bq51013 datasheet shows Figure 5. IC Efficiency from AC Input to DC Output.
Would you specify the condition of efficiency percentage?
Input from secondary coil or IC VRECT input?
Output to OUT current?
The graph in figure 5 shows gradual decline from 3W to 5W.
What is the reason of this gradual decline of the efficiency?
Regards, M. Emi
Figure 5 takes into account losses in the rectifier and LDO over the output power range from 1W to 5W. Output voltage is fixed at 5V so this represents output current from 200mA to 1A. Input power is measured at AC1 and AC2 and output power is measured at Out.
LDO input voltage will change with output current, voltage drop across the LDO will change.
LDO input voltage (V-Rect) / LDO Voltage drop:
Less than 100mA – 7.08V / 2.08V
100mA to 200mA – 6.28V / 1.28V
200mA to 400mA – 5.53V / 0.53V
Greater than 400mA – 5.11V / 0.11V
Rectifier is Half Synchronous below 225mA and Full Synchronous above that point.
By the 3W point rectifier is operating in a fully synchronous mode and LDO drop is fixed at 0.11V. The gradual decline of efficiency is due to increases power loss across Rdson and resistance as output current increases. Increase in die temp will also increase Rdson and increase power loss.
Contact me if you have further questions.
Bill Johns, Applications Engineer
bqTESLA Wireless Power Products
Texas Instruments Inc
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