Application Note

VISHAY SFERNICE
www.vishay.com
Film Resistors
Application Note
Pulse Capabilities for Thick Film Power Resistors
By Yannick Tonnelier and Frederic Lovera
V
Vishay Sfernice offers a wide range of thick film power
resistors. Our resistors are able to dissipate from 5 W up to
1100 W with a large range of ohmic values (10 m up to
1 M).
The pulse capability of our resistors is a key specification for
many customer applications.
The energy curve in the datasheets shows the maximum
energy that can be applied over a given period.
In this application note, we use the example of our LPS 800
resistor to explain a method to evaluate whether the resistor
is appropriate for a given application. This method can be
used for each resistor type using the corresponding pulse
curve or limiting voltage from the corresponding datasheet.
Peak
50 %
0
t1
t2
..t3
t
2 x  t3 - t 1 
2
 V x
 - ------------------------------
1 V

E =  -- x ----- x t 1 +   ----------------- x  e
- 1 
3
   - 2 x R 
R

2
Lightning pulse
If we take the following example:
A capacity of 5 μF charged to 1140 V will be discharged
through an LPS 800 80 with a frequency of 100 Hz. The
ambient temperature is 25 °C (see next page for the
determination of the ambient temperature).
Unity:
E = Energy in J
t = Time in s
V = Voltage in V
1. The maximum pulse voltage for LPS 800 indicated on
the datasheet is 5000 V. This voltage is not exceeded by
the discharge of the capacitor, so LPS 800 is compatible
with this application.
2. Calculation of the energy stored by the capacitor for one
pulse: represented below are the most common voltage
curves and the formula used for each of them.
R = Resistance in 
C = Capacity in F
Square pulse: A constant voltage V is applied to the resistor
R during a period t.
Capacity discharge: A capacitor C is charged to a given
voltage V and discharged into the resistor R.
Lightning pulse: The voltage rises up to Vpeak and decreases
at an exponential rate. This pulse is the pulse defined in the
IEC 61000-4-5 with
V
V
t1 = Time to peak voltage (s)
t
t3 = Time to negligible voltage > 20 x t2
t
2
V t
E = -------R
1
2
E = --- x C x V
2
t = RxC
Square pulse
Capacitor discharge
 = Exponential rate of decay = - (t2 - t1)/ln(0.50)
In our example introduced above, we have C = 5 μF and
V = 1140 V:
E = 1-- CV2 E = 1-- x 5.10-6 x 11402 E = 3.25 J
2
2
t = RC t = 80 x 5.10-6 t = 400 μs
We can now examine the energy curve of proposed use.
Revision: 18-Apr-16
Document Number: 50060
1
For technical questions, contact: sfer@vishay.com
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
APPLICATION NOTE
t2 = Time to 50 % of peak voltage (s)
Application Note
www.vishay.com
Vishay Sfernice
Pulse Capabilities for Thick Film Power Resistors
3. Check of the chosen operating point on the energy curve
of LPS 800:
ENERGY IN J
10 000
1000
Use of the resistor out of the
limits with possible destruction
of the part
100
Maximum energy
(Rth(c - h) - Rth(h - a))max. = --------T
----------- - Rth(j - c) =
P average
T j max. - T a
------------------------ - Rth(j - c)
P average
Rth(j - c) = Thermal resistance value measured between
resistive layer and outer side of the resistor.
Rth(c - h) = Thermal resistance value measured between
outer side of the resistor and upper side of the
heatsink. This is the thermal resistance of the
interface (grease, thermal pad), and the quality of
the fastening device.
Operating conditions
10
To define the size of our heatsink, we take the formula:
3.25
Good conditions of
use for the resistor
1
Rth(h - a) = Thermal resistance of the heatsink.
0.1
0.0001 0.001
0.01
0.1 0.4 1
10
100
1000
Tj max. =
Temperature of the resistive element (maximum
175 °C for LPS 800).
Ta =
Ambient temperature (determinated by the
measurement of the temperature of the junction
without any power or the temperature of the
water for a water cooling heatsink.)
OVERLOAD DURATION IN ms
Each point on the curve corresponds to a single test at
25 °C for the ambient temperature.
The operating conditions are in the zone corresponding
to good conditions of use for the resistor.
Now, we must calculate the average power dissipated
by the component.
4. Calculation of the average power dissipated LPS 800 for
this example:
In case of multiple pulses applications, we need to use
the formula linking the energy of the pulse and the
frequency of repetition of this pulse (f = 100 Hz for this
example):
5. Paverage = E / t = E x Paverage = 3.25 x 100
Paverage = 325 W
Take the example of a thermal interface of 0.2 °C/W for the
interface between the component and the heatsink.
T j max. - T a
Rth(h - a) max. = ------------------------ - Rth(j - c) - Rth(c - h)
P
175 - 25
325
Rth(h - a) max. = -------------------- - 0.112 - 0.2 = 0.15 °C/W
In this example, we must therefore choose a heatsink
with a Rth  0.15 °C/W
Use of the resistor out of
the limits with possible
destruction of the part
800
Good conditions of
use for the resistor
600
Working limitation
for the resistor
Operating conditions
400
325
LA 14 from Fischer Elektronik
(air cooling heatsink with fan)
To avoid any damage to the resistor by excessive pulse
loading, the specifics of the customer’s application must be
checked.
200
0
CP15 from Lytron
(water cooling heatsink)
0
50
75 85
100
150
175
200
BOTTOM CASE TEMPERATURE IN °C
See the following link:
www.vishay,com/resistors/pulse-energy-calculator/
Note
• Our operating conditions are in the acceptable zone for the
component. It remains for us to determine the size of the
heatsink.
Revision: 18-Apr-16
Document Number: 50060
2
For technical questions, contact: sfer@vishay.com
THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT
ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000
APPLICATION NOTE
RATED DISSIPATION IN %
6. With the derating curve, we can see if LPS 800 can be
used at 325 W with a case temperature, backside of the
resistor, at 75 °C for example.