The operating loss of MOSFET can basically be divided into the following parts:
1. Conduction loss Pon
Conduction loss refers to the loss caused by the voltage drop generated by the load current (ie drain-source current) IDS(on)(t) on the on-resistance RDS(on) after the MOSFET is fully turned on.
Calculation of working loss of 8 kinds of switching power supply MOS tubes
Conduction loss calculation:
First obtain the IDS(on)(t) function expression through calculation and calculate its effective value IDS(on)rms, and then calculate through the following resistance loss calculation formula:
Pon=IDS(on)rms2 × RDS(on) × K × Don
illustrate:
The period used to calculate IDS(on)rms is only the on-time Ton, not the entire duty cycle Ts; RDS(on) will vary with the IDS(on)(t) value and the device junction temperature, so The principle is to find the RDS(on) value as close as possible to the expected working conditions according to the specification (that is, multiply by a temperature coefficient K provided by the specification).
2. Cut-off loss Poff
Turn-off loss refers to the loss caused by the leakage current IDSS generated under the drain-source voltage VDS(off) stress after the MOSFET is completely turned off.
Cut-off loss calculation:
First calculate the drain-source voltage VDS(off) that the MOSFET is subjected to when it is turned off, and then search for the IDSS provided in the device specification, and then calculate it with the following formula:
Poff=VDS(off) × IDSS ×( 1-Don )
illustrate:
IDSS will vary according to VDS(off), and the value provided in the specification is a parameter under an approximate V(BR)DSS condition. If the calculated drain-source voltage VDS(off) is so large that it is close to V(BR)DSS, this value can be directly quoted; if it is very small, it can be zero, that is, this item is ignored.
3. Opening process loss
The turn-on process loss refers to the loss caused by the overlapping part of the gradually falling drain-source voltage VDS(off_on)(t) and the gradually rising load current (ie drain-source current) IDS(off_on)(t) during the MOSFET turn-on process.
To enable process loss calculation:
The cross waveform of VDS(off_on)(t) and IDS(off_on)(t) during the turn-on process is shown in the figure above. First, it is necessary to calculate or estimate the VDS(off_end) before the opening time, the IDS(on_beginning) after the opening is completed, which is the Ip1 shown in the figure, and the overlapping time Tx of VDS(off_on)(t) and IDS(off_on)(t). Then it is calculated by the following formula:
Poff_on= fs ×∫ Tx VDS(off_on)(t) × ID(off_on)(t) × dt
There are mainly two kinds of assumptions in the actual calculation—the assumption in Figure (A) that the initial decline of VDS(off_on)(t) and the gradual rise of ID(off_on)(t) occur simultaneously; the assumption in Figure (B) that VDS The decline of (off_on)(t) starts after ID(off_on)(t) rises to the maximum value. Figure (C) is the actual test waveform of a MOSFET in the FLYBACK architecture, which is closer to the assumption of type (A). Based on these two assumptions, two calculation formulas are extended:
(A) class assumes that Poff_on=1/6 × VDS(off_end) × Ip1 × tr × fs
Class (B) assumes that Poff_on=1/2 × VDS(off_end) × Ip1 × (td(on)+tr) × fs
(B) Class assumptions can be used as calculations for worst-case models.
illustrate:
From the actual test waveform in Figure (C), you can see the IDS(on_beginning)>>Ip1 after the turn-on is completed (the Ip1 parameter is often the initial value of the excitation current during power supply use). It is difficult to predict the exact value of the superimposed current peak, which is related to the circuit structure and device parameters. For example, the actual current in FLYBACK should be Itotal=Idp1+Ia+Ib (Ia is the current value induced by the reverse recovery current of the rectifier diode on the secondary side back to the primary electrode -- that is, multiplied by the turn ratio, and Ib is the interlayer of the primary side winding of the transformer The current released by the parasitic capacitance at the moment the MOSFET switch is turned on). This unpredictable value is also one of the main reasons for the calculation error in this part.
4. Turn-off process loss
Turn off process loss. Refers to the loss caused by the cross-overlap portion of the rising drain-source voltage VDS(on_off)(t) and the falling drain-source current IDS(on_off)(t) during MOSFET turn-off.
Calculation of loss in turn-off process:
As shown in the figure above, the loss calculation principle and method of this part are similar to those of Poff_on. First of all, it is necessary to calculate or estimate the drain-source voltage VDS(off_beginning) after the turn-off, the load current IDS(on_end) before the turn-off time, which is the Ip2 shown in the figure, and VDS(on_off) (t) and IDS(on_off)(t ) overlap time Tx.
and then calculated by the following formula:
Poff_on= fs ×∫ Tx VDS(on_off) (t) × IDS(on_off)(t) × dt
In the actual calculation, two calculation formulas are extended based on these two assumptions:
(A) class assumes that Poff_on=1/6 × VDS(off_beginning) × Ip2 × tf × fs
Class (B) assumes that Poff_on=1/2 × VDS(off_beginning) × Ip2 × (td(off)+tf) × fs
(B) Class assumptions can be used as calculations for worst-case models.
illustrate:
IDS(on_end) =Ip2, this parameter is often the end value of the excitation current in the power supply. Due to factors such as leakage inductance, VDS (off_beginning) of the MOSFET after it is turned off often has a large voltage spike Vspike superimposed on it, and this value can be roughly estimated based on experience.
5. Driving loss Pgs
Driving loss refers to the loss caused by the gate being driven by the driving power
Calculation of drive loss:
After determining the driving power supply voltage Vgs, it can be calculated by the following formula:
Pgs= Vgs × Qg × fs
illustrate:
Qg is the total driving power, which can be found through device specifications.
6. The discharge loss Pds of the Coss capacitor
The discharge loss of the Coss capacitor refers to the discharge loss of the electric field energy stored during the cut-off period of the MOS output capacitor Coss on the drain and source during the conduction period.
Calculation of discharge loss of Coss capacitor:
First of all, the VDS before the opening time must be calculated or estimated, and then calculated by the following formula:
Pds=1/2 × VDS(off_end)2 × Coss × fs
illustrate:
Coss is the MOSFET output capacitance, which is generally equal to Cds, and this value can be found through device specifications.
7. The forward conduction loss Pd_f of the parasitic diode in the body
The forward conduction loss of the parasitic diode in the body refers to the loss caused by the forward voltage drop when the parasitic diode in the MOS body carries forward current.
Calculation of the forward conduction loss of the parasitic diode in the body:
In some applications that use internal parasitic diodes to carry current (such as synchronous rectification), it is necessary to calculate the loss of this part. The formula is as follows:
Pd_f = IF × VDF × tx × fs
Among them: IF is the current carried by the diode, VDF is the forward conduction voltage drop of the diode, and tx is the time for the diode to carry the current in one cycle.
illustrate:
It will vary depending on the junction temperature of the device and the magnitude of the current it carries. According to the actual application environment, the value as close as possible can be found on the specification sheet.
8. Reverse recovery loss Pd_recover of internal parasitic diode
The reverse recovery loss of parasitic diodes in the body refers to the loss caused by the reverse recovery caused by the reverse voltage after the parasitic diodes in the MOS body carry forward current.
Calculation of reverse recovery loss of internal parasitic diode:
The principle and calculation method of this loss are the same as the reverse recovery loss of ordinary diodes. The formula is as follows:
Pd_recover=VDR × Qrr × fs
Among them: VDR is the reverse voltage drop of the diode, Qrr is the reverse recovery power of the diode, which can be obtained from the data sheet provided by the device.
Several basic principles of MOS design and selection
Suggested basic steps for the primary election:
1. Voltage stress
In power supply circuit applications, the selection of the drain-source voltage VDS is often considered first. The basic principle here is that the maximum peak drain-source voltage in the actual working environment of the MOSFET is not greater than 90% of the nominal drain-source breakdown voltage in the device specification. Right now:
VDS_peak ≤ 90% * V(BR)DSS
Note: In general, V(BR)DSS has a positive temperature coefficient. Therefore, the V(BR)DSS value under the minimum operating temperature of the equipment should be taken as a reference.
2. Drain current
Next, consider the selection of the drain current. The basic principle is that the maximum periodic drain current in the actual working environment of the MOSFET is not greater than 90% of the nominal maximum drain-source current in the specification; the peak value of the drain pulse current is not greater than 90% of the nominal peak value of the drain pulse current in the specification. :
ID_max ≤ 90% * ID
ID_pulse ≤ 90% * IDP
Note: Generally, ID_max and ID_pulse have negative temperature coefficients, so the ID_max and ID_pulse values of the device at the maximum junction temperature should be taken as a reference. The selection of this parameter of the device is extremely uncertain—mainly due to the mutual constraints of the working environment, heat dissipation technology, and other parameters of the device (such as on-resistance, thermal resistance, etc.). The final judgment is based on the junction temperature (ie, the "dissipated power constraint" in Article 6 below). According to experience, in actual application, the ID in the specification bibliography will be several times larger than the actual maximum operating current, which is due to the limitation of power dissipation and temperature rise. During the preliminary calculation period, this parameter must be continuously adjusted according to the dissipation power constraint in Article 6 below. It is recommended that the initial selection is about 3~5 times ID = (3~5)*ID_max.
3. Driver requirements
The drive requirement of MOSFEF is determined by its gate total charge capacity (Qg) parameter. In the case of meeting the requirements of other parameters, try to choose the one with the smallest Qg to facilitate the design of the driving circuit. The driving voltage is selected to keep Ron as small as possible under the premise of keeping away from the maximum gate-source voltage ( VGSS ) (generally use the recommended value in the device specification)
4. Loss and heat dissipation
A small Ron value is beneficial to reduce the loss during conduction, and a small Rth value can reduce the temperature difference (under the same power dissipation condition), so it is beneficial to heat dissipation.
5. Preliminary calculation of power loss
MOSFET loss calculation mainly includes the following 8 parts:
PD = Pon + Poff + Poff_on + Pon_off + Pds + Pgs+Pd_f+Pd_recover
The detailed calculation formula should be determined according to the specific circuit and working conditions. For example, in the application of synchronous rectification, the loss during the forward conduction period of the body diode and the reverse recovery loss when turning to cut-off should also be considered. For loss calculation, please refer to the "8 Components of MOS Tube Loss" section below.
6. Dissipated power constraints
The steady-state power loss PD,max of the device should be based on the maximum operating junction temperature limit of the device. If the working environment temperature of the device can be known in advance, the maximum power dissipation can be estimated as follows:
PD,max ≤ ( Tj,max - Tamb ) / Rθj-a
Where Rθj-a is the total thermal resistance between the device junction and its working environment, including Rθjuntion-case, Rθcase-sink, Rθsink-ambiance, etc. If there is an insulating material in between, its thermal resistance must also be taken into consideration.
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