APPLICATION NOTE | AN50 |
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Converter Efficiency
Losses due to parasitic resistance in the switches, coil, and | • | |
sense resistor dominate at high | • | |
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 |
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Efficiency = | = | × VOUT + PLOSS- |
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pIN | IOUT |
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PLOSS = PDMOSFET + PDCOIL + PDSENSER + PDGATE + PDDIODE + PDTRAN + PDCAP + PDIC | |||
where PDMOSFET | 2 | × RDS,ON × DutyCycle , where DutyCycle | VOUT + VD |
= IOUT | = | ||
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| VIN + VD – VSW |
PDCOIL = IOUT2 × RCOIL |
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PDSENSER = IOUT2 × RSENSE |
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PDGATE = qGATE × f × 5V , where qGATE is the gate charge and f is the switching frequency | |||
PDDIODE = Vf × ID(1 – Dutycycle) |
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VIN2 × CRSS × ILOAD × f |
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PDTRAN = | IDRIVE | , where CRSS is the reverse transfer capacitance of the | |
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PDCAP = IRMS2 × ESR |
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PDIC = VCC × ICC |
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Example |
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3.3 + 0.5 |
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DutyCycle = |
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5 + 0.5 – 0.3 |
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PDMOSFET = 10 2 × 0.030 × 0.73 = 2.19W |
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PDCOIL = 10 2 × 0.010 = 1W |
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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 |
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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 =
3.3 × 10 + 5.815
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