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It's rare material that an expert can analyze power MOS like this!
Release time:
2021-03-31
Equivalent circuit of power MOSFET in forward conduction
(1): Equivalent circuit
(2): Description
The power MOSFET in forward conduction can be represented by a resistor equivalent, which is temperature-dependent; as the temperature increases, the resistance increases; it is also related to the gate drive voltage; as the drive voltage increases, the resistance decreases. Detailed relationship curves can be obtained from the manufacturer's manual.
Equivalent circuit of power MOSFET in reverse conduction (1)
(1): Equivalent circuit (no control applied to the gate)
(2): Description
This is the equivalent circuit of the internal diode, which can be represented by a voltage drop; this diode is the body diode of the MOSFET, and in most cases, due to its poor characteristics, it should be avoided.
Equivalent circuit of power MOSFET in reverse conduction (2)
(1): Equivalent circuit (control applied to the gate)
(2): Description
The reverse conduction of the power MOSFET under gate control can also be represented by a resistor equivalent, which is temperature-dependent; as the temperature increases, the resistance increases; it is also related to the gate drive voltage; as the drive voltage increases, the resistance decreases. Detailed relationship curves can be obtained from the manufacturer's manual. This operating state is referred to as the synchronous rectification operation of the MOSFET, which is a very important operating state in low-voltage high-current output switch power supplies.
Equivalent circuit of power MOSFET in forward cutoff
(1): Equivalent circuit
(2): Description
The power MOSFET in forward cutoff can be represented by a capacitor equivalent, whose capacitance is related to the applied forward voltage, ambient temperature, etc., and 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
Steady-state operating point of power MOSFET in forward saturation conduction:
When the gate is not controlled, its steady-state operating point in reverse conduction is the same as that of a diode.
(3): Summary of steady-state characteristics
-- The voltage Vgs between the gate and source controls the conduction state of the device; when Vgs < Vth, the device is in the off state, Vth is generally 3V; when Vgs > Vth, the device is in the on state; the on-state resistance of the device is related to Vgs; a larger Vgs results in a smaller on-state resistance; most devices have a Vgs of 12V-15V, with a rated value of ±30V;
-- The rated drain current of the device is specified using its effective or average value; as long as the actual effective drain current does not exceed its rated value and heat dissipation is ensured, the device is safe;
-- The on-state resistance of the device has a positive temperature coefficient, so it is theoretically easy to parallel and expand capacity, but in practice, the symmetry of the drive and dynamic current sharing issues must also be considered;
-- Currently, Logic-Level power MOSFETs require only 5V for Vgs to ensure a very small drain-source on-state resistance;
-- The synchronous rectification operating state of the device has become increasingly widespread due to its very small on-state resistance (currently the smallest is 2-4 milliohms), making it the most critical device in low-voltage high-current output DC/DC converters;
Equivalent circuit of power MOSFET including parasitic parameters
(1): Equivalent circuit
(2): Description
The actual power MOSFET can be represented by three junction capacitances, three channel resistances, one internal diode, and one ideal MOSFET. The three junction capacitances are all related to the magnitude of the junction voltage, while the channel resistance of the gate 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 saturated.
Principle of the turn-on and turn-off process of power MOSFET
(1): Experimental circuit for the turn-on and turn-off process
(2): Voltage and current waveforms of the MOSFET
(3): Principle of the switching process
Turn-on process [t0 ~ t4]:
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Before t0, the MOSFET is in the cutoff state; at t0, the MOSFET is driven to turn on;
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In the [t0-t1] interval, the GS voltage of the MOSFET rises due to charging of Cgs by Vgg, reaching the maintenance voltage Vth at t1, and the MOSFET begins to conduct;
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In the [t1-t2] interval, the DS current of the MOSFET increases, and the Millier capacitor discharges due to the discharge of the DS capacitance, which has little effect on the charging of the GS capacitance;
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In the [t2-t3] interval, by t2, the DS voltage of the MOSFET drops to the same voltage as Vgs, the Millier capacitor increases significantly, and the external drive voltage charges the Millier capacitor, while the voltage of the GS capacitor remains unchanged, and the voltage on the Millier capacitor increases, while the voltage on the DS capacitor continues to decrease;
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In the [t3-t4] interval, by t3, the DS voltage of the MOSFET drops to the voltage during saturation conduction, the Millier capacitor decreases and charges together with the GS capacitor from the external drive voltage, the voltage of the GS capacitor rises, until t4. At this point, the voltage of the GS capacitor has reached a steady state, and the DS voltage has also reached its minimum, which is the stable on-state voltage drop.
Turn-off process [t5 ~t9 ]:
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Before t5, the MOSFET is in the on state; at t5, the MOSFET is driven to turn off;
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In the [t5-t6] interval, the Cgs voltage of the MOSFET decreases due to discharge through the resistance of the drive circuit; at t6, the on-state resistance of the MOSFET slightly increases, and the DS voltage slightly increases, but the DS current remains unchanged;
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In the [t6-t7] interval, at t6, the Millier capacitor of the MOSFET becomes very large again, so the voltage of the GS capacitor remains unchanged, and the discharge current flows through the Millier capacitor, causing the DS voltage to continue to increase.
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In the interval [t7-t8], by the time of t7, the DS voltage of the MOSFET rises to the same voltage as Vgs, the Millier capacitance rapidly decreases, and the GS capacitance begins to continue discharging. At this time, the voltage across the DS capacitance rises rapidly, and the DS current decreases rapidly.
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In the interval [t8-t9], by the time of t8, the GS capacitance has discharged to Vth, and the MOSFET is completely turned off; during this interval, the GS capacitance 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 MOSFET power loss formula
(1): Conduction loss
This formula applies 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 loss
All distributed inductance in the MOSFET device and diode circuit is summed. This loss can generally be regarded as the inductive turn-off loss of the device.
(3): Switching loss
Turn-on loss:
Considering diode reverse recovery:
Turn-off loss:
Drive loss:
Principles and steps for selecting power MOSFETs
(1): Selection principles
(A): Select MOSFET devices reasonably according to the power supply Specification (see table below):
(B): When selecting, if the operating current is large, then under the same device rated parameters,
-- choose MOSFETs with as low forward conduction resistance as possible;
-- choose MOSFETs with as low junction capacitance as possible.
(2): Selection steps
(A): Calculate the steady-state parameters of the MOSFET in the selected converter according to the power supply Specification:
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Maximum forward blocking voltage;
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Maximum effective forward current;
(B): Select suitable MOSFETs from the device manufacturer's DATASHEET, and select multiple for comparison during experiments;
(C): Estimate the maximum loss during operation from other parameters of the selected MOSFET, such as forward on-resistance, junction capacitance, etc., and estimate the efficiency of the converter together with the losses of other components;
(D): Select the final MOSFET device through experiments.
Basic requirements for an ideal switch
(1): Symbols
(2): Requirements
(A): Steady-state requirements
After closing K
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The voltage across the switch is zero;
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The current in the switch is determined by the external circuit;
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The direction of the switch current can be positive or negative;
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The capacity of the switch current is infinite.
After opening K
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The voltage across the switch can be positive or negative;
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The current in the switch is zero;
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The voltage across the switch is determined by the external circuit;
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The capacity of the voltage across the switch is infinite.
(B): Dynamic requirements:
Turn-on of K
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The power of the control signal for turn-on is zero;
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The time for the turn-on process is zero.
Turn-off of K
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The power of the control signal for turn-off is zero;
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The time for the turn-off process is zero.
(3): Waveform
Where: H: Control high level; L: Control low level
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Ion can be positive or negative, its value is determined by the external circuit;
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Voff can be positive or negative, its value is determined by the external circuit.
Limitations of using electronic switches to achieve ideal switches
(1): The voltage and current direction of electronic switches are limited
(2): The steady-state switching characteristics of electronic switches are limited
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There is a voltage drop when conducting; (forward voltage drop, on-resistance, etc.)
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There is leakage current when cut off;
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The maximum on current is limited;
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The maximum blocking voltage is limited;
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Control signals have power requirements, etc.
(3): The dynamic switching characteristics of electronic switches are limited
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Turn-on has a process, the duration is related to the control signal and the internal structure of the device;
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Turn-off has a process, the duration is related to the control signal and the internal structure of the device;
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The maximum switching frequency is limited.
Currently, there are many electronic devices used as switches. In switch power supplies, the most commonly used are diodes, MOSFETs, IGBTs, etc., as well as their combinations.
Four structures of electronic switches
(1): Single quadrant switch
(2): Bidirectional (Quadrant II) switch
(3): Bidirectional (Quadrant II) switch
(4): Four Quadrant switch
Classification of switching devices
(1): Classified by manufacturing materials
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(Si) Power devices;
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(Ga) Power devices;
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(GaAs) Power devices;
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(SiC) Power devices;
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(GaN) Power devices; --- Next generation
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(Diamond) Power devices; --- Next next generation
(2): Classified by controllability
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Completely uncontrollable devices: such as diode devices;
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Controllable turn-on, but cannot control turn-off: such as ordinary thyristor devices;
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Fully controllable switching devices
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Voltage-type control devices: such as MOSFET, IGBT, IGT/COMFET, SIT, etc.;
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Current-type control devices: such as GTR, GTO, etc.
(3): Classified by operating frequency
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Low-frequency power devices: such as thyristors, ordinary diodes, etc.;
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Medium-frequency power devices: such as GTR, IGBT, IGT/COMFET;
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High-frequency power devices: such as MOSFET, fast recovery diodes, Schottky diodes, SIT, etc.
(4): Classified by rated maximum capacity
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Small power devices: such as MOSFET
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Medium power devices: such as IGBT
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Large power devices: such as GTO
(5): Classified by the type of charge carriers
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Majority carrier devices: such as MOSFET, Schottky, SIT, JFET, etc.
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Minority carrier devices: such as IGBT, GTR, GTO, fast recovery, etc.
Comparison of different switching devices
(1): Comparison of power handling capabilities of several controllable devices
(2): Comparison of operating characteristics of several controllable devices
The above data will change continuously with the development of devices and is for reference only.
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