28 of 43
2020-5-22
UG2020-11 REF-Vacuum-C101-2ED User Guide
-
Maximum DCBUS input current of inverter (I
rms
) is 3 A, and already includes 20~30% overload margin
-
Shunt resistor rate power is 1 W (low cost of thin film 2512 package), 60% derating (allow maximum 0.6 W)
The shunt resistor power rating is calculated by the following equation.
𝑃
𝑆𝐻
=
𝐼
𝑟𝑚𝑠
2
×𝑅
𝑆𝐻
𝑑𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑟𝑎𝑡𝑖𝑜
= 0.6 W =
𝐼
𝑟𝑚𝑠
2
× 𝑅
𝑆𝐻
==>
𝑅
𝑆𝐻
= 0.066 𝑂ℎ𝑚 = 66 𝑚𝑂ℎ𝑚
75 m
Ω
or 50 m
Ω
can be considered, supposing maximum input current occurs at maximum PWM modulation
rate, according to the conservation of power:
𝑃𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡 = 𝑉
𝑑𝑐
∗ 𝐼
𝑏𝑢𝑠𝑅𝑀𝑆
= 3 ∗
𝑉
𝑑𝑐
1.414
∗ 𝐼
𝑚𝑜𝑡𝑜𝑟𝑅𝑀𝑆
𝐼
𝑚𝑜𝑡𝑜𝑟𝑅𝑀𝑆
= 0.47 ∗ 𝐼
𝑏𝑢𝑠𝑅𝑀𝑆
Maximum voltage to ADC pin: 3A *0.47* 1.414 * 50 Ohm = 99.7 mV, which is about 1/33 of 3300 mV.
ADC internal gain can be 12X, whose maximum input range is 3300 mV/12 = 275 mV, consider equaled MotorLim
and RegenLim, the maximum motoring and regenerating sensing range is 275/2 = 137.5 mV > 99.7 mV.
Measure margin check: 50 m
Ω
shunt resistor has a margin of (137.5-99.7)/99.7 = 37.9%
Next, checking the real derating of 50 m
Ω
and 75 m
Ω
shunt resistors:
𝐷𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑓𝑜𝑟 50m
Ω
=
𝐼
𝑟𝑚𝑠
2
× 𝑅
𝑆𝐻
𝑅𝑎𝑡𝑒 𝑝𝑜𝑤𝑒𝑟
=
3𝐴 ∗ 3𝐴 × 50𝑚Ω
1𝑊
= 0.45
𝐷𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑓𝑜𝑟 75m
Ω
=
𝐼
𝑟𝑚𝑠
2
× 𝑅
𝑆𝐻
𝑅𝑎𝑡𝑒 𝑝𝑜𝑤𝑒𝑟
=
3𝐴 ∗ 3𝐴 × 75𝑚Ω
1𝑊
= 0.675
The 50 m
Ω
shunt resistor has a higher power margin, and 75 m
Ω
has better SNR. Since the power derating is
highly related to the real thermal resistance of the PCB, we can test both 50 m
Ω
and 75 m
Ω
shunt resistors and
decide which one is better on the tradeoff of SNR and real temperature rising.
6.1.4
Inverter overcurrent protection and motor gatekill configuration
Figure 24 displays the overcurrent protection circuit for the IMC101T application. R29 & R34 sensed voltage (or
external op-amp) is connected to ADC and internal comparators to achieve the overcurrent protection function.
Only internal comparators are used for overcurrent protection.
The reference of the internal comparators is set by IMC101T’s internal DAC, and external C31 is the low pass
filter capacitor. MCEWizard calculate the DAC reference voltage based on the question input of “Device
Overcurrent trigger level setting for Comparator” and “Motor1 Current Input to ADC Offset Voltage”. MCEWizard
output parameter CompRef define the DAC output voltage, which depends on the 3 items listed below and
shown in Figure 24 & 25
A: Expected OC threshold
B: Gain of external current sensing network
