AN-6003 datasheet(3/6 Pages) FAIRCHILD

Part #	AN-6003

Description	Shoot-through in Synchronous Buck Converters

Download	6 Pages

Scroll/Zoom	100%

Manufacturer	FAIRCHILD [Fairchild Semiconductor]

Direct Link	http://www.fairchildsemi.com

Logo

3 / 6 page

Shoot-through in Synchronous Buck Regulators

AN-6003

04/25/2003

3

When the adaptive gate circuit switches, the internal

MOSFET gate voltage will be:

()

V

1

.

4

5

2

.

1

2

V

1

=

Ω

+

•

Ω

In this example, if there were no delay in the circuit,

the HDRV would turn on when the low-side

MOSFET has just begun to discharge, causing a very

high shoot-through current.

Much of the problem in the above circuit is the

damping resistor. If a damping resistance is

necessary, place a Schottky diode across the resistor

(as shown below) to reduce the effect the damping

resistor will have on the adaptive gate drive.

R

DRIVER

R

Damping

H.S. MOSFET

1V

HDRV

LDRV

Delay

H.S. MOSFET

R

GATE

C

GS

C

GD

D

S

G

Q2

Figure 6. Schottky diode reduces damping

resistor error in adaptive gate drive

When using the schottky, the internal gate node will

be at:

()

GATE

DRIVER

)

I

(

GS

R

V

1

5

.

0

V

+

•

+

=

or 2.1V for our example. A dramatic improvement.

Furthermore, the Schottky reduces the duration of the

shoot-through step, since only R

GATE + RDRIVER will be

discharging C

GS, rather than the sum of

R

GATE + RDAMPING + RDRIVER .

Table 1 below illustrates the performance

improvement in our example with and without the

Schottky diode:

No

Schottky

With

Schottky

Comparator Flips @ VGS(INT) =

4.1

2.1

V

VGS(INT) after 20nS delay

2.23

1.14

V

VSTEP Peak

2.50

1.25

V

Peak current

36

0.29

A

Power Loss @ FSW=300KHz

1100

20

mW

Conditions: Typical low-side MOSFET, 25nS

delay from comparator sense to beginning of SW

node rise, 19V

IN, 10nS SW node rise time.

Table 1 . Peak Currents with and without

Schottky with R

DAMPING = 5Ω .

MOSFET Choices

MOSFET characteristics can have a dramatic effect

on how much shoot-through current can be induced

by the gate step. The worst case for shoot-through is

an infinitely fast (0 rise time) on the drain node. The

amount of gate step is largely determined by the

ration of C

GS and CGD .

Once the size of the gate step

is determined (eq. 1 above), the peak magnitude of

the shoot-through current can be calculated as :

(

)

MIN

(

TH

STEP(MAX)

M

)

MAX

(

PEAK

V

G

K

I

−

•

≈

(2)

where G

M is the transconductance (in S, or A/V)

given in the datasheet. While only a small

percentage of MOSFETs exhibit V

TH(MIN) at room

temperature, V

TH goes down with increasing junction

temperature, therefore V

TH(MIN) is a good proxy for the

V

TH at the operating junction temperature of the

MOSFET. Subsequent calculations use V

TH(MIN) for

this reason.

G

M is not really a contstant, however, and its value is

greatly reduced low enhancement voltages (V

GS-VTH).

In these calculations we use a factor "K" from the

graph below, which is typical of G

M with low values

of enhancement. The X axis of Figure 7 is calculated

as

)

MIN

(

TH

)

MIN

(

TH

GS

V

−

0.0

0.2

0.4

0.6

0.8

1.0

0%

50%

100%

150%

200%

250%

300%

Normalized Enhancement Voltage

Figure 7 GM factor (K)

Table 2 shows the relevant MOSFET characteristics

which determine the maximum shoot-through

current.

MOSFET

CGS

CGD

Typical

VTH

Min

VTH

GM

MOSFET1

3,514

307

1.6

1

86

MOSFET2

5,070

230

1.2

0.8

97

MOSFET3

4,942

315

1.6

1

80

MOSFET4

3,888

401

1.6

1

135

MOSFET5

6,324

281

1.15

0.6

90

Table 2 . Low-Side MOSFET Characteristics

Similar Part No. - AN-6003

Manufacturer	Part #	Datasheet	Description
Fairchild Semiconductor	AN-6006	70Kb / 4P	FAN5068 Component calculation and simulation tools

More results

Similar Description - AN-6003

Manufacturer	Part #	Datasheet	Description
Texas Instruments	TPS544B20	2Mb / 74P	[Old version datasheet] Synchronous Buck Converters
Intersil Corporation	ISL80020	708Kb / 16P	Compact Synchronous Buck Converters June 5, 2015
Renesas Technology Corp	ISL80020	1,013Kb / 16P	Compact Synchronous Buck Converters
International Rectifier	IRFHM830DPBF	211Kb / 8P	Synchronous MOSFET for Buck Converters
M.S. Kennedy Corporatio...	MSK4411	775Kb / 8P	Shoot-through, Cross Conduction Protection
International Rectifier	IRFH4209DPBF	491Kb / 9P	Synchronous Rectifier MOSFET for Synchronous Buck Converters
	IRFH4210PBF	500Kb / 9P	Synchronous Rectifier MOSFET for Synchronous Buck Converters
	IRFH5301PBF	242Kb / 9P	Synchronous MOSFET for Buck Converters
	IRFH5302PBF	246Kb / 9P	Synchronous MOSFET for buck converters
	IRFH4210DPBF	256Kb / 8P	Synchronous Rectifier MOSFET for Synchronous Buck Converters

More results

AN-6003 Datasheet(PDF) 3 Page - Fairchild Semiconductor

AN-6003 Datasheet(HTML) 3 Page - Fairchild Semiconductor

Similar Part No. - AN-6003

Similar Description - AN-6003

Html Pages

1 2 3 4 5 6

Datasheet Download

Link URL