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 2004 Microchip Technology Inc. DS00799B-page 1

AN799

INTRODUCTION

There are many MOSFET technologies and silicon

processes in existence today, with new advances being

made every day. To make a generalized statement

about matching a MOSFET driver to a MOSFET based

on voltage/current ratings or die sizes is very difficult, if

not impossible.

As with any design decision, there are multiple vari-

ables involved when selecting the proper MOSFET

driver for the MOSFET being used in your design.

Parameters such as input-to-output propagation delay,

quiescent current, latch-up immunity and driver current

rating must all be taken into account. Power dissipation

of the driver will also effect your packaging decision

and driver selection.

This Application Note discusses the details of MOSFET

driver power dissipation in relation to MOSFET gate

charge and operating frequency. It also discusses how

to match MOSFET driver current drive capability and

MOSFET gate charge based on desired turn-on and

turn-off times of the MOSFET.

Microchip offers many variations of MOSFET drivers in

various packages, which allows the designer to select

the optimal MOSFET driver for the MOSFET(s) being

used in their application.

POWER DISSIPATION IN A MOSFET

DRIVER

Charging and discharging the gate of a MOSFET

requires the same amount of energy, regardless of how

fast or slow (rise and fall of gate voltage) it occurs.

Therefore, the current drive capability of the MOSFET

driver does not effect the power dissipation in the driver

due to the capacitive load of the MOSFET gate.

There are three elements of power dissipation in a

MOSFET driver:

1. Power dissipation due to the charging and

discharging of the gate capacitance of the

MOSFET.

EQUATION 1:

2. Power dissipation due to quiescent current draw

of the MOSFET driver.

EQUATION 2:

3. Power dissipation due to cross-conduction

(shoot-through) current in the MOSFET driver.

EQUATION 3:

As deduced from the equations above, only one of the

three elements of power dissipation is due to the

charging and discharging of the MOSFET gate

capacitance. This portion of the power dissipation is

typically the highest, especially at lower switching

frequencies.

In order to calculate a value for Equation 1, the gate

capacitance of the MOSFET is required. The gate

capacitance of a MOSFET is comprised of two capaci-

tances: the gate-to-source capacitance and the gate-

to-drain capacitance (Miller Capacitance). A common

mistake is to use the Input Capacitance rating of the

MOSFET (CISS) as the total gate capacitance of the

MOSFET. The proper method for determining gate

capacitance is to look at the Total Gate Charge (QG) in

the MOSFET data sheet. This information is typically

shown in the Electrical Characteristics table and as a

typical characteristics curve in any MOSFET data

sheet.

Author: Jamie Dunn

Microchip Technology Inc.

PCCGVDD

2F××=

Where:

CG= MOSFET Gate Capacitance

VDD = Supply Voltage of MOSFET Driver (V)

F = Switching Frequency

PQIQH D IQL 1 D–( )×+×( ) VDD

×=

Where:

IQH = Quiescent current of the driver with

the input in the high state

D = Duty cycle of the switching waveform

IQL = Quiescent current of the driver with

the input in the low state

PSCC F VDD

××=

Where:

CC = Crossover constant (A*sec)

Matching MOSFET Drivers to MOSFETs

AN799

DS00799B-page 2  2004 Microchip Technology Inc.

Table 1 shows a typical example of the data sheet

representation of gate charge for a 500V, 14A, N-chan-

nel MOSFET. Note that the values given in the data

sheet table have conditions associated with them: gate

voltage and drain voltage. These conditions effect the

gate charge value. Figure 1 shows the gate charge typ-

ical characteristic curve for the same MOSFET as it

varies with gate voltage and drain voltage. Make sure

the gate charge value you use for calculating power

dissipation fits the conditions of your application.

Taking a typical value from the graph in Figure 1 for

VGS = 10V, we get a total gate charge of 98 nC

(VDS = 400V). Using the relationship Q = C * V, we get

a gate capacitance value of 9.8 nF, which is signifi-

cantly higher than the 2.6 nF input capacitance that is

specified in Table 1. This illustrates the fact that when

a calculation calls for a gate capacitance value, the

total gate capacitance value should be derived from the

total gate charge value.

FIGURE 1: Total Gate Charge vs. Gate-

to-Source Voltage (500V, 14A, N-channel

MOSFET).

When using maximum values for gate charge from the

Electrical Characteristics table for worst-case design,

the values must be adjusted for the drain-to-source and

gate-to-source voltages in your design.

Using the MOSFET information presented in Table 1

and Figure 1 as an example, the power dissipation in a

MOSFET driver due to the charging and discharging of

the gate capacitance of this MOSFET with a V

GS of

12V, a switching frequency of F = 250 kHz and a drain-

to-source voltage of 400V would be:

The value for CG is arrived at by using the graph in

Figure 1 and finding the value for QG at 12V. QG is then

divided by 12V to get the C

G value. Knowing that QG is

equal to CG * VG

, the equation for PC could be rewritten

as:

A note of importance is that the voltage in this equation

is squared. Therefore, a reduction in the gate drive

voltage can result in a significant reduction in power

loss in the driver. For some MOSFETs, driving the gate

voltage above 8V to 10V does not result in any further

decrease in MOSFET resistance (RDS-ON). Using the

same MOSFET as above as an example, a 10V gate

drive results in the following power dissipation:

The 16% reduction in gate voltage (going from 12V to

10V) resulted in a 28% reduction in power dissipation

due to gate drive. Further savings will also be seen in

the cross-conduction losses due to gate drive voltage

reduction.

Equation 3 represents the power dissipation due to

MOSFET driver cross-conduction, or what is

commonly referred to as shoot-through. This is a result

of the P-channel and N-channel FETs in the output

drive stage being on at the same time as they

transition between the on and off states.

TABLE 1: DATA SHEET REPRESENTATION FOR GATE CHARGE

Name Parameter Min. Typ. Max. Units Test Conditions

QGTotal Gate Charge — — 150

ID = 14A

VDS = 400V

VGS = 10V

QGS Gate-to-Source Charge — — 20

QGD Gate-to-Drain Charge — — 80

CISS Input Capacitance — 2600 —

VGS = 0V

VDS = 25V

f = 1.0 MHz

COSS Output Capacitance — 720 —

CRSS Reverse Transfer Capacitance — 340 —

100

110

120

130

140

QG, Total Gate Charge (nC)

VGS , Gate-to-Source Voltage (V)

VDS = 400V

VDS = 250V

VDS = 100V

PCCGV2F××=

PC9.5 10 9–

×12( )2250 103

×××=

PC342mW=

PCQGV×F×=

PC98 10 9–

×10×250 103

××=

PC245mW=

 2004 Microchip Technology Inc. DS00799B-page 3

AN799

Cross-conduction characteristics are shown in the

MOSFET driver data sheet as a typical characteristic

curve and as “Crossover Energy vs. Supply Voltage”.

An example of this is shown in Figure 2.

FIGURE 2: Crossover Energy vs.

Supply Voltage.

The units for the crossover constant are typically shown

as Amp-Seconds (A*sec). Multiplying this number by

the frequency of operation yields a value for average

current. Figure 2 illustrates a point that was discussed

earlier. Namely, as bias voltage increases, the cross-

over constant increases and, consequently, the power

dissipation in the driver (due to cross-conduction)

increases. Therefore, a decrease in driver voltage will

result in a decrease in driver power dissipation.

One thing to make note of is that when using a dual

driver, the crossover constant is usually shown for both

portions of the driver operating. If only one portion of

the driver is being used, or the two portions of the driver

are operating at different frequencies, be sure to use

only half the value for each portion of the driver.

Using the information illustrated in Figure 2 as an

example, we will assume it is for a single-output driver

operating with a VDD of 12V, at a frequency of 250 kHz.

Based on the graph, the crossover constant is found to

be 5.2*10-9.

For this driver, operating at this voltage and frequency,

the power dissipation is relatively insignificant. Typically,

as the current drive capability of the MOSFET driver

increases, the losses due to shoot-through current will

also increase. These losses can be significant and need

to be taken into account when selecting a package for

the MOSFET driver.

Microchip offers surface-mount and pin through-hole

packages, ranging from 8-pin MSOPs to 8-pin DFNs to

5-pin TO-220s, allowing for the selection of the package

that is most appropriate for your application.

DIE SIZE EFFECT ON GATE

CAPACITANCE

As can be expected, the larger the die size of the

MOSFET, the larger the effective gate charge. For an

illustration of this, browse through any manufacturer's

data book. By relating die size to total gate charge, you

will find that, as die size increases, the total gate charge

will also increase. As advances are made in silicon

technology, new MOSFETs are produced that may

have the same die size as an older device, but with a

lower total gate charge. However, MOSFETs within the

same silicon technology still follow the same general

rule that as die size goes up, so does the gate charge

requirement.

Die sizes will often be referred to in Hex size. Table 2

below gives some typical die sizes and total gate

capacitance values for various MOSFET size Hex

ratings.

TABLE 2: HEX RATINGS AND TYPICAL

GATE CAPACITANCE FOR

MOSFET DIE SIZE

Many suppliers today have also come out with “low

gate charge” versions of MOSFETs that allow for faster

switching times and lower gate charge losses. These

devices allow applications to operate at higher speeds,

with lower switching losses in the power MOSFET, as

well as lower gate charge losses in the MOSFET driver.

A • sec

186 8 10 12 14 16

10-9

10-8

VDD

PSCC F×V×=

PS5.2 10 9–

×250 103

×× 12×=

PS15.6mW=

MOSFET

Size

Die Size

(mm)

Total C of MOSFET

(pF)

Hex 0 0.89 X 1.09 400

Hex 1 1.75 x 2.41 750

Hex 2 3.40 x 2.21 1500

Hex 3 4.44 x 2.79 3000

Hex 4 7.04 x 4.32 6000

Hex 5 6.45 x 6.45 12000

Hex 6 283 x 348 mil 15000

Hex 7 283 x 348 mil 16000

Parallel

Modules

Various Up to 48,000

Produktspezifikationen

Marke:	Microchip
Kategorie:	Nicht kategorisiert
Modell:	TC4432

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