EETimes India - November 16, 2008 - (Page 8)

In Focus | Data acquisition Function gens simplify measurement continued from page 1 be configured to match different source and/or load impedances. Commonly, boost amplifiers do not feature a display so the effective output amplitude must be monitored with an oscilloscope or other measurement device. This adds to the complexity of the measurement set-up and requires additional time, especially when amplitude levels need to be adjusted and verified before and during the test. MOSFET switching time Power MOSFETs are used in a variety of automotive motion control, power management and climate control applications. They drive small motors, solenoids, anti-lock brake, electrical power steering, and more. They are also a key component of integrated starter/alternators. When used as a switch, the MOSFET’s basic function is to control the drain current via the gate signal. In these applications, switching time is an important criteria considered by circuit designers during the component selection. A MOSFET’s switching performance is determined by the time required to establish voltage changes across its internal capacitances. Note that the gate-to-source voltage must first charge the MOSFET’s input capacitance to its characteristic threshold level before drain current conduction can start. Time related parameters of interest are turn-on and turn-off delay, as well as rise and fall time. To measure these parameters, the MOSFET’s gate is stimulated with a narrow pulse from the signal generator input. The gate and drain voltages are measured with an oscilloscope (see figure 2). Using an arbitrary/function generator with integrated high amplitude output stage instead of an external amplifier gives the user direct visibility of the effective signal amplitude at the MOSFET’s input circuit without the need to measure it with an oscilloscope. The turn-on delay can now be determined conveniently via cursor measurements on the trace displayed on the oscilloscope screen. Turn-on delay is the time it takes from the moment the gate-to-source voltage reaches 10 per cent of its final value until the drain-tosource voltage declines to 90 per cent of its initial value. Similarly, turn-off delay is the time taken from the moment the gate-tosource voltage declines to 90 per cent of its previous level until the drain-to-source voltage has risen to 10 per cent of the supply voltage. For the measurement of the drain signal’s rise and fall times, modern oscilloscopes offer convenient automated measurements. Switching IGBT waveforms In recent years, insulated gate bipolar transistors (IGBTs) have been increasingly used in industrial and automotive applications as replacement of MOSFETs, thanks to their high switching speed, high current capabilities, large blocking voltages, and simple gate drive characteristics, in addition to lower conduction losses and lower voltage drop in the on-state. Industrial applications for IGBTs include traction, variable speed motor drives, UPS, induction heating, welding, and highfrequency switch mode power supplies in telecom and server systems. In automotives, IGBTs are used for ignition coil driver circuits, motor controllers and safety related systems. IGBTs are a cross between bipolar transistors and MOSFETs. In terms of output switching and conduction characteristics, the IGBT resembles the bipolar transistor. However, while bipolar transistors are current controlled, IGBTs are voltage controlled like a MOSFET. To assure full saturation and limit short circuit current, a gate drive voltage of +15V is recommended. Like a MOSFET, an IGBT has capacitances between gate, emit- Figure 1: Measurement setup with external amplifier. Figure 2: Set-up for measuring switching time of power MOSFET. Figure 3: Switching waveforms of an IGBT. ter and collector. When voltage is applied between the gate and emitter terminals, the input capacitance is charged up through the gate resistor R G in an exponential fashion until the IGBT’s characteristic threshold voltage is reached where collector-to-emitter conduction is established. Likewise, the input gate-to-emitter capacitance must be discharged to a specific plateau voltage, before collector-to-emitter conduction is interrupted, and the IGBT turns off. The size of the gate resistor significantly impacts the dynamic turn on and turn off characteristics of the IGBT. A smaller gate resistor charges and discharges the IGBT’s gate-to-emitter capacitance faster, resulting in short switching times and small switching losses. However, a small gate resistor value can also cause oscillations due to the gate-toemitter capacitance of the IGBT and parasitic inductance of the leads. To reduce turn-off losses continued on page 11 EE Times-India | November 16-30, 2008 | www.eetindia.com http://www.eetindia.co.in/SEARCH/SUMMARY/technical-articles/oscilloscope.HTM?ClickFromNewsletter_081116 http://www.eetindia.com/STATIC/REDIRECT/Newsletter_081116_EETI02.htm?ClickFromNewsletter_081116

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EETimes India - November 16, 2008 Contents Farnell Embedded System Eases Rail Maintenance National Semiconductor Working With IT for Networked DAQ Digital Telemetry Advances Torque Measurement Events

EETimes India - November 16, 2008

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