Wednesday, July 28, 2010

Recent Developments

Motion control industry went through a revolutionary phase after 1980 following the remarkable development took place in the fields of power switching devices, high energy permanent magnets and microelectronics.

Advanced and sophisticated power switching devices such as power MOSFET and IGBT came into being during that time. This made a significant change in the trend of power electronic control, producing inverters and converters capable of switching at extremely high frequencies well into MHz range. (The old Thyristor converters could not do more that about 1 KHz at that time.) This level of very high switching frequency is essentials to realize complex control functions.

Circuit symbol of IGBT

Modern IGBTs can handle collector current Ic upto several thousands of amperes in the ON state. Collector - Emitter voltages Vcc upto several 1000s of amperes in the OFF state. In the ON state an ideal IGBT has Vce = 0 (a practical IGBT may however have a small voltage around 1V). To ON the IGBT should apply a small CD voltage about 15V between gate and emitter (i.e. give Vge = 15V), to OFF the IGBT we should apply Vge = 0V. This is a strong device that can accept short time over current. If needs only minimal protection for reliable operation such as over current protection, thermal protection, spike voltage protection...etc. We have standard procedures to arrange these protections. Switching frequency capability is closed to MHz range.

Driver circuits for an IGBT is the circuit that can produce 15V and 0V as Vge, starting from a logic signal coming from a computer.

Driver Circuit

We can construct the driver circuit from custom assembled components, but the best option would be to use a standard IGBT-drivers, which is an integrated circuit specifically built for driving IGBTs.
Circuit Symbol of Power MOSFET

MOSFET can be handle drain current Id upto several 100s of Amperes in the ON state, and drain-source voltage Vds up to about 800V in the OFF state. Theoretically MOSFETs can block voltage drop Vds(ON) will be higher, about 4-5V. This is the reason for fixing an upper limit for the maximum working Vds. Switching ON and OFF of a MOSFET is similar to that of the IGBT in every sence. Infact, an IGBT driver can drive a MOSFET too. Switching frequency capability of the MOSFET is at the top, deep into MHz range.

MOSFET is little stronger that the IGBT and hence the same scheme of protection used with IGBT applies.

Power BJT is also an option for modern converter designs and it posses voltage and current ratings in between those of IGBT and MOSFET. Lower ON-state voltage drop, about 0.8V is and attraction of the BJT.

The level protection required by BJT is however higher and this is a kind of drawback. The need of relatively larger base current Ib to switch ON a BJT is another crucial drawback.

Circuit Symbol of BJT

(To switch on Ic = 100A. We may need a base current Ib about 4A, this is not a small current)

Darlington BJT is a variant of power BJT has removed a problem of larger base current requirement. (By fabricating another BJT at the base end in cascade). But this has led to some other issues, such as longer switching times (Eg: increased switching losses), reduced switching frequency limits and lower life span. Nevertheless, Darlington BJT finds applications more than the power BJT in practice.

Thyristor is no longer a choice in the design of present day converters in the ow to medium power range. At high power range we have no option but to go for thyristors.

Circuit Symbol of Thyristor

To switch ON a thyristor we should apply a small current pulse, about 500 mA and 100 us duration, to the gate terminal while the thyristor is in forward-biased state. To switch OFF a thyristor we have no direct method but wait until anode current Ia falls down to zero, at which point it is switched off automatically. We can try to expedite the bridging of Ia down to zero by some external support, when possible; this is what commonly known as "forced communication".

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