table of Contents
1. The equivalent model of the mos tube is:
Six, changes to the Miller platform
1. The equivalent model of the mos tube is:
2. Simple navigation
3. Detailed guidance
The waveform diagram at turn-on is
Leading process analysis:
T0~T1 : Drive to charge Cgs through Rgate , the voltage Vgs rises exponentially, and there is no change in Id at this time
~ T2 Tl : Vgs of reach MOSFET threshold voltage, MOSFET enters the linear region, Id slowly increased to T2 time Id reaches the saturated or maximum current load. During this period, almost the full voltage Vdd is still borne between the drain and the source .
T2~T3 : At time T2, Id reaches saturation and maintains a stable value, the MOS tube works in the saturation region, Vgs is fixed, and the voltage Vds begins to decrease. During this period, Cgs no longer consumes charge, and VDD starts to provide discharge current to Cgd . At this time, the Vg voltage reaches the Miller platform and the Cgs capacitor is full. During this period, the Id current does not change, Vd continues to decrease, and the mos tube is turned on.
T3~T4 : The voltage Vds drops to 0V , and VDD continues to charge Cgs until Vgs=VDD , and the MOSFET completes its turn-on process.
Leading process measured
Fourth, the shutdown process
The MOSFET turn-off process is the reverse process of the turn-on process
Five, important instructions
The time span of each stage of Vgs is proportional to the grid consumption charge (because △ Q = IG △ T , and IG is the output of the constant current source here).
The span of T0 ~ T2 represents the charge consumed by Ciss ( VGS+CGD ), which corresponds to the parameter Qgs (Gate to Source Charge) provided in the device specification .
The span of T2 ~ T3 represents the charge consumed by CGD (or called Miller capacitor), which corresponds to the parameter Qds (Gate to Drain ("Miller") Charge) provided in the device specification .
All the electric charge consumed before T3 is the minimum electric charge required for a MOSFET with a driving voltage of Vdd and a current of Id to be fully turned on. The extra charge consumed after T3 does not mean the charge necessary for driving, but only the excess charge provided by the driving circuit .
Switching loss: When the MOSFET is turned on, the voltage at both ends has a decreasing process, and the current flowing has a rising process. During this time, the MOS tube loses the product of voltage and current, which is called switching loss.
Conduction loss: After the MOS tube is turned on, the current consumes energy on the conduction resistance, which is called conduction loss
Overall performance:
Drive power requirements:
△Q t0 ~ t4= (t4－t0 )IG = VG（CGS + CGD）+ VDDCGD
Drive current requirements:
IG = △ Q t0 ~ t4 / (t4 - t0) ≈ △ Q t0 ~ t3 / (t3 - t0) ≈Qg / (Td (on) + Tr)
Drive power requirements:
Pdrive=∫t4－t0 vg(t)ig(t)≈VG△Q≈VG〔VG（CGS+CGD）+ VDDCGD〕
Drive resistance requirements:
RG = VG / IG
Generally, the following parameters provided in the device specification can be used as the calculation assumptions for the initial drive design
a) Qg (Total Gate Charge) : As the minimum drive power requirement.
b) Correspondingly, the minimum drive current requirement is IG ≈Qg/(td(on)+tr) .
c) Pdrive=VG *Qg as the minimum drive power requirement.
d) Correspondingly, the average drive loss is VG *Qg*fs
Six, changes to the Miller platform
The Miller platform of the mos tube will be different under different Id currents. The higher the current, the higher the Miller platform.
In the case of different Vds voltage, the time when the Miller platform starts to rise is different. The higher the voltage, the longer the Miller platform lasts.
