AN-53 Datasheet, PDF (9/20 Pages) Fairchild Semiconductor – Implementing an RC5051 DC-DC Converter on Pentium II Motherboards

APPLICATION NOTE
AN53
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 typical order of
importance, are:
• MOSFET I ² R losses
• Transition losses
Formulae for Calculation of Converter Efficiency
• Sense Resistor losses
• Input Capacitor losses
• Coil Losses
• Losses due to the operating supply current of the IC.
• Diode-conduction losses
• Gate-charge losses
Efficiency = -P---^O----^U---^T-- = ------------V----^O----^U----^T---I--^O----^U----^T------------
p_IN
V_OUTI_OUT + P_LOSS
P_LOSS = P_RDS + (P_RISE + P_FALL) + P_INDUCTOR + P_SENSE + P_GATE + P_DIODE + P_CAPS + P_IC
P_RDS
=
I_O
U
2
T
R_DS,ON
D
C
for the high-side MOSFET
P_RDS
=
I_O
U
2
T
R_DS,ON
(
1
–
DC)
for
the
low-side
MOSFET
P_RISE + P_FALL
=
V-----_{I--N----I--O----U---T----t-R----I--S---E---F----S-}
2
+
V-----_{I--N----I--O----U---T----t-F---A----L---L----F---S-}
2
for
the
high-side
MOSFET
P_RISE + P_FALL
=
-V----^f--I--^O----^U----^T---t--^R---^I--^S---^E---F----^S-
2
+
-V----^f--I--^O----^U----^T---t--^F---^A----^L---^L---F----^S-
2
for
the
low-side
MOSFET
P_INDUCTOR = I_OUT2R_INDUCTOR
P_SENSE
=
I_O
U
2
T
R_SENS
E
P_GATE = CV²F_S for each MOSFET
P_DIODE = I_OUTV_FT_DTF_S , with T_DT the deadtime
P_CAPS = ESR ´ I_OUT2 ´ DC ´ (1 – DC)
P_IC = 25mA^*V_CC
Example Efficiency Calculation
As an example, the efficiency of a synchronous 14A converter based on the RC5051 will be calculated. The converter produces
2.8V output from a 5V input, switches at 300kHz, has MOSFETs with a 4nF gate capacitance, RDS,on = 10mW for both MOS-
FETS and a rise and fall time of 50nsec. The inductor has a winding resistance of 3mW, and the sense resistor used is 5.2mW. The
schottky paralleled with the synchronous rectifier has a forward voltage of 400mV at 14A. The input capacitors have a total ESR
of 15mW.
P_RDS = (14A)²10mW = 1.96W
P_RISE + P_FALL
=
-5---V------^´----1---4----A----⁽---5---0---n----s--e---c----+-----5---0---n---s---e--c---⁾---3---0---0---k---H-----z-
2
=
1.05W
P_RISE + P_FALL = -4--0---0----m-----V------^´----1---4----A----⁽---5---0---n----s-2-e---c----+-----5---0---n---s---e--c---⁾---3---0---0---k---H-----z- = 84mW
P_INDUCTOR = (14A)²3mW = 590mW
P_SENSE = (14A)²5.2mW = 1.02W
P_GATE = 4nF(5V)²300kHz ´ 2 = 60mW
P_DIODE = 14A ´ 400mV ´ 50nsec ´ 300kHz = 84mW
P_CAPS = 15mW ´ (14A)² ´ 0.56 ´ (1 – 0.56) = 724mW
P_IC = 25mA ´ 5V ´ 125mW
P_LOSS = 1.96W + 1.05W + 0.084W + 0.59W + 1.02W + 0.06W + 0.084W + 0.724W + 0.125W = 5.70W
Efficiency = _{2---.--8---V----2--´}-.-8---1-V--4---A-^´----1-+--4---5-A--.-7---0----W---- = 87%
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