Performance Without Penalties

Designers of today’s industrial and appliance applications are continually faced with the challenge of reducing application size and component count. At the same time, they’ve got to improve system performance and reliability. The latest high-voltage IC (HVIC) technologies can help by streamlining the design of the inverter-based, variable-speed motor drive solutions that increasingly find their way into these applications. Furthermore, the same integrated technologies can also deliver benefits to applications such as general-purpose inverter circuits, switch-mode power supplies (SMPSs), and uninterruptible power supplies (UPSs).

Variable-speed motor drives deliver a number of benefits in domestic appliances and industrial or commercial equipment (e.g., air conditioners): higher energy efficiency, improved reliability, lower vibration, and reduced electrical and acoustic noise. Implementing these variable-speed drives efficiently and cost-effectively is made possible mainly due to advances in power-semiconductor technology, via 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 (short circuits, overcurrent conditions, and ground faults).

Sensing of inverter-stage and motor-phase currents is another critical requirement in these designs, since it’s the basis for both current-mode control and overcurrent protection. Current-mode control requires high precision and linearity, while overcurrent protection requires fast response. Practically, the current signals can be sampled in series with the positive or negative DC bus, individual IGBT phase leg, or motor-phase lead (Fig. 1). Current signals sampled in either DC bus are the vector sum of all 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.

Therefore, rather complicated “sample-and-hold” plus DSP circuits must 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 can’t 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 one significant complication here is that the small differential signal in the millivolt range is floating on top of a 600V to 1200V common- mode voltage. In addition, the common-mode voltage swings from –dc to +dc at a dV/dt rate of up to 10V/ns due to the action of the IGBT inverter phase.

High-voltage IC technology

With the latest developments in HVIC technology, designers now can employ elegant, space-saving, and low-component-count solutions that address the issues of protection and current sensing in modern drive designs. IR’s proprietary HVIC technology, for example, 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 (Fig. 2). The LDMOS performs level shifting to transfer control signals across the high-voltage barrier between the low-side and high-side circuits. What ultimately results 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 makes it possible to sense a small differential voltage floating on top of a large common-mode voltage, even when this includes fast transients. By using its HVIC technology, International Rectifier developed a new suite of rugged, highspeed, high voltage IGBT control ICs and sensor ICs that enable small circuits with full protection. These ICs offer a set of protection functions that includes ground fault protection—functionality that was once available only in high-end systems. In addition to these advanced functions, the ICs feature high noise immunity with up to 30% fewer components in half the footprint compared to discrete optocoupler- or transformer-based solutions. As a result, designers can reduce PCB area by as much as 50%.

1200V HVICs a good fit for industrial applications

The IR22381 is an analogue, three-phase IGBT gate driver. With a deadtime of 0.5μs, the device operates ten times faster than comparable optocoupler-based drivers. In addition, the IR22381 minimises temperature drift and changes in performance over time. An integrated de-saturation feature provides for all modes of overcurrent protection, including ground fault, shoot-through, and short-to-supply-rails protection. Soft shutdown is initiated in the event of an overcurrent condition followed by the turnoff of all six outputs. A shutdown input is included to enable custom shutdown functions.

The IR22381 includes programmable deadtime, and the output drivers have separate turn-on/turn-off pins with a two-stage turn-on output to achieve the desired IGBT dV/dt switching level. Voltage feedback provides accurate measurements, and a bootstrap power supply eliminates an auxiliary power supply.

The IR2277 and IR22771 are high-speed, single-phase current sense interface ICs with synchronous sampling for motor drive applications. The current is sensed through an external shunt resistor that converts the analogue voltage into a time interval through a precise dual ramp system. The time interval is level-shifted and provides digital PWM output that’s suitable for digital signal-processing and analogue-to-digital interfaces without additional logic circuits. Maximum throughput is 40ksamples/s (suitable for up to 20kHz asymmetrical PWM modulation) and maximum delay is less than 7.5°s at 20kHz.A noise-immune, bidirectional, level-shifting circuit is used to avoid false common-mode, dV/dt noise up to 50V/ns. The IR2277 provides both analogue and PWM output, while the IR22771 supplies PWM output.

The IR2214 and IR22141 are designed to drive single halfbridge circuits in power switching applications as well as three-phase 380VAC circuits at up to 50A at 80°C. Unlike solutions that use optocouplers, the IR2214 and IR22141 gate drivers deliver stability during their lifetime and include parameter matching, which includes propagation delay for high- and lowside channels as well as deadtime insertion. Their low quiescent current on the high side enables economical and spacesaving bootstrap supply topology. Fault feedback on IGBT de-saturation automatically shuts down the IGBT when used in multiphase configurations.

Desaturation detection for both sides as well as an internal biasing resistor are included in the IR2214, while the IR22141 features an additional active de-saturation diode bias. The IR2214 and IR22141 can be connected together via a dedicated pin to protect the drive system from phase-to-phase short circuits. Input and output pins are 3.3V CMOS-compatible to simplify microprocessor interfacing. Separate power and signal ground pins enable emitter shunt configurations to simplify low-side IGBT current sensing. They’re packaged in an SSOP-24.

600V HVICs targeted for appliance applications

To best fit appliance motor drives and a wide range of general-purpose inverter circuits, the 1200V HVICs come in 600V versions. The IR21381 is an analogue three-phase driver, similar to the IR22381. The IR2177 and IR21771 are 600V versions of the IR2277 and IR22771. Rounding out the set, the IR2114 and IR21141 represent 600V versions of the IR2214 and IR22141.

TAGS: Digital ICs
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