It is well known that in some applications Power MOSFETs must operate inside the linear zone, that is the area of the output characteristics ID vs VDS with high current and voltage values. Such mode of operation differs from the traditional way of using MOSFETs which are normally made to function like “switches”, that is, in on-off switching mode.
When used in linear mode, MOSFETs are subject to thermal stresses in low drain current conditions and current crowding phenomena is involved. From a datasheet standpoint, this involves a reduction in the forward-biased safe operating area (FBSOA). |
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| The MOSFET’s ability to overcome a thermal stress related to linear operation is strongly dependent upon its physical structure. It has been observed that the latest competitors’ “Trench” technologies designed for ever-decreasing on-resistance are extremely weak when used in the linear zone.
ST’s planar technologies are much more suitable than Trench variants in yielding robust devices. This statement can be demonstrated by analyzing the variation of the threshold voltage as temperature increases. In ST’s planar technology this parameter is less prone to decrease with temperature.
This in turn implies that the drain current rise is limited more easily, making the MOSFET less susceptible to thermal runaway.
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Typical applications in linear mode are encountered in automotive applications such as fan motors or in industrial where they can be employed in UPS converters.
It has been shown that power MOSFET devices like the STP130NS04ZB and the STP140NF55 realized using an optimized version of STripFET behave significantly better than similar devices from competition as proven by a detailed analysis of the thermal coefficient behaviour.
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 175°C as maximum operating temperature;
Higher current capability;
Standard threshold devices;
100% avalanche tested;
Optimized for linear mode operation.
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Electrical Characteristics |
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| The parameter measuring how well or bad a MOSFET is in linear mode can be identified by the thermal coefficient. It in fact represents the rate of change of drain current caused by an increase in temperature. It is sometimes normalized with respect to the chip area to allow benchmark ing with other devices. |
A typical motor control application using STP140NF55 |
In this schematic, a DC voltage supplied by a battery drives a motor in series with two power MOSFET devices connected in parallel on the same heatsink. The two MOSFETs work in the linear zone and, acting on the gate-source voltage, it is possible to fix the drain-source voltage. Consequently the voltage across the motor terminals is established by the difference between the battery and the drain-source voltages. The regulation is performed by a suitable driver that checks the current flowing in the MOSFETs and establishes the right gate-source voltage.
P/N |
IC (A) |
BVDSS
(V) |
RTHJ-C
(C/W) |
RDS(on) max
(mOhm) |
Package |
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120 |
55 |
0.5 |
< 8 |
D2PAK TO-220 |
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80 |
Clamped
@ 33V |
0.5 |
< 9 |
D2PAK
TO-220 TO-247 |
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