### AN830 Current Ratings for Vishay Siliconix MOSFETs - Digi-Key

```AN830
Vishay Siliconix
Current Ratings for Vishay Siliconix MOSFETs
By Spiro Zefferys and Dave MacDonald
SUMMARY
Vishay uses three approaches in providing the continuous
drain current (ID) rating for its MOSFETs. The values are
printed in the "Absolute Maximum Ratings" table on the first
page of the datasheet, and are intended to provide designers
with sample conditions to determine if the part is being used
properly in their applications. A general approach is taken to
the calculation of this value. Yet every PCB layout and
design is different, and every MOSFET is constructed
differently. So no general process can be provided to
calculate the maximum allowable current for every
application. Instead, the values provided are numbers that
Vishay guarantees under given conditions, and it is left to
designers to model the performance of the MOSFET in their
specific applications based on these values.
Please note that the methods below are for calculating the
maximum allowable continuous DC current. This value
cannot be used for dc-to-dc converters that utilize pulsed
current with high peak currents. For this type of application,
the Root Mean Squared current (IRMS) must be calculated
and then compared to the ID rating on the data sheet with
IRMS << ID MAX. Formulas for calculating IRMS can be found
in most power electronics texts. In addition, high transient
spike currents seen at turn on or turn off must be compared
to the SOA curve to determine if they will damage the
MOSFETs.
FORMULA METHOD FOR ID RATINGS
The first approach is to calculate ID with the standard current
calculation, which is shown below. In this formula, TJMAX is
the maximum junction temperature specified on the
datasheet (either 150 °C or 175 °C) and TA is the maximum
ambient temperature allowed when the MOSFET is in
steady-state operation. rDS(on) is the maximum on-resistance
rating at a specific temperature and a specific drive voltage
(such as 4.5 V or 10 V), and RthJA is the steady-state
junction-to-ambient thermal resistance of the MOSFET as
specified on the datasheet.
ID
TJMAX
=
- TA
rDS (on) R thJA
The following example is based on the Si7884DP, which is a
40 V MOSFET in the PowerPAK® SO-8 package.
ABSOLUTE MAXIMUM RATINGS TA = 25 °C, unless otherwise noted
Parameter
Drain-Source Voltage
Gate-Source Voltage
Symbol
VDS
VGS
Continuous Drain Current (TJ = 150 °C)a
Pulsed Drain Current
Avalanche Current
TA = 25 °C
TA = 70 °C
ID
L = 0.1 mH
IDM
IAS
IS
Continuous Source Current (Diode Conduction)a
Maximum Power
TA = 25 °C
TA = 70 °C
Dissipationa
TJ, Tstg
Operating Junction and Storage Temperature Range
Soldering Recommendations (Peak Temperature)
PD
10 sec
20
16
4.7
40
± 20
12
10
50
30
1.7
5.2
3.3
1.9
1.2
- 55 to 150
b,c
260
Unit
V
A
W
°C
THERMAL RESISTANCE RATINGS
Parameter
Maximum Junction-to-Ambienta
Maximum Junction-to-Case (Drain)
Document Number 74670
07-May-07
Symbol
t ≤ 10 sec
RthJA
RthJC
Typical
19
52
1.2
Maximum
24
65
1.8
Unit
°C/W
www.vishay.com
1
AN830
Vishay Siliconix
From the "Absolute Maximum Ratings" table we get the following values:
Parameter
TJMAX
TA
RthJA MAX
Value
150 °C
25 °C
65 °C/W
rDS(on) MAX at 10 V (at Temp)
0.0126 Ω
Maximum junction temperature; note that some devices are rated at 175 °C
Ambient temperature; used with RthJ-A
Maximum value at steady state
From 0.007 Ω x 1.8; 0.007 Ω is from the "Electrical Characteristics" section of the data
sheet; 1.8 is a typical factor used to rate rDS(on) at higher temperatures; an actual curve can
be found in the "Typical Performance Curves" section of the data sheet
ID = SQR ROOT OF (150-25)/(0.007x1.8)x65 = 12 A
The ID of 12 A is specified in the "Absolute Maximum
Ratings" table for the steady-state value at a TA of 25 °C.
The formula can also use RthJC in the denominator instead of
RthJA. The RthJC value provided on datasheets is a bestcase thermal resistance value, while the RthJA value is
usually more representative of actual board layout
conditions. Therefore, the designer can calculate a range to
determine the boundaries of a device's operation. Vishay
usually specifies the RthJA since it gives the worst-case
condition using a 1-in-by-1-in. PCB area on an FR4 board,
with 2 oz of copper or more as a pad. In most board designs
today, even smaller guidelines are used for the PCB layout.
Since it is impractical to calculate values for every possible
design, Vishay uses the standard 1-in-by-1-in condition for
the RthJA rating.
The formula can also be stated as TJ = TA + ID² x rDS(on)
x RthJA. There is no difference in the result; however, this
method can be easier for understanding the dynamics at
work.
ID² x rDS(on) is the standard calculation for power loss in
Watts (W). RthJA is specified in °C/W and the product of
power and thermal resistance results is measured in °C,
since the Watts cancel out. Adding this value to TA, which is
also measured in °C, results in the temperature rise of the
junction (TJ). As long as this value is below the maximum
rating (such as 150 °C), the design is thermally within the
boundaries of the safe operating area for the MOSFET.
Normally design engineers will place a de-rating value on the
maximum junction temperature, such as 80 %, so that
designs don't come near the maximum value. This is a good
practice and is recommended by Vishay.
PACKAGE LIMITATION METHOD FOR ID
RATINGS
The second method for determining the ID rating for a
MOSFET is to determine how much current the package can
handle. This is the next step before stating a value in the
"Absolute Maximum Ratings" table. After using the standard
current calculation to determine a value, engineering will
determine if the package can handle that amount of current.
For MOSFETs with higher rDS(on) ratings, the formula
calculation is usually good enough. However, for ultra-low
rDS(on) ratings, the calculated value usually exceeds the
current-handling capability of the MOSFET package.
www.vishay.com
2
The weakest part of the MOSFET's package is the wires
used to bond the silicon die to the lead frame. Based on the
material of the wires and the number of wires used, a certain
current-carrying value can be determined. If the current
exceeds this value, the wires can melt, resulting in
catastrophic destruction of the device. To strengthen this
part of the package, a clip is used instead of wires. Clips
usually have greater current-carrying capability since there is
more metallic mass. However, different clips are used based
on the size of the silicon die, which means that the current
capability also varies. No specific guideline can be provided
other than to rely on the maximum ID ratings on the data
sheet. If the ratings are lower than values designers
calculate, then the capability of the package has been
factored into the rating.
MEASUREMENT METHOD FOR ID RATINGS
The third method for determining the maximum ID rating of a
MOSFET is to drive the device to destruction by applying
current methodically until breakdown. This method is usually
used to determine the pulsed drain current value (IDM), but it
can also be used for the continuous current ratings.
When this method is applied, the MOSFET is driven into
saturation and no more current can pass through the device.
After measuring this value on a typical sample size of parts,
the rating will be provided on the datasheet with a guard
band. This guard band can be as much as 50 %.
CONCLUSION
Three methods are used to arrive at the ID rating for Vishay's
MOSFET products - formula, package limitations, and actual
measurements. The number on the data sheet is the limiting
value based on the three approaches. The conditions used
may not replicate every application in the field, but are a
departure point for designers to understand the limitations of
the device. Followed carefully, they will help designers
ensure that they are using MOSFETs well within the
operating conditions required for robust and reliable
operation.
Document Number 74670
07-May-07
```