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AN606
Vishay Siliconix
Current-Sensing Power MOSFETs
Kandarp Pandya
INTRODUCTION
Vishay Siliconix current-sensing power MOSFETs offer a simple means of incorporating a protection feature into an electronic control circuit and avoiding catastrophic failures resulting from overcurrent (overload) and/or short-circuit conditions. The device package is a modified D2PAK with five pins. The MOSFET termination retains the standard D2PAK footprint for a three-pin device. The additional two pins provide termination for a current-sense output and an internal Kelvin connection to the source. For current sensing, the MOSFET design employs a small number of the total number of MOSFET cells in a known ratio. The latter define the current-sense parameters. A typical control interface uses a simple circuit with an op-amp or a comparator. This approach offers the freedom of control-level setting and facilitates its incorporation into the main control system. between gate and drain-stub and between drain-stub and source, respectively. See Application Note 826, Recommended Minimum Pad Patterns With Outline Access for Vishay Siliconix MOSFETs (http://www.vishay.com/doc?72286), for the recommended PCB layout dimensional details of the pad pattern. Modified-part library symbols for schematic symbol and PCB layout are available on the "Protel" (PCB design software) platform. For soft copy, please contact Vishay Siliconix in Santa Clara, Calif., in the United States, by phoning 1-408-567-8927. The Principle Behind the Current-Sensing Feature The most efficient way to sense the drain-source current is to use the ratio-metric measurement. In a power MOSFET, it is possible to implement this method easily. The cell density, a favored term within the power MOSFET industry, conveys that the power MOSFET structure consists of many cells connected in parallel. In principle, these cells constitute a resistive path for drain-source current. Electrically, these cells are parallel connected resistors, rDS(on) s. Each cell - being identical in structure and electrical characteristics shares the current equally when the device is on. This property enables design of a MOSFET with a current-sensing feature.
DEVICE DESCRIPTION AND PRINCIPLE OF OPERATION
D2PAK-5
D (Tab, 3)
G 12345 SENSE
(1)
(4)
KELVIN
(2)
S (5) G SENSE D S KELVIN N-Channel MOSFET
Dividing the MOSFET cells in a known ratio creates two paths that share the drain-source current. The path with the smaller number of cells constitutes the sense current, which is much smaller than the current conducting through the rest of the cells. A very simple, low-power, external circuit can measure this current. Multiplying this value with the cell ratio gives the total drain-source current. The classic Kelvin termination for the return of sense current to the main source connection insures the measurement accuracy. This terminal not only eliminates the ground loop, but also minimizes the imbalance of internal structures with two current paths. The Current-Sensing Parameters, Table 1, and the Current-Sense Die Characteristics and Schematic, Figure 2, help to demonstrate the current-sensing operation and circuit implementation.
FIGURE 1. Package Information and Schematic Symbol
Package Information and Schematic Symbol, Figure 1, shows a partial reproduction of a datasheet for a current-sensing MOSFET, SUM50N03-13C. Gate, drain-stub/tab, and source (pins 1, 2, and 3) are in the same position as in a standard D2PAK (TO-263) MOSFET. However, pin-out modification is required to incorporate current-sense (pin 2) and Kelvin-to-source (pin 4)
TABLE 1: Current Sense Characteristics
Current Sensing Ratio Mirror Active Resistance Document Number: 71991 17-Dec-03 r rm(on) ID = 1 A, VGSS = 10 V, RSENSE = 1.1 W VGS = 10 V, ID = 10 mA 420 520 3.5 620 W www.vishay.com
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AN606
Vishay Siliconix
TYPICAL CHARACTERISTICS (25_C UNLESS NOTED)
10
SENSE DIE
10
On-Resistance vs. Sense Current
On-Resistance vs. Gate-Source Voltage
8 rm(on) - On-Resistance (W) rm(on) - On-Resistance (W)
8 ID = 10 mA
6
VGS = 4.5 V VGS = 10 V
6
4
4
2
2
0 0.00
0 0.02 0.04 0.06 0.08 0.10 0 2 4 6 8 10 ISENSE (A) VGS - Gate-to-Source Voltage (V)
Current Ratio (I(MAIN)/IS) vs. Gate-Source Voltage (Figure 1)
1200 RS = 6.6 W 1000 800 Ratio RS = 2.2 W 600 400 200 0 0 4 8 12 16 20 VGS - Gate-to-Source Voltage (V) RS = 1.1 W RS = 0.5 W VG RS = 4.7 W G
SENSE RS
S
KELVIN
FIGURE 2. Current-Sensing Die Characteristics and Schematic
Definition of Current-Sensing Parameters The current-sense ratio, r, is the quotient of the number of cells terminated on the sense terminal to the total number of cells on the MOSFET die. To derive the value of r using the above definition requires detailed die design. However, the quotient of drain current to the sense current provides the same value because these current values are the sum of cell current in each path. Mathematically: r = ID/ISENSE ID is drain current
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ISENSE is the current flowing out of the sense terminal and into the sense resistor, RSENSE Mirror active resistance, rm(on), is the resistance of parallel connected cells used in the sense chain when the device is on. Being rDS(on) as in any other MOSFET, the value depends on the gate drive, drain current, and junction temperature. Accordingly, rm(on) is defined at given values of VGS, IDRAIN, and TJ junction. By definition, for the sense die, refer to Figure 2. Mirror active resistance rm(on) is specified at the gate-source voltages, VGS at 4.5 V and 10 V, corresponding drain-source current ISENSE up to 0.1 A, and junction temperature TJ at 25 _C. The temperature coefficient of rm(on) is the same as that of rDS(on). Refer to the on-resistance vs. junction temperature curve in Figure 3.
Document Number: 71991 17-Dec-03
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AN606
Vishay Siliconix
2.0 1.8 rm(on) - On-Resistance (W) Normalized 1.6 1.4 1.2 1.0 0.8 0.6 -50
On-Resistance vs. Junction Temperature
VGS = 10 V ID = 25 A
2. use a minimum value of the ISENSE signal at the maximum value of ID; and 3. use a fast comparator with hysterisis to control and protect the MOSFET. Typical schematic configurations for implementing the current sense are shown in Figure 4 and Figure 5. The Virtual Earth Sensing Scheme, Figure 4, is suitable for applications aiming at higher noise immunity and speed. This approach also improves measurement accuracy by eliminating the sense resistor. However, a dual power supply and inverted (negative) output signal are the price designers pay for deriving these benefits.
-25
0
25
50
75
100
125
150
175
TJ - Junction Temperature (_C)
FIGURE 3. Normalized rm(on) for the Sense Die
The Resistor Sensing Scheme shown in Figure 5 is a quite simple and economical approach. The accuracy of current measurement is affected by the introduction of an external sense resistor RS. However, the latter aids in lowering the temperature sensitivity of the current-sense signal.
VDD Load IL D Kelvin G +V Sense - + -V S V1
DESIGN EQUATIONS
The following three equations enable circuit design and analysis. ISENSE = x ID/r VDS = ISENSE x [rm(on) + RSENSE] or VDS = ID x rDS(on)/(rm(on) + RSENSE) VSENSE = ISENSE x RSENSE or VSENSE = VDS x RSENSE/(rm(on) + RSENSE) Where ISENSE r ID VDS rm(on) RSENSE Current flowing out of sense terminal Current-sensing ratio Drain-source current Drain-source voltage Mirror active resistance External current-sense resistor
FIGURE 4. Virtual Earth Sensing Scheme
VDD Load IL D R3 Kelvin G R1 Sense S Stray Resistance R2 - + V1
Application Aspects and Design Examples The current-sense ratio r, even though fixed by design, is dependent on manufacturing process variations. Furthermore, mirror active resistance rm(on) depends on circuit parameters VGS and ID and junction temperature TJ. As a result, a practical design can realize an accuracy of 15% -- 20% for current sensing. Accordingly, the current-sensing MOSFET is most suitable for supervisory functions such as overcurrent and/or short-circuit protection. Three keys to a successful design are to: 1. have an adequate margin between the normal operating-current value and the trip-current value;
Document Number: 71991 17-Dec-03
FIGURE 5. Resistor Sensing Scheme
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AN606
Vishay Siliconix
TABLE 2: Current Product Range
Part # Si6862DQ Si4730EY Channel Type
N N
VDS (VDC)
20 30
rDS (W)
0.026/4.5 V 0.015/10 V
IDS (A)
6.6 11.7
PD (W)
1.8 3.6
Package
TSSOP-8* SO-8*
SUM50N03-13LC SUM60N08-07C
N N
30 75
0.013/10 V 0.007/10 V
50 60
83 300
D2PAK-5
Recommended minimum pads for current-sensing MOSFETs in TSSOP-8 and SOIC-8 packages see application note AN826 (http://www.vishay.com/doc?72286).
CONCLUSION
Vishay Siliconix current-sensing power MOSFETs enable implementation of a simple solution for incorporating supervisory protection features such as overcurrent and/or short circuit. This approach offers the freedom and flexibility of control-circuit design, though the accuracy of measurement is not suitable for current-control applications. Virtually any power MOSFET from the Vishay Siliconix product range can be supplied with a current-sensing feature.
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Document Number: 71991 17-Dec-03

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