CS5132 datasheet(13/19 Pages) CHERRY

13 / 19 page

input RMS current IIN(RMS). CIN discharges during the on-

time.

The discharge current is given by:

ICINDISRMS =

= 10.2A.

CIN charges during the off-time, the average current

through the capacitor over one switching cycle is zero:

ICIN(CH) = ICIN(DIS) ´

ICIN(CH) = 10.2A ´

= 6.8A.

So the total Input RMS current is:

ICIN(RMS) = (ICIN(DIS)2 ´ D) +(ICIN(CH)2 ´ (1-D)),

ICIN(RMS) =

The number of input capacitors required is given by:

NCIN =.

For Sanyo capacitors type GX:

1200µF/10V, IRIPPLE = 1.25A.

Hence,

NCIN =

= 6.6.

The number of input capacitors can be rounded off to 6.

Calculate the Input Capacitor Ripple Voltage:

VRMS = IRMS ´ Total ESR = 8.3A ´ 7.3m½ = 60mV.

Calculate the Input Capacitor Power Loss:

PCIN = IRMS2 ´ Total ESR = 0.504W.

Step 5b: VI/O Buck Regulator Input Capacitors

Repeating for the 3.3V output, we select 3 GX 1200µF/10V

capacitors.

Step 6: Power MOSFETs

FET Basics

The use of the MOSFET as a power switch is propelled by

two reasons: 1) Its very high input impedance and 2) Its

very fast switching times. The electrical characteristics of a

MOSFET are considered to be those of a perfect switch.

Control and drive circuitry power is therefore reduced.

Because the input impedance is so high, it is voltage driv-

en. The input of the MOSFET acts as if it were a small

capacitor, which the driving circuit must charge at turn on.

The lower the drive impedance, the higher the rate of rise

of VGS, and the faster the turn- on time. Power dissipation

in the switching MOSFET consists of 1) conduction losses,

2) leakage losses, 3) turn-on switching losses, 4) turn-off

switching losses, and 5) gate-transitions losses. The latter

three losses are proportional to frequency. For the conduct-

ing power dissipation rms values of current and resistance

are used for true power calculations.

The fast switching speed of the MOSFET makes it indis-

pensable for high-frequency power supply applications.

Not only are switching power losses minimized, but the

maximum usable switching frequency is considerably

higher. Switching time is independent of temperature.

Also, at higher frequencies, the use of smaller and lighter

components (transformer, filter choke, filter capacitor)

reduces overall component cost while using less space for

more efficient packaging at lower weight.

The MOSFET has purely capacitive input impedance. No

DC current is required. It is important to keep in mind the

drain current of the FET has a negative temperature coeffi-

cient. Increase in temperature causes higher on-resistance

and greater leakage current.

For switching circuits, VDS(ON) should be low to minimize

power dissipation at a given ID, and VGS should be high to

accomplish this. MOSFET switching times are determined

by device capacitances, stray capacitances, and the

impedance of the gate drive circuit. Thus the gate driving

circuit must have high momentary peak current sourcing

and sinking capability for switching the MOSFET. The

input capacitance, output capacitance and reverse-transfer

capacitance also increase with increased device current

rating.

Two considerations complicate the task of estimating

switching times. First, since the magnitude of the input

capacitance, CISS, varies with VDS, the RC time constant

determined by the gate-drive impedance and CISS changes

during the switching cycle. Consequently, computation of

the rise time of the gate voltage by using a specific gate-

drive impedance and input capacitance yields only a rough

estimate. The second consideration is the effect of the

"Miller" capacitance, CRSS, which is referred to as Cdg in the

following discussion. For example, when a device is on,

VDS is fairly small and VGS is about 12V. Cdg is charged to

VDS(ON) - VGS, which is a negative potential if the drain is

considered the positive electrode. When the drain is "off",

Cdg is charged to quite a different potential. In this case the

voltage across Cdg is a positive value since the potential

from gate-to-source is near zero volts and VDS is essentially

the drain supply voltage. During turn-on and turn-off,

8.3

1.25

ICIN(RMS)

IRIPPLE

(10.22

´ 0.4) + (6.82 (´ 0.6)) = 8.3A

0.4

(1-0.4)

1-D

(IL(PEAK)2 + (IL(PEAK) ´ IL(VALLEY)) + IL(VALLEY2) ´ D

Application Information: continued

CS5132 датащи(PDF) 13 Page - Cherry Semiconductor Corporation

CS5132 датащи(HTML) 13 Page - Cherry Semiconductor Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

датащи скачать

ссылки URL