The system parses the MOS tube in the switching power supply--15 big knowledge points!

Positive guide circuit for power MOSFET

(1): equivalent circuit

(2): Description:

The power MOSFET is guided by a resistor equivalent, the resistance is temperature-related, the temperature increases, the resistance becomes larger; it is also related to the size of the gate drive voltage, the driving voltage increases, the resistance becomes smaller. Detailed relationship curves can be obtained from the manufacturer's manual.

Reverse-guide equivalent circuit for power MOSFETs (1)

(1): Equivalent circuit (door is not controlled)

(2): Description:

The equivalent circuit of the internal diode, which can be used as a voltage drop equivalent, this diode is a MOSFET body diode, in most cases, because of its poor characteristics, to avoid the use.

Reverse-guide equivalent circuit for power MOSFETs (2)

(1): Equivalent circuit (gate plus control)

(2): Description:

The power MOSFET in the gate-level control of the counter-guide, can also be equivalent to a resistor, the resistance is temperature-related, the temperature increases, the resistance becomes larger; it is also related to the size of the gate drive voltage, the driving voltage increases, the resistance becomes smaller. Detailed relationship curves can be obtained from the manufacturer's manual. This working condition, known as the synchronous rectification of the MOSFET, is a very important working condition in the low voltage high current output switching power supply.

Forward cutoff equivalent circuit of power MOSFET

(1): equivalent circuit

(2): Description:

The power MOSFET can be used as a capacitor equivalent in the forward cutoff, and its capacity is related to the added 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 at steady state of power MOSFET

(2): Description:

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

When the gate is not controlled, its counter-guide steady-state working point is the same as the diode.

(3): steady-state Characteristics Summary:

-The voltage between the gate and the source VGs control the conduction state of the device; When Vgs<vth, the device is in a disconnected state, the Vth is generally 3V, when the Vgs>vth, the device is on, the device's on-state resistance is related to VGS, VGs is large, the on-State resistance is small The VGS of most devices is 12v-15v and the rated value is +-30v;

-The device's drain current rating is nominal with its RMS or average value, as long as the actual drain current RMS does not exceed its rated value, to ensure that the heat dissipation is not a problem, the device is safe;

--The on-state resistance of the device is a positive temperature coefficient, so it is easy to expand the shunt in parallel, but the symmetry of the drive and the dynamic current-sharing problem should be considered in the actual parallel operation.

Current Logic-level Power MOSFET, its vgs as long as 5V, it can ensure that the leakage source on-state resistance is very small;

-The synchronous rectification of the device has become more and more widespread due to its very small on-state resistance (currently the smallest is 2-4 MW), which is the most critical device in the DC/DC output of low voltage and high current.

Equivalent circuit of power MOSFET with parasitic parameters

(1): equivalent circuit

(2): Description:

The actual power MOSFET can be used with three junction capacitors, three channel resistors, and an internal diode and an ideal MOSFET to be equivalent. The three junction capacitors are related to the junction voltage, and the gate channel resistance is generally small, the drain and the source of the two channel resistance is the MOSFET saturation of the on-State resistor.

Principle of power MOSFET opening and shutdown process

(1): Open and shut-off process experimental circuit

(2): voltage and current waveform of MOSFET:

(3): Switch process principle:

opening process [t0 ~ T4]:

--Before T0, the MOSFET works in the cut-off state, t0, the MOSFET is driven to open;

--[T0-T1] interval, the MOSFET GS voltage by Vgg to the CGS charge and rise, in T1 time, reached the maintenance voltage vth,mosfet start conductive;

--[T1-t2] interval, the DS current of the MOSFET increases, Millier capacitance in the interval due to the discharge of the DS capacitor, the charge of the GS capacitor is not very significant;

--[T2-T3] interval, to T2 time, the MOSFET DS voltage to VGs the same voltage, Millier capacitance greatly increased, the external driving voltage to the Millier capacitor to charge, GS capacitor voltage unchanged, Millier capacitance on the voltage increase, and the voltage on the DS capacitor continues to decrease;

--[T3-T4] interval, to T3 time, the MOSFET's DS voltage to the saturation conduction of the voltage, Millier capacitance is small and with the GS capacitor by the external driving voltage charge, GS capacitor voltage rise, up to T4 time. At this time the GS capacitor voltage has reached the steady state, the DS voltage is also the smallest, that is, stable on-state pressure drop.

Shutdown process [T5 ~T9]:

--Before T5, the MOSFET works on the conduction state, T5, the MOSFET is driven off;

--[T5-T6] interval, the CGS voltage of the MOSFET is decreased by the resistance discharge of the driving circuit, at T6 time, the MOSFET's on-state resistance rises slightly, the DS voltage tip slightly increases, but the DS current is unchanged;

--[T6-t7] interval, in T6 time, the MOSFET millier capacitance has become very large, so the voltage of GS capacitor is constant, discharge current through millier capacitance, so that the DS voltage continues to increase;

--[T7-t8] interval, to T7 time, the DS voltage of the MOSFET rises to the same voltage as VGS, the Millier capacitance decreases rapidly, the GS capacitor starts to discharge, the voltage on the DS capacitor rises rapidly, and the DS current drops rapidly;

--[T8-t9] interval, to T8 time, GS capacitance has been discharged to Vth,mosfet completely shut off, the interval GS capacitor continues