# Converter efficiency – Fairchild SEMICONDUCTOR RC5051 User Manual

## Page 9 APPLICATION NOTE

AN50

9

Converter Efficiency

Losses due to parasitic resistance in the switches, coil, and
sense resistor dominate at high load-current level. The major
loss mechanisms under heavy loads, in usual order of impor-
tance, are:

• MOSFET I

2

R Losses

• Coil Losses
• Sense Resistor Losses

• gate-charge losses
• diode-conduction losses
• transition losses
• Input Capacitor losses
• losses due to the operating supply current of the IC.

Calculation of Converter Efficiency Under Heavy Loads

where

, where

, where q

GATE

is the gate charge and f is the switching frequency

, where C

is the reverse transfer capacitance of the high-side MOSFET.

Example

Efficiency

P

OUT

p

IN

-------------

I

OUT

V

OUT

×

I

OUT

V

OUT

P

LOSS

+

×

--------------------------------------------------------

=

=

P

LOSS

PD

MOSFET

PD

COIL

PD

SENSER

PD

GATE

PD

DIODE

PD

TRAN

PD

CAP

PD

IC

+

+

+

+

+

+

+

=

PD

MOSFET

I

OUT

2

R

DS,ON

×

DutyCycle

×

=

DutyCycle

V

OUT

V

D

+

V

IN

V

D

V

SW

+

------------------------------------------

=

PD

COIL

I

OUT

2

R

COIL

×

=

PD

SENSER

I

OUT

2

R

SENSE

×

=

PD

GATE

q

GATE

f

5V

Ч

Ч

=

PD

DIODE

V

f

I

D

×

1

Dutycycle

(

)

=

PD

TRAN

V

IN

2

C

×

I

×

f

×

I

DRIVE

-------------------------------------------------------------

=

PD

CAP

I

RMS

2

ESR

×

=

PD

IC

V

CC

I

CC

×

=

DutyCycle

3.3

0.5

+

5

0.5

0.3

+

------------------------------

0.73

=

=

PD

MOSFET

10

2

0.030

Ч

0.73

Ч

2.19W

=

=

PD

COIL

10

2

0.010

×

1W

=

=

PD

SENSER

10

2

0.0065

×

0.65W

=

=

PD

GATE

CV

f

×

5V

×

1.75nf

9

1

(

)

V

285Khz

5V

Ч

Ч

Ч

0.019W

=

=

=

PD

DIODE

0.5

10 1

0.73

(

)

×

1.35W

=

=

PD

TRAN

5

2

400pf

Ч

10

Ч

285khz

×

0.7A

----------------------------------------------------------------

0.010W

=

PD

CAP

7.5

2.5

(

)

2

0.015

×

0.37W

=

=

PD

IC

0.2W

=

PD

LOSS

2.19W

1.0W

0.65W

0.019W

1.35W

0.010W

0.37W

0.2W

+

+

+

+

+

+

+

5.789W

=

=

Efficiency

3.3

10

Ч

3.3

10

5.815

+

Ч

---------------------------------------

85%

=