RC5041
PRODUCT SPECIFICATION
• Inductor coil losses
• Sense resistor losses
• Gate-charge losses
• Diode-conduction losses
• Transition losses
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 importance, are:
• Input capacitor losses
• MOSFET I2R Losses
• Losses due to the operating supply current of the IC.
Efficiency of the converter under heavy loads can be calculated as follows:
POUT OUT × VOUT
I
Efficiency = ------------- = ------------------------------------------------------- ,
pIN OUT × VOUT + PLOSS
I
where PLOSS = PDMOSFET + PDINDUCTOR + PDRSENSE + PDGATE + PDDIODE + PDTRAN + PDCAP + PDIC
Design Equations:
(1) PDMOSFET = IOUT2 × (RDS(ON) × 1.5) × DutyCycle where 1.5 is the temperature multiplier
V
OUT + VD
where DutyCycle = -----------------------------------------
IN + VD – VSW
V
(2) PDINDUCTOR = IOUT2 × RINDUCTOR
(3) PDRSENSE = IOUT2 × RSENSE
(4) PDGATE = qGATE × f × 5V , where qGATE is the gate charge and f is the switching frequency
(5) PDDIODE = Vf × IOUT(1 – Dutycycle)
VIN2 × CRSS × ILOAD × f
(6) PDTRAN = ------------------------------------------------------------- , where C
is the reverse transfer capacitance of the MOSFET.
RSS
IDRIVE
(7) PDCAP = IRMS2 × ESR
(8) PDIC = VCC × ICC
Example:
PDMOSFET = 102 × (0.010 × 1.5) × 0.70 = 1.05W
PDRSENSE = 102 × 0.0065 = 0.65W
3.3 + 0.5
DutyCycle = ----------------------------- = 0.70
5 + 0.5 – 0.1
PDINDUCTOR = 102 × 0.010 = 1W
PDGATE = CV × f × 5V = 1.75nf × (9 – 1)V × 300Khz × 5V = 0.021W
PDDIODE = 0.5 × 10(1 – 0.70) = 1.5W
52 × 400pf × 10 × 300khz
----------------------------------------------------------------
0.074W
PDTRAN
=
0.7A
PDCAP = (7.5 – 2.5)2 × 0.015 = 0.37W
PDIC = 0.2W
PDLOSS = 1.05W + 1.0W + 0.65W + 0.021W + 1.50W + 0.074W + 0.37W + 0.2W = 4.865W
3.3 × 10
3.3 × 10 + 4.865
---------------------------------------
Efficiency =
87%
10
RAYTHEON [ RAYTHEON COMPANY ]
