ACPL-C870 查看數據表（PDF） - Avago Technologies

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ACPL-C870

Precision Optically Isolated Voltage Sensor

Avago Technologies 

Measurement Accuracy and Power Dissipation of the Resistive Divider

The input stage of the typical application circuit in Figure

19 can be simplified as the diagram shown in Figure 20.

R2 and RIN, input resistance of the ACPL-C87X, create a

current divider that results in an additional measurement

error component that will add on to the tot on top of the

device gain error. With the assumption that R1 and RIN

have a much higher value than R2, the resulting error can

be estimated to be R2/RIN.

With RIN of 1 GW for the ACPL-C87X, this additional mea-

surement error is negligible with R2 up to 1 M, where the

error is approximately 0.1%. Though small, it can be further

reduced by reducing the R2 to 100 k (error of 0.01%

approximately), or 10 k (error of 0.001% approximately).

However with lower R2, a drawback of higher power dis-

sipation in the resistive divider string needs to be consid-

ered, especially in higher voltage sensing applications. For

example, with 600 V DC across L1 and L2 and R2 of 100 k

for 0.01% measurement error, the resistive divider string

R1

RIN

+

R2

GND

ACPL-C87x

Figure 20. Simplified Input Stage.

consumes about 12 mW, assuming VIN is set at 2 V. If the R2

is reduced to 10 k to reduce error to 0.001%, the power

consumption will increase to about 120 mW. In energy

efficiency critical applications such as PV inverters and

battery-powered applications, this trade-off between

measurement accuracy and power dissipation in the

resistive string provides flexibility in design priority.

Isolated Temperature Sensing using Thermistor

IGBTs are an integral part of a motor or servo drive system

and because of the high power that they usually handle,

it is essential that they have proper thermal management

and are sufficiently cooled. Long term overload conditions

could raise the IGBT module temperature permanently or

failure of the thermal management system could subject

the module to package overstress and lead to catastrophic

failures. One common way to monitor the temperature

of the module is through using a NTC type thermistor

mounted onto the IGBT module. Some IGBT module man-

ufacturers also have IGBTs that comes with the thermistor

integrated inside the module. In some cases, it is necessary

to isolate this thermistor to provide added isolation and

insulation due to the high power nature of the IGBTs. The

ACPL-C87x voltage sensor can be used to easily meet

such a requirement, while providing good accuracy and

non-linearity. Figure. 21 shows an example of such an

implementation. The ACPL-C87x is used to isolate the

thermistor voltage which is later fed by the post amp

stage to an ADC onboard the microcontroller (MCU) to

determine the module temperature. The thermistor needs

to be biased in way that its voltage output will optimize

the 2 V input range of the ACPL-C87x across the intended

temperature measurement range.

HV+

HV-

IGBT Module

Figure 21. Thermistor sensing in IGBT Module

U

V Vdd

W

NTC Thermistor

+

GND

Post ADC

ACPL-C87x Amp

MCU

12

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