APPLICATION NOTE

AN50

 

 

Converter Efficiency

Losses due to parasitic resistance in the switches, coil, and

gate-charge losses

sense resistor dominate at high load-current level. The major

diode-conduction losses

loss mechanisms under heavy loads, in usual order of impor-

transition losses

tance, are:

Input Capacitor losses

• losses due to the operating supply current of the IC.

MOSFET I2R Losses

Coil Losses

Sense Resistor Losses

Calculation of Converter Efficiency Under Heavy Loads

POUT

IOUT × VOUT

 

Efficiency = -------------

= ------------------------------------------------------

× VOUT + PLOSS-

 

pIN

IOUT

 

PLOSS = PDMOSFET + PDCOIL + PDSENSER + PDGATE + PDDIODE + PDTRAN + PDCAP + PDIC

where PDMOSFET

2

× RDS,ON × DutyCycle , where DutyCycle

VOUT + VD

= IOUT

= -----------------------------------------

 

 

 

VIN + VD – VSW

PDCOIL = IOUT2 × RCOIL

 

 

PDSENSER = IOUT2 × RSENSE

 

PDGATE = qGATE × f × 5V , where qGATE is the gate charge and f is the switching frequency

PDDIODE = Vf × ID(1 – Dutycycle)

 

VIN2 × CRSS × ILOAD × f

 

PDTRAN = -------------------------------------------------------------

IDRIVE

, where CRSS is the reverse transfer capacitance of the high-side MOSFET.

 

 

 

PDCAP = IRMS2 × ESR

 

 

PDIC = VCC × ICC

 

 

 

Example

 

 

 

3.3 + 0.5

 

 

DutyCycle = -----------------------------= 0.73

 

5 + 0.5 – 0.3

 

 

PDMOSFET = 10 2 × 0.030 × 0.73 = 2.19W

 

PDCOIL = 10 2 × 0.010 = 1W

 

PDSENSER = 102 × 0.0065 = 0.65W

PDGATE = CV × f × 5V = 1.75nf × (9 – 1)V × 285Khz × 5V = 0.019W

PDDIODE = 0.5 × 10(1 – 0.73)

= 1.35W

PDTRAN =

5 2

× 400pf × 10 × 285khz

∼ 0.010W

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

0.7A

 

PDCAP = (7.5 – 2.5)2 × 0.015 = 0.37W

PDIC = 0.2W

PDLOSS = 2.19W + 1.0W + 0.65W + 0.019W + 1.35W + 0.010W + 0.37W + 0.2W = 5.789W

3.3 × 10 Efficiency = --------------------------------------- 85%

3.3 × 10 + 5.815

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Fairchild RC5051, RC5050 specifications Calculation of Converter Efficiency Under Heavy Loads