Optocouplers Safely Isolate Integrated Power Modules

Jun 1, 2008 12:00 PM
By Junhua He, Product Manager, Avago Technologies, Singapore

Optocouplers provide high-voltage isolation between a low-voltage device like a microcontroller or a pulse-width modulation (PWM) generator and a high-voltage device like an intelligent or integrated power module (IPM). The optocoupler is a key interface device because every high-voltage circuit must be compliant with equipment safety standards, such as IEC 60950 for IT equipment and IEC 60335 for household appliances. In testing for these standards, a high voltage is usually applied between low-voltage and high-voltage ports of the equipment. In these systems, the optocoupler isolates the low- to high-voltage interface to meet safety and protection standards.

Some common semiconductor component electrical safety standards applicable to optocouplers are IEC 60747-5-2 and UL 1577. A designer can select the appropriate optocouplers based on the relevant equipment safety standards. The table lists the characteristics of some optocouplers intended for high-voltage isolation. The key optocoupler parameters related to equipment safety ratings are working voltage, polluting degree, installation class and insulation level.

Various safety standards for industrial, home, office and IT equipment require a reinforced insulation level for electrical equipment powered by the ac line. Some parameters specified by equipment safety standards include external clearance, creepage, distance through insulation (DTI) or internal clearance, and comparative tracking index (CTI).

Fig. 1 shows a diagram of a motor-drive circuit between a microcontroller unit (MCU) and an IPM. Seven units of digital optocoupler ACPL-W456 isolate the IPM's seven gate driver inputs, one for the brake and six for the IGBTs. Using voltage sampling from two shunt resistors, two HCPL-7840 isolated amplifiers provide linear feedback from two motor phases to the MCU. Four HCPL-817 general-purpose phototransistor-type optocouplers isolate the IPM's fault feedback signals. All these optocouplers are compliant with reinforced safety-protection levels, because they secure a galvanic isolation boundary between low- and high-voltage circuits.

A three-phase IPM employs six gate drivers each for six high- and low-side IGBTs. Each gate driver needs a 10-V to 30-V power supply. The emitters of the low-side IGBT connect to the dc bus HV- as common reference ground, which allows all low-side gate drivers to share the same power supply (V_{CC_L} - GND1). Also integrated are overtemperature and overcurrent protection functions that feedback a fault signal to the host microcontroller.

The emitter of the high-side IGBT and the collector of the low-side IGBT connect to form one leg of a three-phase switch. By alternately turning high-side and low-side IGBTs on and off, the HV dc bus voltage switches the output to the respective phase of U, V or W load. The three-phase vectors are 120 degrees apart. With ground connecting to the collector of the low-side IGBTs, the ground of each high-side gate-driver circuit swings between HV- and HV+. Thus, the ground of each power supply for the high-side IGBT gate-driver circuit must float and be separated from each other.

A more robust solution is to have three isolated power supplies to each high-side gate-driver circuit. Bootstrapping the power supply with individual floating grounds is a cost-effective alternative. You can derive a bootstrapping power supply from either dc bus voltage or low-side power supply V_{CC_L}. Conventional IPM input logic and gate-driving circuits are integrated on a monolithic IC, and their power supply ranges from 15 V to 20 V.

This conventional IPM has an inverted logic. When the input voltage is high, the IGBT turns off; when the input voltage is low, the IGBT turns on. The ACPL-W456 optocoupler has an open-collector transistor output. Before the input side and MCU power up, the ACPL-W456 output logic level is high and keeps all IGBTs off. Typically, both the high-side floating power supplies (V_{CC_UH}, V_{CC_VH}, V_{CC_WH}) and low-side power supply (V_{CC_L}) are 15 V. The IPM driver input can operate at 15-V logic levels.

The ACPL-W456 high-voltage output can be calculated from:

V_OH = V_CC - I_OH × R_L,

where V_CC equals the power-supply voltage, I_OH equals the output high transistor leakage current, V_OH equals the high-voltage output and R_L equals the output transistor pull-up resistor.

Select a moderate pull-up resistor value to retain sufficient V_OH and to drive the IGBT on or off without errors from the PWM logic. For example, at a maximum 50-µA leakage current, use a 20-kΩ pull-up resistor for a 15-V power supply or a 3-kΩ resistor for a 5-V power supply. Use the minimum V_CC voltage if it fluctuates.

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.