Useful Information | A technical expert shares 15 tips on power MOSFET [Collect it]

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Useful Information | A technical expert shares 15 tips on power MOSFET [Collect it] [Copy link]

Forward conduction equivalent circuit of power MOSFET

Equivalent Circuit

illustrate:

When the power MOSFET is forward-conducting, it can be equivalent to a resistor. This resistor is related to temperature. As the temperature rises, the resistor increases. It is also related to the gate drive voltage. As the drive voltage increases, the resistor decreases. Detailed relationship curves can be obtained from the manufacturer's manual.

Reverse conduction equivalent circuit of power MOSFET

Equivalent circuit (gate not controlled)

illustrate:

That is, the equivalent circuit of the internal diode, which can be equivalent to a voltage drop. This diode is the body diode of the MOSFET. In most cases, its characteristics are very poor and it should be avoided.

Reverse conduction equivalent circuit of power MOSFET

Equivalent circuit (gate plus control)

illustrate:

The reverse conduction of the power MOSFET under gate-level control can also be equivalent to a resistor. This resistor is related to temperature. As the temperature rises, the resistor increases. It is also related to the gate drive voltage. As the drive voltage increases, the resistor decreases. The detailed relationship curve can be obtained from the manufacturer's manual. This working state is called the synchronous rectification of the MOSFET, which is a very important working state in low-voltage and high-current output switching power supplies.

Power MOSFET forward cutoff equivalent circuit

Equivalent Circuit

illustrate:

When the power MOSFET is forward-cut off, a capacitor can be used as an equivalent. The capacity of the capacitor is related to the applied forward voltage, ambient temperature, etc. The size can be obtained from the manufacturer's manual.

Summary of Steady-State Characteristics of Power MOSFET

(1): Current/voltage curve of power MOSFET in steady state

(2) Description

The steady-state operating point of the power MOSFET during forward saturation conduction:

When the gate is not controlled, its reverse conduction steady-state operating point is the same as that of the diode.

(3): Summary of steady-state characteristics

-- The voltage Vgs between the gate and the source controls the on-state of the device; when VgsVth, the device is in the on-state; the on-resistance of the device is related to Vgs, the larger the Vgs, the smaller the on-resistance; the Vgs of most devices is 12V-15V, and the rated value is +-30V;

-- The device's drain current rating is specified by its effective value or average value; as long as the actual drain current effective value does not exceed its rated value and heat dissipation is guaranteed, the device is safe;

-- The on-state resistance of the device is a positive temperature coefficient, so in principle it is easy to connect in parallel to expand the capacity, but when actually connecting in parallel, the symmetry of the drive and the dynamic current sharing must also be considered;

-- The current logic-level power MOSFET only needs 5V Vgs to ensure a very low drain-source on-state resistance;

-- The synchronous rectification working state of the device has become more and more widespread because its on-state resistance is very small (the minimum is currently 2-4 milliohms), and it has become the most critical device in low-voltage and high-current DC/DC output;

Power MOSFET equivalent circuit including parasitic parameters

Equivalent Circuit

illustrate:

The actual power MOSFET can be equivalent to three junction capacitors, three channel resistors, an internal diode and an ideal MOSFET. The three junction capacitors are all related to the size of the junction voltage, and the gate channel resistance is generally very small. The sum of the two channel resistances of the drain and source is the on-state resistance of the MOSFET when it is saturated.

The principle of power MOSFET turn-on and turn-off process

(1): Opening and closing process experimental circuit

(2): MOSFET voltage and current waveforms

(3): Switching process principle

Opening process [t0 ~ t4]:

-- Before t0, MOSFET works in the cut-off state. At t0, MOSFET is driven to turn on;

-- In the interval [t0-t1], the GS voltage of MOSFET rises through Vgg charging Cgs. At t1, it reaches the maintenance voltage Vth and MOSFET starts to conduct;

-- In the interval [t1-t2], the DS current of the MOSFET increases, and the Millier capacitor discharges due to the discharge of the DS capacitor in this interval, which has little effect on the charging of the GS capacitor;

-- [t2-t3] interval, at time t2, the DS voltage of MOSFET drops to the same voltage as Vgs, the Millier capacitance increases greatly, the external drive voltage charges the Millier capacitance, the voltage of the GS capacitance remains unchanged, the voltage on the Millier capacitance increases, and the voltage on the DS capacitance continues to decrease;

-- [t3-t4] interval, to time t3, the MOSFET's DS voltage drops to the saturated on voltage, the Millier capacitance becomes smaller and is charged by the external drive voltage together with the GS capacitance, and the GS capacitance voltage rises until time t4. At this time, the GS capacitance voltage has reached a steady state, and the DS voltage has also reached a minimum, that is, a stable on-state voltage drop.

Shutdown process [t5 ~t9]:

-- Before t5, MOSFET is in the on state. At t5, MOSFET is driven off.

-- In the interval [t5-t6], the Cgs voltage of the MOSFET decreases due to discharge through the drive circuit resistor. At t6, the on-resistance of the MOSFET increases slightly, the DS voltage increases slightly, but the DS current remains unchanged;

-- [t6-t7] interval, at time t6, the Millier capacitance of MOSFET becomes very large again, so the voltage of GS capacitor remains unchanged, and the discharge current flows through the Millier capacitance, causing the DS voltage to continue to increase;

-- [t7-t8] interval, at time t7, the DS voltage of MOSFET rises to the same voltage as Vgs, the Millier capacitance decreases rapidly, and the GS capacitance begins to continue discharging. At this time, the voltage on the DS capacitance rises rapidly, and the DS current decreases rapidly;

-- [t8-t9] interval, by time t8, the GS capacitor has discharged to Vth and the MOSFET is completely turned off; in this interval, the GS capacitor continues to discharge until it reaches zero.

MOSFET switching waveform caused by diode reverse recovery

(1): Experimental circuit

(2): MOSFET switching waveform caused by diode reverse recovery

Power loss formula for power MOSFET

(1): Conduction loss

This formula is applicable to both controlled rectification and synchronous rectification.

This formula is applicable when the body diode is conducting.

(2): Capacitive turn-on and inductive turn-off losses

is the sum of all distributed inductances in the MOSFET device and the diode loop. Generally, this loss can also be regarded as the inductive turn-off loss of the device.

(3): Switching loss

Turn-on loss:

After considering the reverse recovery of the diode:

Turn-off loss:

Driver loss:

Principles and steps for selecting power MOSFET

(1): Selection principle

(A): According to the power supply specifications, reasonably select MOSFET devices (see the table below):

(B): When selecting, if the operating current is large, then under the same device rated parameters,

-- Choose a MOSFET with a small forward resistance as much as possible;

-- Choose a MOSFET with as small a junction capacitance as possible.

(2): Selection step

(A): Based on the power supply specifications, calculate the steady-state parameters of the MOSFET in the selected converter:

-- Maximum forward blocking voltage;

-- Maximum forward current effective value;

(B): Select a suitable MOSFET from the device manufacturer's DATASHEET. You can select more than one for comparison during the experiment.

(C): Estimate the maximum loss of the selected MOSFET from other parameters such as forward resistance, junction capacitance, etc., and together with the losses of other components, estimate the efficiency of the converter;

(D): The final MOSFET device is selected by experiment.

Basic requirements for an ideal switch

(1): Symbol

(2): Requirements

(A): Steady-state requirements

After closing K

-- The voltage across the switch is zero;

-- The current in the switch is determined by the external circuit;

-- The direction of the switching current can be positive or negative;

-- The capacity of the switching current is unlimited.

After disconnecting K

-- The voltage across the switch can be positive or negative;

-- The current in the switch is zero;

-- The voltage across the switch is determined by the external circuit;

-- The voltage capacity across the switch is infinite.

(B): Dynamic requirements

K Opening

-- The signal power of the control opening is zero;

-- The time for the opening process is zero.

K shut-off

-- The signal power controlling the shutdown is zero;

-- The time of the shutdown process is zero.

(3): Waveform

Where: H: control high level; L: control low level

-- Ion can be positive or negative, and its value is determined by the external circuit;

-- Voff can be positive or negative, and its value is determined by the external circuit.

Limitations of achieving an ideal switch with electronic switches

(1): The voltage and current direction of the electronic switch are limited

(2): The steady-state switching characteristics of electronic switches are limited

-- There is a voltage drop when conducting; (forward voltage drop, on-state resistance, etc.)

-- There is leakage current when cut off;

-- The maximum on-state current is limited;

-- The maximum blocking voltage is limited;

-- Control signals have power requirements, etc.

(3): The dynamic switching characteristics of electronic switches are limited

-- There is a process for opening, and its length is related to the control signal and the internal structure of the device;

-- There is a shutdown process, the length of which is related to the control signal and the internal structure of the device;

-- The maximum switching frequency is limited.

There are many electronic devices used as switches. In switching power supplies, the most commonly used are diodes, MOSFETs, IGBTs, etc., and their combinations.

Four structures of electronic switches

(1): Single quadrant switch

(2): Current bidirectional (dual-quadrant) switch

(3): Voltage bidirectional (dual-quadrant) switch

(4): Four single quadrant switches

Classification of switching devices

(1): Classification by production materials

-- (Si) power devices;

-- (Ga) power devices;

-- (GaAs) power devices;

-- (SiC) power devices;

-- (GaN) power devices; --- Next generation

-- (Diamond) power devices; --- The next generation

(2): Classification by whether it is controllable

--Completely uncontrolled devices: such as diode devices;

-- Can be controlled to turn on, but not turned off: such as ordinary thyristor devices;

-- Fully controlled switching devices

-- Voltage-type control devices: such as MOSFET, IGBT, IGT/COMFET, SIT, etc.;

-- During current type control: such as GTR, GTO, etc.

(3): Classification by operating frequency

-- Low-frequency power devices: such as thyristors, ordinary diodes, etc.;

-- Medium frequency power devices: such as GTR, IGBT, IGT/COMFET;

-- High frequency power devices: such as MOSFET, fast recovery diode, Schottky diode, SIT, etc.

(4): Classification by rated maximum achievable capacity

-- Low power devices: such as MOSFET

-- Medium power devices: such as IGBT

-- High power devices: such as GTO

(5): Classification by conductive carrier particles:

-- Multi-sub devices: such as MOSFET, Schottky, SIT, JFET, etc.

-- Minority carrier devices: such as IGBT, GTR, GTO, fast recovery, etc.

Comparison of different switching devices

(1): Comparison of power handling capabilities of several turn-off devices

(2): Comparison of the operating characteristics of several turn-off devices

The above data will change with the development of devices and is for reference only.

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