基于A788的变频器短路电流及过载检测隔离放大器
2009-03-12 21:36阅读：
云洋电子：www.sjzyunyang.com
功能介绍：
Features
• Output Voltage Directly Compatible with A/D Converters (0 V to
VREF)
• Fast (3 ms) Short Circuit Detection with Transient Fault
Rejection
• Absolute Value Signal Output for Overload Detection
• 1 mV/°C Offset Change vs. Temperature
• SO-16 Package
• 25 kV/ms Isolation Transient Immunity
• Regulatory Approvals (Pending): UL, CSA, VDE 0884 (891 Vpeak
Working Voltage)
▲Low Cost Three Phase Current Sensing with Short Circuit and
Overload Detection
Hewlett-Packard’s Isolation Amplifier with Short Circuit and
Overload Detection makes motor phase current sensing compact,
affordable and easy-to-implement while satisfying worldwide safety
and regulatory requirements.
▲Description
The HCPL-788J isolation amplifier is designed for current sensing
in electronic motor drives. In a typical implementation, motor
currents flow through an external resistor and the resulting analog
voltage drop is sensed by the HCPL-788J. A larger analog output
voltage is created on the
other side of the HCPL-788J’s optical isolation barrier. The output
voltage is proportional
to the motor current and can be connected directly to a
singlesupply A/D converter. A digital
over-range output (FAULT) and an analog rectified output (ABSVAL)
are also provided. The
wire OR-able over-range output (FAULT) is useful for quick
detection of short circuit conditions on any of the motor phases.
The wire-OR-able rectified output (ABSVAL), simplifies
measurement of motor load since it performs polyphase
rectification. Since the common-mode voltage swings several hundred
volts in tens of nanoseconds in modern electronic motor drives, the
HCPL-788J was designed to ignore very high common-mode transient
slew rates (10 kV/ms).
Figure 1. Current Sensing Circuit.
引脚功能说明：
内部框图
▲Applications Information
Production Description
The internal block diagram of the HCPL-788J.The analog input (VIN)
is converted
to a digital signal using a sigma-delta (å-D) analog to digital
(A/D) converter. This A/D samples the input 6 million times per
second and generates a high speed 1-bit output representing the
input very accurately. This 1 bit data stream is transmitted via a
light emitting diode (LED) over the optical barrier after encoding.
The detector converts the optical
signal back to a bit stream. This bit stream is decoded and drives
a 1 bit digital to analog (D/A) converter. Finally a low pass
filter and output buffer drive the output signal (VOUT) which
linearly represents the analog input. The output signal full-scale
range is determined by the external reference voltage (VREF). By
sharing this reference voltage
(which can be the supply voltage), the full-scale range of the
HCPL-788J can precisely match the full-scale range of an external
A/D converter.In addition, the HCPL-788J compares the analog input
(VIN) to both the negative and positive full-scale values. If the
input exceeds the full-scale range, the short-circuit fault output
(FAULT) is activated quickly. This feature operates independently
of the å-D A/D converter in order to provide the highspeed response
(typically 3 ms) needed to protect power transistors. The FAULT
output is wire OR-able so that a short circuit on any one motor
phase can be detected using only one signal. One other output is
provided — the rectified output (ABSVAL). This output is also wire
OR-able. The motor phase having the highest instantaneous rectified
output pulls the common output high. When three sinusoidal motor
phases are combined, the rectified output (ABSVAL) is essentially a
DC signal representing the rms motor current. This single DC signal
and a threshold comparator can indicate motor overload conditions
before damage to the motor or drive occur.Figure 22 shows the
ABSVAL output when 3 HCPL-788Js are used to monitor a sinusoidal 60
Hz current. Figures 23 and 24 show the ABSVAL output when only 2 or
1 of the 3 phases are monitored, respectively. The HCPL-788J’s
other main function is to provide galvanic isolation between the
analog input and the analog output. An internal voltage reference
determines the full-scale analog input range of the modulator
(approximately ± 256 mV); an input range of ± 200 mV is recommended
to achieve optimal performance.
