Power MOSFET transistors. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Power semiconductor devices. Power MOSFET transistors. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Electrician's Handbook

MOSFET is an abbreviation of the English phrase Metal-Oxide-Semiconductor Field Effect Transistor (Metal Oxide Semiconductor Field Effect Transistors).

This class of transistors differs, first of all, in the minimum control power with a significant output (hundreds of watts). It is also necessary to note the extremely low values of the resistance in the open state (tenths of an ohm at an output current of tens of amperes), and, consequently, the minimum power released on the transistor in the form of heat.

The designation of this type of transistors is shown in fig. 7.1. Also, to reduce the number of external components, a powerful high-frequency snubber diode can be built into the transistor.

Power MOSFETs
Rice. 7.1. Designation of MOSFET transistors (G - gate, D - drain, S - source): a - designation of an N-channel transistor; b - designation of the P-channel transistor

Advantages of MOSFET transistors over bipolar ones

To the undeniable the benefits MOSFET transistors before bipolar can be attributed to the following:

minimum control power and high current gain ensures the simplicity of control circuits (there is even a kind of MOSFET driven by logic levels);
high switching speed (at the same time, turn-off delays are minimal, a wide area of safe operation is provided);
the possibility of simple parallel connection of transistors to increase the output power;
resistance of transistors to large voltage pulses (dv/dt).

Application and manufacturers

These devices are widely used in high-power load control devices, switching power supplies. In the latter case, their scope is somewhat limited by the maximum drain-source voltage (up to 1000 V).

N-channel MOSFET™ are the most popular for switching power circuits. The control voltage, or the voltage applied between gate and source to turn on the MOSFET, must exceed the UT threshold of 4V, in fact 10-12V is needed to reliably turn on the MOSFET. Reducing the control voltage to the lower threshold UT will turn off the MOSFET.

Power MOSFET release various manufacturers:

HEXFET (NATIONAL);
VMOS (PHILLIPS);
SIPMOS (SIEMENS company).

MOSFET internal structure

On fig. 7.2 shows the similarity internal structure HEXFET, VMOS and SIPMOS. They have a vertical four-layer structure with alternating P and N layers: This structure is caused by the heavy duty N-channel MOSFETs.

If the voltage applied to the gate pins is above the threshold level, the gate is biased relative to the source, creating an inverted N-channel under the silicon oxide film that connects the source to the drain for current to flow.

The conduction of the MOSFET is ensured by the majority carriers, since there are no injected minority carriers in the channel. This does not lead to charge accumulation, which speeds up the switching process. In the on state, the relationship between current and voltage is almost linear, similar to the resistance, which is considered as the resistance of the channel in the open state.

Power MOSFETs
Rice. 7.2. Internal structures of transistors: a - transistor of the HEXFET structure; b - transistor of the VMOS structure; c - SIPMOS structure transistor

The MOSFET equivalent circuit is shown in fig. 7.3. The two capacitances between gate and source, gate and drain, result in a switching delay if the driver cannot support a high turn-on current. Another capacitance in the transistor is between drain and source, but due to the internal structure of the transistor, it is shunted by a parasitic diode formed between drain and source. Unfortunately, the parasitic diode is not fast and should not be taken into account, and an additional shunt diode is introduced to speed up the switching.

Power MOSFETs
Rice. 7.3. MOSFET equivalent circuit: a - the first version of the equivalent circuit; b - the second version of the equivalent circuit with the replacement of the transistor by a diode; c - internal structure corresponding to the first option

MOSFET parameters

Consider the main parameters that characterize MOSFET transistors.

Maximum drain-source voltage, SHE IS_DS - maximum instantaneous operating voltage.

Continuous drain current, The_D is the maximum current that the MOSFET can carry due to the junction temperature.

Maximum surge current drain, The_DM - more than I_D and is defined for an impulse of a given duration and duty cycle.

Maximum gate-source voltage age, SHE IS_GS is the maximum voltage that can be applied between gate and source without damaging the gate insulation.

Additionally, take place:

gate threshold voltage, U_T {U_TH, SHE IS_GS};
U_T is the minimum gate voltage at which the transistor turns on.

Author: Koryakin-Chernyak S.L.

See other articles Section Electrician's Handbook.

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