual curve inside the working area
imposed by the application conditions (see
Figure 3).
The line intersects the horizontal axis
at the voltage V
CL0
. The slope of the line is inversely proportional to the dynamic resistance
R
D
. The equivalent circuit that models this equation is shown in
Figure 4.
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AN4209
STRVS parameters
Figure 3. Simplified characteristics of an STRVS device
V
CL2 (I CL2 ; T j)
V
CL1 (I CL1 ; T j)
V
CL0 (T j)
V(V)
V
BR(T j)
I
PP1
I
PP2
1/R
D
(I
PP1
; I
PP2
; T
j
)
Operating region
I (A)
Figure 4. Simplified electrical model of an STRVS device
V
CL
K
I
PP
A
V
CL
(IPP; Tj)
I
PP
R
D
V
CL0
(IPP1; IPP2; Tj) (IPP1; IPP2; Tj)
A simple rule to calculate R
D
and V
CL0
is to use I
PP2
=I
peak
of the application and
I
PP1
= I
PP2
/ 2
The clamping voltage is defined in
Equation 2.
Equation 2
V
CL
( I
PP
; T
j
) = V
CL0
( I
PP1
; I
PP2
; T
j
) + R
D
( I
PP1
; I
PP2
; T
j
) · I
PP
DocID023949 Rev 2
5/19
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