Power Electronics Technology April 2005 www.powerelectronics.com34

ICs Protect IGBTs and SenseCurrents in Motor Drives

In industrial and appliance motor-drive applica-tions, high-voltage ICs can provide full IGBT pro-tection in the inverters and sensorless measure-ment of motor currents at low cost.

By David Tam, Vice President, Consumer Industrial BusinessUnit, International Rectifier, El Segundo, Calif.

New industrial and appliance motordrives have become more advanced inperformance and more compact insize. This is largely due to advancementin power silicon technology, such as

IGBTs and high-voltage integrated circuits (HVICs). Thistrend is particularly accelerated in low-power industrialor servo drives that are less than 5 kW and in inverter drivesfor energy-saving appliances, such as air conditioners,washers and refrigerators.

IGBT protection is one enabler of these performanceadvancements. Today’s state-of-the-art small inverters areequipped with full IGBT protection, including ground-faultprotection that was once required only in high-end drivesystems. These inverters require additional complexity inthe detection circuit as well as sensors. Therefore, the costassociated with the implementation could be prohibitivefor the low-end industrial or servo drive and especially inultra cost-competitive appliance drives.

Another performance enhancement is the ability tosense motor current to provide high bandwidth andaccurate feedback to the torque control loop. As the driveperformance advances toward adoption of sensorlessfield-oriented control (FOC), a low-cost method to sensemotor current is required.

HVIC technology is uniquely able to integrate advancedcircuit functions while permitting operation in thehigh-voltage and noisy environment of motor-drivecircuits. As a result, HVICs are changing the landscapeof motor-drive design, providing high-performance yetlow-cost IGBT protection and current-sensing features.

Methods of IGBT ProtectionAn overcurrent condition is one of the fatal drive faults

that could destroy IGBT devices in a motor-drive system.IGBT overcurrent conditions basically fall into threecategories—line-to-line short, ground fault and shoot-through. Table 1 lists overcurrent conditions and theirpotential causes.

When considering an IGBT overcurrent protectionscheme, two important factors must be evaluated. The firstfactor is what type of overcurrent protection the systemshould provide and how the system can be shut down. Thesecond factor is the control architecture. Controlarchitecture significantly influences the method and imple-mentation of the overcurrent protection.

Protection of IGBT devices is normally implementedin the hardware circuit. However, the circuit implementa-tion and the type of overcurrent-sensing device variesdepending on which overcurrent condition is beingaddressed. That reflects the different path and flow of thecurrent in each condition (Fig. 1).

The short-circuit current in both the shoot-through andthe line-to-line short condition always flows from/to thedc bus capacitor(s). However, the ground-fault currentnormally flows from the ac line input through the positivedc bus and a high-side IGBT to the earth ground wherethe failure occurs. There is no current flow across dc buscapacitor(s).

The protection scheme also depends on the controlarchitecture. For protection against line-to-line short andTable 1. Overcurrent conditions and their likely causes.

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MOTOR DRIVES

shoot-through, either a Hall-effect sensor or linear opto-isolation device across the shunt resistor, inserted in serieswith the negative dc bus line, detects the overcurrent con-dition. If the system needs ground-fault protection too, anadditional Hall-effect leakage current sensor can be placedon the ac line input or the positive dc bus. Protection cir-cuitry can be implemented using fast comparators.

Two comparators are used for each Hall-effect sensor ifthe sensors are located in the motor-phase output. That’sbecause both positive and negative polarity of current flowexists in the event of the line-to-line short condition. Totalpropagation delay of shutdown also is important. Thedelay time associated with the optical isolation device inthe gate drive and Hall-effect sensor is typically more than2 µs. Therefore, no matter how the protection circuit isimplemented, this delay time should be added to the cir-cuit delay to meet the IGBT short-circuit duration time.

The circuit requires two Hall-effect sensors and/orlinear opto-isolation devices and an additional protectioncircuit. The protection circuit contains two comparators,voltage references, capacitors and resistors. Additionally,each Hall-effect sensor would require its own separateisolated power supply.

An IGBT desaturation circuit is the alternative way toprotect the IGBTs. The discrete circuit can be constructed

in the secondary side of the opto gate driver or the opto-isolated device that has a desaturation circuit. The circuitdetects voltage buildup across the collector and emitterwhile the device is fully on. If the voltage exceeds a certainlimit, the desaturation circuit shuts off the associated gatesignal. When implemented as a discrete circuit, the requiredcomponents are a comparator with voltage reference, ahigh-voltage diode, and resistors and capacitors.

When an overcurrent condition is detected, the methodof IGBT turn-off becomes important. It is sometimes pref-erable to add softness to the turn-off. This will help reducethe high-voltage spike that appears across the collector andthe emitter of the IGBT. This spike is associated with theparasitic inductance in the circuit.

Creating a soft turn-off provides more safety margin inthe reverse-biased safe operating area (RBSOA) limit givena short-circuit condition. If successfully implemented,the snubber circuit can be significantly minimized oreliminated. For each IGBT gate-drive circuit, an addi-tional fast opto-isolation device and circuit with weak turn-off capability and dedicated totem-pole buffer transistorsare required.

When choosing between a snubber circuit and adiscrete soft shutdown circuit, there are tradeoffs. In aconventional inverter system, the snubber circuit may befavored because of its lower complexity and cost. In

Fig. 1. IGBT overcurrent conditions can occur as a line-to-line short (a), aground fault (b) or as shoot-through (c).

(a)

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particular, for small drives the snubber circuit can be imple-mented using a high-frequency capacitor across the dc busnear the IGBTs.

In contrast, Hall-effect sensors and opto isolators arerelatively large and bulky, thus requiring a large space whencompared with other components, such as a monolithicIC. In addition, if the system requires a soft shutdown func-tion, then an additional six opto isolators and six buffercircuits with the provision of weak turn-off capability arerequired.

The discrete shutdown circuit solution does not prom-ise simplicity or further integration of the gate drive andprotection circuit. This can be problematic given today’sdemands for smaller size and robustness in IGBT protec-tion in low-power industrial or appliance drives. The totalcost of the discrete circuit, including the assembly cost ofthe inverter system, is high because the solution requiresmany discrete and bulky components. Moreover, somecomponents such as Hall-effect sensors are still subject tomanual assembly.

Methods of Motor Current SensingPerhaps the most difficult function in the drive design

is sensing currents in the inverter stage and motor phases.The current signals are used for current-mode control,which require high precision and linearity, as well as forovercurrent protection, which requires fast response.

Different circuit techniques have to be used to processthe current signals depending on where in the inverter stagethe current signals are sampled. Precision, low ohm value,sensing resistors are typically used in the current path togenerate a sensing voltage signal with a maximum rangeof ±300 mV to minimize power dissipation. Alternatively,Hall-effect sensors are used. They are generally bulkier andmore expensive than sense resistors, but they have theadvantages of lossless sensing.

In practice, the current signals can be sampled in serieswith +dc bus, –dc bus, individual IGBT phase leg ormotor phase lead (Fig. 2). Current signals sampled in the

+dc or –dc bus are the vector sum of all the IGBT phase legcurrents. Also, the signal content is the pulse-width-modu-lated envelope at fixed carrier frequency of the fundamen-tal variable frequency motor current. Therefore, rathercomplicated sample-and-hold plus digital signal process-ing circuits have to be used to extract useful current infor-mation with good linearity and accuracy.

Individual IGBT phase leg current is somewhat easierto process, but cannot escape from dealing with carrier fre-quency sampling. By far, the simplest current signal avail-able is from the motor phase lead. The signal content isonly the fundamental variable frequency motor current.However, the one significant complication is that the smalldifferential signal in the millivolt range is floating on topof a 600-V to 1200-V common-mode voltage. In addition,the common-mode voltage is swinging from –dc to +dc ata dV/dt rate of up to 10 V/ns.

HVIC TechnologyIGBT protection and motor current-sensing circuits can

be simplified substantially by the monolithic integrationof the gate drive, protection and sensing functions. Thisintegration has been implemented in HVIC technology.The structure used in this HVIC supports a versatile set ofdevices and circuits as well as the unique capability of in-tegrating high-voltage level shifting and floating CMOS ina single piece of silicon.

In this HVIC, 600-V or 1200-V n-channel and p-chan-nel LDMOS silicon are used for level-shifting functionsfrom low-voltage to high-voltage and vice versa. The CMOScircuits can be referenced to a high-voltage floating supplylevel for high-side gate-drive and signal-processing circuits.This floating capability allows differential-mode signal pro-cessing on top of a 600-V or 1200-V common-mode volt-age. Other uses for the high-voltage devices include start-

Fig. 2. A motor current can be sensed by sampling current signals inseries with the dc bus, in an IGBT phase leg or in a motor phase lead.

MOTOR DRIVES

Table 2. Solution comparison of gate drive and IGBT protection.

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MOTOR DRIVES

up bias supply and synchronous bootstrapping supply.Gate-drive circuits are implemented in enhanced volt-

age CMOS with supply rail up to 25 V and provide neces-sary gate-drive voltage to the IGBT. Analog and digital cir-cuit blocks are implemented in BiCMOS. The circuit li-brary includes pulse-width modulator (PWM), voltagecontrol oscillator, precision sense amplifier, fast fault com-parator and other power conversion control functions.

Another HVIC design provides a monolithic solutionfor all six IGBT gate drivers with full IGBT protection withsoft shutdown. Table 2 shows a com-parison of this device with a discretesolution.

An application circuit for thisHVIC, which combines 3-phase gatedrive with IGBT desaturation protec-tion in each high-side and low-sideoutput, is shown in Fig. 3a. The chip’sinternal circuitry for desaturation de-tection (DSD) is shown in Fig. 3b.Maximum floating high-side voltagecan be as much as 600 V to 1200 V.Overcurrent is detected sensing thecollector-emitter voltage of IGBT atthe on-condition through an externaldiode, then the V

CE is compared with

a fixed 8-V threshold; afterward, theresulting signal is filtered for 1 µs. Ablanking filter of 3 µs is used to ig-nore the effect of the tail at the turn-on of the IGBT.

Once desaturation is detected,the output stage goes immediatelyinto a high-impedance state and theSSD driver is activated, turning offthe IGBT through the SSDH/L pinwith the proper impedance. SSDevents holds for 7 µs to achievesmooth IGBT gate discharge at highcollector current level. An externalresistor can control discharge imped-ance through the dedicated SSD pinon top of a minimum 75-� resistorinternally provided.

The short-circuit detection infor-mation is shared with the other high-or low-side drivers through theSY_FLT I/O pin. In this way, the maindriver—the one detecting the shortcircuit—communicates with theother drivers in the local network.Once active, this signal freezes all theother drivers’ status on the output, re-gardless of the status on the inputs.The main driver itself freezes its sta-tus until the SSD takes place.

When the soft shutdown is over, SY_FLT signal is dis-abled and diagnostic information is sent by FAULT/SD pinto microcontroller. Then, the main driver pulls down theFAULT/SD line, forcing a hard shutdown. By means of theFAULT/SD pin, all of the other drivers in the local networkare switched off and the faulty condition is reported to themain controller for diagnostic purposes.

To sense IGBT desaturation, the collector voltage is readby an external high-voltage diode. The diode is normallybiased by an internal pull-up resistor connected to the lo-

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MOTOR DRIVES

Fig. 4. The IR2277 linear motor-phase current-sensing HVIC combinesthe current-sensing function with filtering, PWM synchronization andcircuitry for generating analog or PWM output. This application circuitshows the shunt resistor (CSR) used to sense current in the motorphase lead along with the waveform that appears across CSR.

cal supply line (VB or VCC).When the transistor is “on,”the diode is conducting andthe amount of current flow-ing in the circuit is deter-mined by the internal pull-upresistor value.

In the high-side circuit, thedesaturation biasing currentmight become relevant fordimensioning the bootstrapcapacitor. In fact, using toolow of a value for the pull-upresistor may result in highcurrent significantly discharg-ing the bootstrap capacitor.For that reason, typical pull-up resistors are in the range of100 k�. This is the value of theinternal pull up.

While the impedance ofDSH/DSL pins is very low

when the transistor is on (low imped-ance path through the external diodedown to the IGBT), the impedance isonly controlled by the pull-up resistorwhen the IGBT is off. In that case, rel-evant dV/dt applied by the IGBT dur-ing the commutation at the output re-sults in a considerable current injectedthrough the stray capacitance of the di-ode into the desaturation detection pin(DSH/L). This coupled noise may beeasily reduced using an active bias forthe sensing diode.

An active bias structure is availableat DSH/L pin. The DSH/L pins presentan active pull-up, respectively, to VB/VCC and a pull-down, respectively, to

VS/COM. The dedicated biasing circuit reduces the im-pedance on the DSH/L pin when the voltage exceeds theVDESAT threshold. This low impedance helps in rejectingthe noise providing the current inject by the parasitic ca-pacitance. When the IGBT is fully on, the sensing diodegets forward biased and the voltage at the DSH/L pin de-creases. At this point, the biasing circuit deactivates in or-der to reduce the bias current of the diode.

Fig. 4 shows an example of an HVIC that integrates themotor-phase current-sensing function with advanced fil-tering, PWM synchronization and either analog or PWMoutput. The current signal is sensed using an external shuntresistor in the path of the motor-phase current such thatonly fundamental variable frequency motor current is pro-cessed. The HVIC converts the small differential voltage—±250 mV—into a time interval through a precise circuitthat also performs very good ripple rejection showing small

Fig. 3a. An application circuit for the IR22381Q HVIC depicts the external circuitry required for desaturationdetection on each high-side and low-side output.

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Fig. 3b. To detect desaturation, circuitry within the IR22381Q HVIC reads the collector voltageon the IGBT via a high-voltage diode (labeled DSD).

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MOTOR DRIVES

group delay.The time interval is level shifted and fed to the output.

An analog output voltage also is provided. The voltage isproportional to the measured current and is ratio metricwith respect to an externally provided voltage reference. Themax throughput is 40 ksamples/s suitable for up to 20-kHzasymmetrical PWM modulation and the max delay is lessthan 7.5 µs at 20 kHz. Also, a fast overcurrent signal is pro-vided for IGBT protection.

One of the particularlydifficult challenges is theability of the HVIC to sensesmall differential voltagefloating on top of largecommon-mode voltage(600 V to 1200 V max) thatalso switches at very fasttransient from the action ofthe IGBT inverter phase. Anoise immune bidirectionallevel-shifting circuit is usedto avoid false common-mode dV/dt noise up to50 V/ns.

In theory, the currentsensed in the motor phaselead should be purelysinsuoidal, as shown inFig. 2. But in reality, a trian-gular wave (an artifact of thePWM signal) is superim-posed on that current-sense signal, creating the jaggedwaveform drawn in Fig. 4. That superimposed triangularwave must be filtered out for accurate current sensing.However, doing so with simple capacitive filtering addsphase delay.

To avoid this problem, the HVIC in Fig. 4 integrates anadvanced filtering stage with PWM synchronization. Thesignal path consists of four stages in series (Fig. 5). Thefirst two stages perform the filtering action. The third andfourth stages generate a PWM output signal and analogreconstruction to interface with the torque control loop ofthe MCU or DSP.

The input filter stage is a self-adaptive resettable inte-grator that performs accurate ripple cancellation. This stageautomatically extracts the PWM frequency from sync sig-nal and puts transmission zeros at even harmonics. Theintegral on a half PWM period has the frequency responsesimilar to a single real pole placed at the PWM frequency,so high-frequency noise is rejected. Furthermore, multipletransmission zeroes are placed exactly on even PWM har-monics, resulting in high attenuation (Fig. 6).

The second stage samples the result of the first stage atdouble SYNC frequency. This action can be used to fullyremove the odd harmonics from the input signal. To per-form this cancellation, a shift of 90 degrees of the SYNC

Fig. 5. The HVIC in Fig. 4 performs filtering and PWM synchronizationusing the four stages of signal processing shown here.

Fig. 6. The input filter stage depicted in Fig. 5 is a self-adaptive resettable integrator that passes the PWMfundamental frequency, but cancels its even harmonics (a). Simulation of the input filter’s amplituderesponse reveals that high-frequency noise in the megahertz range is greatly attenuated (b).

Fig. 7. The second stage shown in Fig. 5 samplesthe result of the first stage at double the SYNCfrequency, removing the odd harmonics fromthe input signal.

signal is neces-sary with respectto the triangularcarrier edges(SYNC2). Aphase voltage andsimplified phasecurrent of a mo-tor drive areshown togetherwith the triangu-lar wave in Fig. 7.

The trend to-ward using high-voltage IC tech-nology is acceler-ating as new in-dustrial and ap-pliance motordrives have be-come more ad-vanced in performance and more compact in size. TheseHVICs are uniquely capable of integrating advanced cir-cuit functions while operating in the high-voltage, noisyenvironment of motor-drive circuits. PETech

Multiple transmission zeroes at PWM frequency (20 kHZ) and its harmonics

Baseband signalis left unchanged