The HCPL-788J isolation amplifier is designed for current sensing electronic motor drives. In a typical implementation, motor current flows through an external resistor and the resulting analog voltage drop is sensed by the HCPL-788J. The larger analog output voltage is generated on the other side of the optical isolation barrier of the HCPL-788J. The output voltage is proportional to the motor current and can be connected directly to a single supply A/D converter. A digital overrange output (FAULT) and an analog rectified output (ABSVAL) are also available. The wire-OR-capable overrange output (FAULT) can quickly and efficiently detect short-circuit con- ditions during any motor stage. Line-OR - capable of rectifying the output (ABSVAL), which simplifies the measurement of the motor load due to its multiphase rectification. Since the common-mode voltage swings several hundred volts in tens of nanoseconds in modern electronic motor drives, the HCPL-788J is designed to ignore high common-mode transient slew rates (10 kV/µs) .

Current detection circuit

Application Information

Figure 1 shows a block diagram of the internal HCPL-788J. The analog input (VIN) is configured to be verted using a digital signal sigma-delta (Σ-Δ) analog-to-digital (A/D) converter. The A/D samples the input 6 million times per second and produces a high-speed 1-bit output indicating that the input is very accurate. This 1-bit data stream is transmitted through a light-emitting diode (LED) above the encoded light barrier. The detector feeds back the optical signal into a bit stream. This bitstream is decoded and drives a 1-bit digital-to-analog (D/A) con-converter. The final low pass filter and output buffer drive the output signal (VOUT) to the linear REP-full analog input. The full-scale range of the output signal is determined by the external reference voltage (VREF). By sharing the full-scale range of the reference voltage (which can be the supply voltage) the HCPL-788J can precisely match the full-scale range of an external A/D converter. Additionally, the HCPL-788J compares the analog input (VIN) to negative and positive full-scale values. If the input exceeds the full-scale range, the short-circuit fault output (FAULT) is rapidly activated. This function works independently of the dently ∑-? A/D converter in order to provide a high-speed response (usually 3μs) to protect the resistance value of the power tube. The FAULT output line OR - enables a short circuit in any one motor phase to be detected using only one signal. One other output is provided - the rectified output ABSVAL). This output is also wire-OR-enabled. The output terminal with the highest instantaneous correction of the motor phase will turn the common output high. When three sinusoidal motors are combined, the rectified output (ABSVAL) is essentially a DC signal representing- ing the rms motor current. This single DC signal and threshold comparison can indicate motor overload conditions before damage to the motor or frequency converter occurs. Figure 2 shows the ABSVAL when the 3HCPL-788Js outputs a sinusoidal 60Hz current for monitoring. Figures 3 and 4 show the ABSVAL output when only 2 or 1 of the 3 phases are monitored, respectively. Another main function of this HCPL-788J is to provide isolation between the electrical analog input and analog output. The internal reference determines the full-scale analog input range of this modulator (approximately ± 256mV ); an input range of ±200mV is recommended for optimum performance.

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Analog Interface Technology

The recommended supply CON- wiring for power and bypass is shown in Figure 6. The floating supply (which in many applications may be, is used to drive the high-side power transistor on the same supply) is regulated to 5V with a simple zener diode (D1); the value of resistor R4 should be chosen from supplying sufficient current available floating power supply. The voltage resistor (Rsense) from the current sense is applied to the HCPL-788J through the input of an RC anti-aliasing filter (R2 and C2). Although the application circuit is relatively simple, a few recommendations should be followed to ensure optimum performance. The power supply for the HCPL-788J is most commonly obtained from the gate electric drive circuit of the power transistor with the same power supply. If a dedicated supply is required, in many cases it is possible to add an additional wrapping the former to the existing transmission. Otherwise, some simple isolated power source can be used, such as a linear motor transformer or a high frequency DC-DC converter. The inexpensive 78L05 three-terminal regulator can also be used to reduce the floating supply voltage to 5V. To help mitigate high frequency supply noise or ripple, a resistor or inductor can be used in series with the regulator's input to form a low pass filter with the regulator input bypass capacitor. As shown in Figure 5, 0.1µF bypass capacitors (C1, C3, C4 and C6) should be placed as close as possible to the pins of the HCPL-788J. Bypass capacitors are required because of the high-speed digital nature of the signals within the HCPL-788J. The addition of a 0.01µF bypass capacitor (C2) is recommended at the input due to the nature of switched capacitor input circuits. The input bypass capacitor also forms part of the antialiasing filter, which is recommended to prevent high frequency noise from aliasing down to lower frequencies and interfering with the input signal. The input filter also performs an important reliability function - it reduces transient spikes from ESD events flowing through the current sense resistor.

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Figure 4

PC board layout

The design of the printed circuit board (PCB) should follow good layout practices such as keeping the power pins close to the bypass capacitor, keeping the output away from the signal of the input signal, using ground and power shelves, etc. In addition, the layout of the printed circuit board, The isolation transient immunity (CMTI) of the HCPL-788J can also affect the output circuit due to mainly stray capacitance coupling between the inputs. For optimum CMTI performance, the PC board layout should minimize stray coupling by maintaining the maximum distance possible between the input and output side circuits, and ensuring that any ground or power planes on the PC board do not pass directly below or above In the main body of HCPL-788J which is extended more widely. The load current should have no effect on the measured voltage. When laying out the current sensing resistors on the PCB, a two point should be kept in mind. Kelvin connection resistors should be drawn together under the body resistors and then run very close to each other on the inputs of the HCPL-788J; this minimizes the connection loop area and reduces the possibility of stray interference from magnetic fields with the measured signal. If the sense resistor is not on the same HCPL-788J circuit on the PC board, a tight twisted pair can accomplish the same thing. Additionally, multiple PC laminates can be used to increase current carrying capacity. Numerous plated through holes should be placed around each end of the non-open sense resistor to help distribute the current between the layers of the PC board. The PC board should use a layer of 2 or 4 oz copper to produce a current carrying capacity in excess of 20A. Making the current carrying traceable on the PC board can also improve the power dissipation of the sense resistor by sinking as the thermal capability. Extensive use of vias, where the load current enters and leaves the PC board is also recommended.

Figure 5

Sense Resistor Connection

The recommended method for connecting the HCPL-788J to the current sense resistor is shown in Figure 6. VIN+ (pin 1 of the HCPL-788J) is connected to the sense positive terminal resistor, while VIN- (pin 2) is shorted to ground 1 (8 pin), with the power return path used as the sense wire to the negative terminal of the current sense resistor. This allows a single wire pair or PCB trace to connect the sense resistor of the HCPL-788J circuit. By referring to the negative-side current-sense resistor of the input circuit, any load current induced in the noise transient resistor is seen as a common-mode signal and will not interfere with the current-sense signal. This is because significant large load currents flowing through the motor drive, along with the parasitic inductance inherent in the wiring circuit, can generate both noise spikes and offsets that are clearly large compared to the small voltage being measured across the current path. sense resistor. If the same power supply is used both for the gate drive circuit and for the current sense circuit, it is very important to connect 1 of the HCPL-788J from GND to the sense resistor for the only return path for supplying the current to the gate drive power supply supply to eliminate potential ground loop problems. The only direct connection between the HCPL-788J circuit and the gate drive circuit should be the positive power line.

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