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 Steady State 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 Steady State Steady State 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 Comments 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