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HVICs deliver improved performance

Posted: 17 Dec 2007 ?? ?Print Version ?Bookmark and Share

Keywords:high-voltage IC? IGBTs? HVICs? power MOSFETs? DSP?

Designers of modern industrial and consumer applications are under ever-increasing pressure to reduce size and component count while improving overall system performance and reliability. At the same time, price and time-to-market demands mean that these benefits must be achieved without incurring significant cost penalties or increased project development time. Now, the latest high-voltage IC (HVIC) technologies are helping engineers address these demands by streamlining the design of the inverter-based variable speed motor drive solutions that are increasingly used in these applications.

Variable speed motor drives deliver a number of benefits. These include higher energy efficiency, improved reliability, lower vibration, and reduced electrical and acoustic noise. Efficient and cost-effective implementation of these variable speed drives has been made possible largely because of the advances in power semiconductor technology through devices such as IGBTs and power MOSFETs. A key aspect of these designs is that the IGBT- and MOSFET-based power stages are protected from failures such as short circuits, overcurrent conditions and ground faults.

Sensing of inverter stage and motor phase currents is another critical requirement in these designs as it is the basis for current mode control and overcurrent protection. Current mode control requires high precision and linearity, while overcurrent protection requires fast response. The current signals can be sampled in series with the positive or negative DC bus, individual IGBT phase leg or motor phase lead. Current signals sampled in either DC bus are the vector sum of all the IGBT phase leg currents. Also, the signal content is the pulse-width modulated envelope, at fixed carrier frequency, of the fundamental variable frequency motor current. Thus, rather complicated "sample and hold" plus DSP circuits have to be used to extract useful current information with good linearity and accuracy.

Sampling current in the individual IGBT phase leg current presents an easier processing proposition, but cannot eliminate the need to deal with carrier frequency sampling. By far, the simplest current signal available is from the motor phase lead. The signal content is only the fundamental variable frequency motor current. The significant complication here is that the small differential signal in the mV range is floating on top of a 600-1,200V common mode voltage. In addition, the common mode voltage is swinging from -DC to +DC at a dV/dt rate of up to 10V/ns due to the action of the IGBT inverter phase.

Protection
The latest developments in HVIC technology have allowed designers to use elegant, space-saving and low component count solutions that address the issue of protection and current sensing in modern drive designs. For example, IR's proprietary HVIC technology allows a low-side grounded CMOS circuit to be fabricated alongside a high-side floating CMOS, separated by an N- or P-channel lateral double-diffused MOS (LDMOS) region. The LDMOS performs level shifting to transfer control signals across the high voltage barrier between the low-side and high-side circuits.

The result is a technology that enables the design of single monolithic chip solutions for driving and protecting MOSFETs and IGBTs. At the same time, HVIC technology provides the ability to sense a small differential voltage floating on top of a large common mode voltage, even when this includes fast transients. As a result, HVIC technology is the ideal basis for creating current sensing interface ICs.

By using its HVIC technology, IR has been able to develop a new suite of rugged, high-speed, high-voltage IGBT control and sensor ICs that enable small circuits with full protection. These ICs provide a sophisticated set of protection functions including ground fault protection, a functionality that was once available only in high-end systems. The ICs also feature high noise immunity with up to 30 percent fewer components in half the footprint compared with discrete optocoupler- or transformerbased solutions. As a result, designers can reduce PCB area by as much as 50 percent.

An example of an analog, three-phase IGBT gate driver is the IR22381. With a dead time of 0.5?s, the device is 10 times faster than comparable optocoupler- based drivers. It also minimizes temperature drift and changes in performance over time.

IR's proprietary HVIC technology allows a low-side grounded CMOS circuit to be fabricated alongside a high-side floating CMOS, separated by an N- or P-channel LDMOS region.

Meanwhile, the IR2277 and IR22771 are high-speed, single-phase current sense interface ICs with synchronous sampling for motor drive apps. The current is sensed through an external shunt resistor that converts the analog voltage into a time interval through a precise dual ramp system. The time interval is level-shifted and provides digital PWM output suitable for DSP and A/D interfaces without additional logic circuits.

- David Tam
International Rectifier Corp.




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