AN-6005 [FAIRCHILD]

MOSFET损耗计算,Synchronous buck MOSFET loss calculations with Excel model; MOSFET的损耗计算，同步降压MOSFET的损耗计算与Excel模型


型号：	AN-6005
厂家：	FAIRCHILD SEMICONDUCTOR
描述：	MOSFET损耗计算,Synchronous buck MOSFET loss calculations with Excel model MOSFET的损耗计算，同步降压MOSFET的损耗计算与Excel模型
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中文：	中文翻译
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www.fairchildsemi.com

AN-6005

Synchronous buck MOSFET loss calculations

with Excel model

Jon Klein Power Management Applications

Abstract

The synchronous buck circuit is in widespread use to

provide “point of use” high current, low voltage

power for CPU’s, chipsets, peripherals etc.

High-Side Losses:

The power loss in any MOSFET is the combination

of the switching losses and the MOSFET’s

conduction losses.

Typically used to convert from a 12V or 5V “bulk”

supply, they provide outputs as low as 0.7V for low

voltage CPUs made in sub-micron technologies.

= P

+ P

COND

(1)

MOSFET

Q1 (Figure 1) bears the brunt of the switching losses,

since it swings the full input voltage with full current

through it. In low duty cycle converters (for

example: 12V_INto 1.8V_OUT) switching losses tend to

dominate.

The majority of the power lost in the conversion

process is due to losses in the power MOSFET

switches. The profiles of loss for the High-Side and

Low-Side MOSFET are quite different.

These low output voltage converters have low duty

cycles, concentrating the majority of the conduction

loss in the low-side MOSFET.

High-Side Conduction Losses:

Calculating high-side conduction loss is

straightforward as the conduction losses are just the

I²R losses in the MOSFET times the MOSFET’s duty

cycle:

V _IN

High-Side

OUT

= I

•R •

DS(ON)

(2)

COND

OUT

SW NODE

where R_DS(ON)is @ the maximum operating

MOSFET junction temperature (T_J(MAX)).

V_OUT

–

The maximum operating junction temperature is

equation can be calculated by using an iterative

technique. Since

Low-Side

R_DS(ON)rises with T_Jand

Figure 1. Synchronous Buck output stage

T_Jrises with P_D(dissipated power) and

P_Dis largely being determined by I²x R_DS(ON)

The spreadsheet calculator iterates the die

For the examples in the following discussion, we will

be analyzing losses for the following synchronous

buck converter:

temperature and accounts for the MOSFET's positive

R_DS(ON)temperature coefficient. Iteration continues

in the "DieTemp" custom function until the die

temperature has stabilized to within 0.01°C.

System Parameters

V_IN

1.5

V_OUT

I_OUT

F_SW

300

kHz

High-Side Switching Losses:

The switching time is broken up into 5 periods (t1-t5)

as illustrated in Figure 3. The top drawing in Figure

3 shows the voltage across the MOSFET and the

current through it. The bottom timing graph

represents V_GSas a function of time. The shape of

this graph is identical to the shape of the Q_Gcurve

contained in MOSFET datasheets, which assumes the

gate is being driven with a constant current. The Q_G

Table 1. Example Synchronous Buck

A spreadsheet to aid in the estimation of synchronous

buck losses is available on Fairchild’s web site on

(click here to download):

http://www.fairchildsemi.com/collateral/AN-

6005.zip. Operation of the spreadsheet is described

in the Appendix at the end of this document.

1.0.1 01/04/2006

Synchronous Buck Loss Calculation

AN-6005

notations indicate which QG is being charged during

the corresponding time period.

During this time the current is constant (at I_OUT) and

the voltage is falling fairly linearly from V_INto 0,

therefore:

V_IN

V_DRIVE(VDD)

• I

⎛

⎜

⎝

⎞

⎟

⎠

OUT

= t3 •

(4)

C_RSS

R_GATE

C_GS

C_OSS

R_DRIVER

During t4 and t5, the MOSFET is just fully

HDRV

enhancing the channel to obtain its rated R_DS(ON)at a

rated V_GS. The losses during this time are very small

compared to t2 and t3, when the MOSFET is

simultaneously sustaining voltage and conducting

current, so we can safely ignore them in the analysis.

Figure 2. Drive Equivalent Circuit

The switching loss for any given edge is just the

power that occurs in each switching interval,

multipied by the duty cycle of the switching interval:

ISS

RSS

V_DS

Switching losses

are in Shaded

section

• I

⎛

⎜

⎝

⎞

⎟

⎠

OUT

(

)

(

)

t2 + t3 F

(5)

(6)

Now, all we need to determine are t2 and t3. Each

period is determined by how long it takes the gate

driver to deliver all of the charge required to move

through that time period:

I_D

G(x)

DRIVER

V_GS

Q_GS

Q_GD

4.5V

V_SP

V_TH

Most of the switching interval is spent in t3, which

occurs at a voltage we refer to as “V_SP”, or the

“switching point” voltage. While this is not

Q_{G(SW )}

specifically specified in most MOSFET datasheets, it

can be read from the Gate Charge graph, or

approximated using the following equation:

C_ISS= C_GS+ C_RSS

Figure 3. High-Side Switching losses and Q_G

OUT

≈ V

(7)

The switching interval begins when the high-side

MOSFET driver turns on and begins to supply

current to Q1’s gate to charge its input capacitance.

There are no switching losses until V_GSreaches the

MOSFET’s V_TH. therefore P_t1= 0.

where G_Mis the MOSFET’s transconductance, and

_THis its typical gate threshold voltage.

With V_SPknown, the gate current can be determined

by Ohm’s law on the circuit in Figure 2:

When V_GSreaches V_TH, the input capacitance (C_ISS) is

being charged and I_D(the MOSFET’s drain current)

is rising linearly until it reaches the current in L1 (I_L)

which is presumed to be I_OUT. During this period (t2)

the MOSFET is sustaining the entire input voltage

across it, therefore, the energy in the MOSFET

during t2 is:

V_DD- V_SP

I_{DRIVER(L−H)}

(8A)

R_{DRIVER(PULL-UP)}+ R_GATE

V_SP

I_{DRIVER(H−L)}

(8B)

R_{DRIVER(PULL-DOWN)}+ R_GATE

The rising time (L-H) and falling times (H-L) are

treated separately, since I_DRIVERcan be different for

each edge.

•I

⎛

⎜

⎝

⎞

⎟

⎠

IN OUT

= t2 •

(3)

The V_GSexcursion during t2 is from V_THto V_SP.

Now, we enter t3. At this point, I_OUTis flowing

Approximating this as V_SPsimplifies the calculation

considerably and introduces no significant error.

This approximation also allows us to use the Q_G(SW)

term to represent the gate charge for a MOSFET to

move through the switching interval. A few

through Q1, and the V_DSbegins to fall. Now, all of

the gate current will be going to recharge C_GD. C_GDis

similar to the “Miller” capacitance of bipolar

transistors, so t3 could be thought of as “Miller time”.

1.0.1 01/04/2006

AN-6005

Synchronous Buck Loss Calculation

MOSFET manufacturers specify Q_G(SW)on their data

sheets. For those that don't, it can be approximated

by:

Driver dissipation calculates to :

500 * 5

2(8.5)

= 147mW

= 91mW

(11E)

DR(H-L)

Q_GS

Q_G(SW)≈ Q_GD

(9)

500 * 2

2(5.5)

(11F)

DR(L-H)

so the switching times therefore are:

= P

+P = 238mW (11G)

DR(L-H)

DRIVER

DR(H-L)

G(SW)

(10A)

(10B)

S(L−H)

S(H−L)

DRIVER(L-H)

2. The power to charge the MOSFET’s output

capacitance:

G(SW)

C_oss• V •F_SW

DRIVER(H-L)

P_COSS

≈

(11H)

The switching loss discussion above can be

summarized as:

where C_OSSis the MOSFETs output capacitance,

(C_DS+C_DG).

_INXI_OUT

⎛

⎞

(

)

(

)

P_SW

F_SWt_S(L-H)+ t_S(H−L)(10C)

⎜

⎝

⎟

⎠

3. If an external schottky is used across Q2, the

Schottky’s capacitance needs to be charged

during the high-side MOSFET’s turn-on:

There are several additional losses that are typically

much smaller than the aforementioned losses.

Although their proportional impact on efficiency is

low, they can be significant because of where the

dissipation occurs (for example, driver dissipation).

They are listed in order of importance:

C_SCHOTTKY• V •F_SW

P_C(SCHOTTKY)

(12A)

If a Schottky diode is not used:

4. Reverse recover power for Q2’s body diode:

1. The power to charge the gate:

P_QRR= Q_RR• V_IN•F_SW

(12B)

P_GATE= Q_GX V_DDXF_SW

(11A)

where Q_RRis the body diode’s reverse recovery

charge. If the MOSFET contains an integrated body

diode (like SyncFET™), the Q_RRfigure in the

datasheet is actually Q_OSS, or the charge required by

the MOSFET’s C_OSS. If a SyncFET™ is used, then

set Q_RRto 0 in the companion spreadsheet.

Note that P_GATEis the power from the VDD supply

required to drive a MOSFET gate. It is independent

of the driver's output resistance and includes both the

rising and falling edges.

_GATEis distributed between R_DRIVER, R_GATE, and

R_DAMPINGpropoprtional to their resistances.

Dissipation in the driver for the rising edge is:

Low-Side Losses

Low-side losses (P_LS) are also comprised of

conduction losses and switching losses.

•R

DRIVER(PULL-UP)

GATE

(11B)

(11C)

DR(L-H)

2(R

)

TOTAL

P_LS= P_SW+ P_COND

(13)

where

Switching losses are negligible, since Q2 switches on

and off with only a diode drop across it, however for

completeness we will include the analysis.

R_TOTAL= R_DRIVER+ R_GATE+ R_DAMPING

Similarly, dissipation in the driver for the falling edge

is:

Conduction losses for Q2 are given by:

•R

DRIVER(PULL-DOWN)

P_COND

(

1− D

)

XI_OUTXR_DS(ON)

(14)

GATE

(11D)

DR(H-L)

2(R

)

TOTAL

where R_DS(ON)is the R_DS(ON)of the MOSFET at the

anticipated operating junction temperature and

V_OUT

For an output stage (Driver + MOSFET) with the

following parameters:

D =

is the duty cycle for the converter.

V_IN

P_GATE

500 mW

The junction temperature (T_J) of the MOSFET can be

calculated if the junction to ambient thermal

resistance (θ_JA) and maximum ambient temperature

are known.

R_{DRIVER (PULL-UP)}

R_{DRIVER (PULL-DOWN)}

R_DAMPING

Ω

R_GATE

1.5

1.0.1 01/04/2006

Synchronous Buck Loss Calculation

AN-6005

the R_DS(ON)is typically 110% of the specified R_DS(ON)

T_J= T_A+ (P_LS•θ_JA

)

(15)

P_CONDdominates P_LS. Since R_DS(ON)determines

_COND, and is a function of T_J, either an iterative

The rising edge transition times for the low-side (t2

and t3 in Figure 4) can now be calculated from the

RC equations.

calculation can be used, or T_Jcan be assumed to be

some maximum number determined by the design

goals. The calculations in the accompanying

spreadsheet use T_Aand θ_JAiteratively to determine

the low-side operating T_Jat full current, assuming a

MOSFET R_DS(ON)temperature coefficient of

0.4%/°C, which is typical for the MOSFETs used in

this application.

t2_R= K_2R(R_DRIVER+ R_GATE)C_ISS

(17A)

where

⎛

⎞

⎟

⎠

⎛

⎞

⎟

⎠

DRIVE

= ln

− ln

(17B)

(17C)

⎜

⎝

⎜

− V

⎝

DRIVE

DRIVE TH

t3_R= K_3R(R_DRIVER+ R_GATE)C_ISS

Low-side Switching Losses

where

ISS

⎛

⎞

⎟

⎠

⎛

⎞

DRIVE

− 0.9V

= ln

− ln

⎜

⎝

⎜

⎝

⎟

− V

⎠

DRIVE

SPEC

DRIVE SP

(17D)

– I_OUT

x R_DS(ON)

I_D

and where C_ISSis the MOSFET’s input capacitance

(C_GS+ C_GD) when V_DSis near 0V. If the MOSFET

datasheet has no graph of capacitance vs. V_DS, use

1.25 times the typical C_ISSvalue, which is usually

given with ½ of the rated V_DSacross the MOSFET.

V_DS

– 0.6

– I_OUT

The turn-off losses are the same, but in reverse, so

the switching waveforms are:

V_DRIVER

ISS

0.9 x V_SPEC

V_SP

V_GS

– I x R_DS(ON)

V_TH

I_D

V_DS

– 0.6

– I

Figure 4. Low-Side turn-on switching loss

waveforms

V_DRIVER

Low-side switching losses for each edge can be

calculated in a similar fashion to high-side switching

losses:

0.9 x V_SPEC

V_SP

V_GS

P_SW(LS)

≈

V_TH

V_F+ I_OUT•1.1•R_DS(ON)

⎛

⎜

⎝

⎞

⎟

⎠

t2• V + t3•

I_OUT•F_sw

(16)

but instead of V_INas in eq. 3, we use V_F, the schottky

diode drop (approximated as 0.6V) in the equation.

Also, there is almost no Miller effect for the low-side

MOSFET, since V_DSis increasing (becoming less

negative) as we turn the device on, the gate driver is

not having to supply charge to C_GD. The voltage

collapse for Q2 is caused by the R_DS(ON)going from

Figure 5. Low-Side turn-off switching loss

waveforms

The falling edge transition times for the low-side

(t3 and t2 in Figure 5) can now be calculated from the

RC equations:

t3_F= K_3F(R_DRIVER+ R_GATE)C_ISS

(18A)

0.6

@ V_GS= V_SP, to 90% of V_SPEC, the gate voltage

where

I_OUT

for the highest specified R_DS(ON). At 90% of V_SPEC

1.0.1 01/04/2006

AN-6005

Synchronous Buck Loss Calculation

⎛

⎞

⎟

⎠

0.9V_SPEC

V_SP

≈

(20)

K3 = ln

(18B)

(18C)

⎜

2 •I

⎝

LDRV

This approximation holds, since prior to reaching

threshold, the gate voltage is low enough that I_LDRV

can be approximated with a constant current of

t2_F= K_2F(R_DRIVER+ R_GATE)C_ISS

where

⎛

⎞

⎟

⎠

V_SP

K2 = ln

(18D)

⎜

⎛

⎞

⎝

V_DRIVER− ⎜

⎟

⎜

⎝

⎠

I_LDRV

≈

(21)

R_GATE+ R_DRIVER

Dead-Time (Diode Conduction) Losses

The dead-time is the amount of time that both

Q_GS

and typically Q_G(TH)

≈

MOSFETs are off. During this time the diode (body

diode or parallel schottky diode) is in forward

conduction. It's power loss is:

The diode's total on-time on the falling edge is then:

(

)

+ R

= t

•F • V •I

OUT

(19)

GS GATE

DRIVER

DIODE

DEADTIME

≈ t

DELAY(F)

DEADTIME(F)

⎛

⎞

⎠

where t_DEADTIME= t_DEADTIME(R)+ t_DEADTIME(F), which

are the deadtimes associated before the SW Node

(Figure 1) rises, after Q2 turns off, and after SW

Node falls, before Q2 turns on, respectively.

−

⎜

⎟

DRIVER

⎝

(22)

On the rising edge, t_DELAY(R)is usually much longer to

allow the low-side MOSFET’s gate to discharge

completely. This is necessary since charge is coupled

into the low-side gate during the rising edge of the

SW node. The peak of the resultant voltage “spike”

at the low-side gate is the sum of the amplitude of the

injected spike and the voltage the gate has discharged

to when the SW node begins to rise. Sufficient delay

is necessary to avoid having the resultant peak rise

significantly above the low-side’s V_TH, turning on

both MOSFETs, and inducing “shoot-through”

losses.

To determine t_DEADTIME, we need to consider how the

driver controls the MOSFET gate drives. Most

drivers use "adaptive dead-time circuits, which wait

for the voltage of the opposite MOSFET to reach an

"off" voltage before beginning to charge its own

MOSFET. Most drivers add a fixed delay to prevent

shoot-through, especially on the low to high

transition.

HDRV

H.S. MOSFET

C_GD

40nS

LDRV

R_GATE

C_GS

SW NODE VOLTAGE

R_Damping

R_DRIVER

LS MOSFET GATE

Figure 6. Typical Adaptive Gate drive (low-high

transition)

-2

For the t_DEADTIME(F)the diode will be conducting the

full load current from the time the switch node falls,

until the Low-side MOSFET reaches threshold. This

"deadtime" consists of 2 portions:

t (nS)

Figure 7. Coupled voltage spike on Low-side

MOSFET gate from SW node rising edge

1. t_DELAY(F): The driver’s built in delay time from

detection of 1V_GSat the high-side MOSFET

gate until beginning of low-side MOSFET turn-

on, plus

The other component of deadtime on the rising edge

is the time it takes for the high-side MOSFET’s gate

to charge to V_SP. This is typically less than 10% of

t_DELAY(R)so we will ignore it and set:

t_TH: the time for the driver to charge the low-

side MOSFET's gate to reach threshold (V_TH).

t_DEADTIME(R)≈ t_DELAY(R)

(23)

t_THcan be approximated by:

1.0.1 01/04/2006

Synchronous Buck Loss Calculation

AN-6005

Summary of results

A spreadsheet which contains MOSFET parameters

is used to compute the losses for our example circuit

(Table 1) using a 5V gate drive with 6Ω pull-up and

2Ω pull-down strength.

High-Side Low-Side

MOSFET

FDD6644 FDB6676 Total

Switching Loss

Conduction Loss

Other Losses

Total Losses

Output Power

Efficiency

1.09

0.21

0.31

1.15

1.40

1.36

0.26

3.02

22.5

88%

1.30

1.46

Table 2. Results for 15A example (Table 1)

It’s instructive to review the results in order to

observe a few key points:

•

Switching losses for the low-side MOSFET are

only 15% of low-side MOSFET’s total losses.

•

Unless the switching frequency is very high

(above 1 Mhz.), the loss contribution due to

diode conduction (deadtime loss) is minimal.

•

High-side losses are dominated by switching

losses since the duty cycle is low.

1.0.1 01/04/2006

AN-6005

Synchronous Buck Loss Calculation

Appendix: Using the efficiency and loss calculation spreadsheet tool

The spreadsheet is contained in the following file:

http://www.fairchildsemi.com/collateral/AN-6005.zip

The spreadsheet implements the loss calculations described in this app note. To see the sheet in action press the

"Run" button on the "EfficiencySummary" sheet.

The controller/driver database models several Fairchild products driver products. A listing of these is found in the

ControllerDriver tab.

These detailed instructions can also be found in the " General Instructions" tab of the spreadsheet:

Be sure to turn off Macro Protection in Excel to allow the custom functions and macros in this sheet to run.

For Excel 2000, this is done through "Tools|Macro|Security". Set the level to "low".

In Excel 97, you can do this through "Tools|Options. In the "General" tab, the "Macro Virus Protection" box

should not be checked.

Notes:

RDS(ON) is a function of die temperature, and the die temperature is a function of Power Dissipation which is in

turn dependent on RDS(ON). To solve for dissipation or die temperature, the conduction loss calculations in

this spreadsheet use an iterative calculation method to arrive at the die temperature and dissipation.

DieTemp function

Tabs

Function / Description

Provides guidance on what MOSFET parametric data to enter in the "MOSFETDatabase" tab. This sheet is a

hotlink destination from some of the column headings in the MOSFETDatabase table.

Definitions

Plots efficiency data from the table in EfficiencySummary tab.

Plots power loss data from the table in EfficiencySummary tab.

EfficiencyChart

LossChart

Database for the IC Controllers. Fairchild's portable PWM controllers are featured in this database. Any

controller can be added by using the "Add" button and filling in the appropriate fields.

Database for the MOSFETs. Many popular Fairchild MOSFETs are featured in this database. Any MOSFET

can be added by using the "Add" button and filling in the appropriate fields.

ControllerDriver

MOSFETDatabase

EfficiencySummary

The main sheet where the system requirements and MOSFET choices can be entered, and the data is stored

for graphing. To run the graphing routine, push the "RUN" button at the top of the sheet.

The calculations contained in the "Synchronous buck MOSFET loss calculations" app note are programmed

into this sheet. The EfficiencySummary macro uses this sheet as its calculator. If a particular operating point

needs to be examined in more detail, then use this sheet, and enter the parameters by hand. Be sure to save a

copy of this workbook before overwriting formulas in "Output" tab.

Output

Cells are color coded as follows:

Indicates user input parameters

Indicates calculated values that can be overwritten with selected values if desired.

These fields default to the calculated value directly above them.

These fields are written into, or contain formulae that were input on the "EfficiencySummary" sheet.

Macro Security Note:

"AN-6005 Switching Loss Calculation.xls" uses macros extensively. For the spreadsheet to operate properly, check

the “Always trust macros from this source” box if a security warning appears, then click the “Enable Macros”

button..

This is only required the first time you run a Fairchild spreadsheet tool with macros.

1.0.1 01/04/2006

AN-6005 [FAIRCHILD]

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