NCP1034(2008) 데이터 시트보기 (PDF) - ON Semiconductor

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NCP1034
(Rev.:2008)

100V Synchronous PWM Buck Controller

ON Semiconductor 

NCP1034

ǒ Ǔ IOUT

@

VOUT

VIN

@

1 * VOUT

VIN

CIN w

f @ DVIN

(eq. 6)

Where DVIN is the input voltage ripple and the

recommended value is about 2% − 5% of VIN. The input

capacitor must be large enough to handle the input ripple

current. Its value should be calculated using Equation 7:

Ǹ ǒ Ǔ VOUT @

1

*

VOUT

VIN

IRMS + IOUT @

VIN

(eq. 7)

Power MOSFET Selection

The NCP1034 uses two N−channel MOSFET’s. They can

be primary selected by RDS(on), maximum drain−to−source

voltage and gate charge. RDS(on) impacts conductive losses

and gate charge impacts switching losses. The low side

MOSFET is selected primarily for conduction losses, and

the high−side MOSFET is selected to reduce switching

losses especially when the output voltage is less than 30% of

the input voltages. The drain−to−source breakdown voltage

must be higher than the maximum input voltage. Conductive

power losses can be calculated using the Equations 8 and 9:

PCOND*HIGHFET

+

I2OUT

@

RDS(on)

@

VOUT

VIN

(eq. 8)

ǒ Ǔ PCOND*LOWFET + I2OUT @ RDS(on) @

1 * VOUT

VIN

(eq. 9)

Switching losses are depended on drain−to−source

voltage at turn−off state, output current and switch−on and

switch−off time as is shown by Equation 10.

PSW

+

VDS(off)

2

@

ǒtON

)

tOFFǓ

@

f

@

IOUT

(eq. 10)

tON and tOFF times are dependent on the transistor gate.

The MOSFET output capacitance loss is caused by the

charging and discharging during the switching process and

can be computed using Equation 11.

PCOSS

+

COSS

@ VIN

2

2

@

f

(eq. 11)

Where COSS = CDS + CGS.

Significant power dissipation is caused by the reverse

recovery charge in the low−side MOSFET body diode,

which conducts at dead time. This charge is needed to close

the diode. The current from the input power supply flows

through the high−side MOSFET to the low−side MOSFET

body diode. This power dissipation can be calculated using

Equation 12.

PQRR + QRR @ VIN @ f

(eq. 12)

QRR is the diode recovery charge as given in the

manufacturer’s datasheet. For some types of MOSFETs, this

dissipation may be dominant at high input voltages. It is

necessary to take care when selecting a MOSFET. An

external Schottky diode across the low−side MOSFET can

be used to eliminate the reverse recovery charge power loss.

The Schottky diode’s forward voltage should be lower than

that of the body diode, and reverse recovery time (trr) should

be lower then that of the body diode. The Schottky diode’s

capacitance loss can be calculated as shown in Equation .

CSchottky @ VIN 2 @ f

PC(Schottky) +

2

(eq. 13)

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13

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