Maxim Integrated 78M6610+PSU Hardware Design Guidelines - Manual (Page 5)

AN_6610_107 78M6610+PSU Hardware Design Guidelines
Rev. 0 5
2.1.2 Shunt Resistor Selection
The value of the shunt to be used is a tradeoff between a higher shunt value that utilizes the full analog
sensor input range of the IC and the lowest possible value that minimizes the power loss in the shunt.
For best utilization of the device’s input range (+/-250 mVpp) and performance at lower currents, a larger
shunt value that supports the maximum input current without clipping peak currents should be used. The
maximum shunt value should be calculated at the lowest operating voltage and at maximum input power
(accounting for max output power ratings, conversion efficiency, and standby power consumption of the
PSU). As an example, with 90 VAC as a minimum operating voltage and a maximum input power of
1.0 KW, the maximum RMS input current is:
𝐼𝑚𝑎𝑥(𝑟𝑚𝑠) =
𝑃𝐼𝑁𝑚𝑎𝑥
𝑉𝐴𝐶𝑚𝑖𝑛 =
1.0𝐾𝑊
90𝑉𝐴𝐶 = 11.11 𝐴
Using the maximum supported current as input full scale, the maximum shunt value would be:
𝑅𝑠ℎ𝑢𝑛𝑡 =
𝑉𝐴𝐷𝐶𝑚𝑎𝑥
𝐼𝑚𝑎𝑥(𝑝𝑒𝑎𝑘) =
250 𝑚𝑉𝑝𝑘
√2 ∗ 11.12 𝐴𝑟𝑚𝑠
= 15 𝑚Ω
While a 15 m
Ω shunt value may fully utilizes the ADC input range, this shunt value produces a dissipated
power of 1.85 W at maximum load. In order to ensure more ADC signal margin due to higher crest factors
and to lower the power dissipation in the shunt resistor, the lowest possible shunt resistance is often
desirable. To find the minimum shunt value, the minimum current to be accurately measured should be
considered. As an example, with 265 VAC as a maximum operating voltage and a minimum input power
of 5W, the minimum RMS input current to be measured is:
𝐼𝑚𝑖𝑛(𝑟𝑚𝑠) =
𝑃𝐼𝑁𝑚𝑖𝑛
𝑉𝐴𝐶𝑚𝑎𝑥
=
5𝑊
265𝑉𝐴𝐶
= 18.87 𝑚𝐴(rms)
Using this minimum supported current as the 2000:1 limit for current accuracy, the minimum shunt
resistance value would be:
𝑅𝑠ℎ𝑢𝑛𝑡 =
𝑉𝐴𝐷𝐶𝑚𝑎𝑥
2000 ∗ 𝐼𝑚𝑖𝑛(𝑝𝑒𝑎𝑘) =
250 𝑚𝑉𝑝𝑘
2000 ∗ √2 ∗ 18.87 𝑚𝐴(𝑟𝑚𝑠)
= 4 𝑚Ω
Other considerations involved in the shunt resistor selection include the power dissipation in the shunt
and the device’s temperature coefficient. In the case of a 4m
Ω shunt, the power dissipated in the shunt at
maximum load current is 0.5 W. A 2 W rated device package is recommended for good long-term
reliability and minimize the self heating of the resistor. The initial shunt tolerance is often compensated
during calibration. The temperature coefficient, however, plays an important role in the overall accuracy
and cannot be easily compensated. For example, a temperature coefficient of 100 ppm/°C causes a
resistance variation of 1% over the 100 °C operating temperature environment.