I've got a question:
The TIP122 Drives a 5amp load and IRLZ44N drives a 40 amps load, but in some designes(PICLIST, PMINMO) i saw that they are used for a very smaller currents.
I can't undrestand this,shoud i mention this in my designes? or i can use the IC's in their nominal current,without any problem?
I've got about 20 darlington array(STA403A)
how can i get more current by using them in parallel or serial configuration.
how can i get a current about 4 amps from STA403.
can i use darlington configuration for driving unipolar motors?how?i need a complete description,please help me.
thanks.
There are dramatic differences in FETS (IRLZ44) and Bipolar Transistors. It starts at the difference in gate/base drive techniques and includes the existence of the FET body (intrinsic) diode for dumping reverse current from inductive loads like motors. FET's are voltage devices.
The gate of a FET is turned on with volts (usually at least 5 and and high as 15VDC above the Source. Bipolar transistors are current devices with a predictable "Gain" (Hfe). Typical gain at higher currents is only in the 10 to 20 times range so you have to provide high current base drivers. Most power transistors do not have the high frequency range of FET's (won't turn on as fast). Slower turn on comes with higher dissipation per pulse. All of these factors means the designer has to look at the application before committing to the power output stage. General rule of thumb is that when the voltage is 100V or lower and the current is high or the load is highly inductive the FET wins. If the voltage is high and load is high the bipolar starts to look attractive if nothing else that high voltage power devices are usually cheaper.
You cannot simply substitute a Transistor in a circuit design for FETS or vice versa.
(exception: IBGT's for FETS for high voltage switching)
Because transistors are current devices you cannot easily run them in parallel to gain higher current through multiple parallel devices. If the current is slightly different the transistor with the lower emitter to collector turn-on spec will conduct more current. Driving parallel transistors is a nightmare. FETS are easy to parallel with virtually little change in the driver or problems with current imbalance. There are extensive articles in the Mfg's specs (see International Rectifier application notes for driving FETS).
Running either device type in series makes the drive electronics much more complex.
Bipolar transistors are easy to destroy with reverse current flow so you need external "freewheeling" diodes to allow reverse current to flow back to the power source around the emitter-collector junction for inductive loads (Snubbers).
None of the current ratings mean anything as far as the amount of power the device can handle. You have to look at the Safe Area of Operation curves. You are basically limited by the physical internal connections (in Amps) for pulsed current BUT the amount of power dissipated for the given package limits the CONTINUOUS current rating. The power dissipation across a Transistor is the voltage dropped times the Current times the duty cycle of the waveform. In a transistor the voltage drop is relatively predictable (VCE[sat] being from 1.2 for smaller currents up to 4V for larger darlington devices) In a FET the voltage drop is a function of the "on resistance) of the device. Typically high current lower voltage FETS have very low on resistance (IRLZ44 = .028 ohm) so in the sub-100V range the FET is more efficient than the same size transistor. Add to that the faster turn-on (time to get in and out of saturation) of the FET and it proves to be superior in switching PWM type loads.
At say 10A of current with a 70% duty cycle the IRLZ44 would dissipate 1.32Watts plus about 10% switching loses making it need little or no heatsink.
At 10A with a 70% duty cycle the TIP122 (actually outside its SOA curves) would drop approx 4V (from the spec sheet the power dissipation is 28W plus about 20% switching loses to make it around 31W of power. Free air (no heatsink; it will get very hot in a few seconds)
In an H bridge application getting the transistors to not cross conduct and destroy each other at frequencies above about 5Khz creates the need for wider dead time control.
So there is a short tutorial on just one tiny aspect of power circuit design.
TOM Caudle www.CandCNC.com
OBOPE (Old Burned Out Power Engineer)
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stepper drive+irlz44 or tip122
