Re: Speed control

[Subscuba@aol.com]

I've been out of town for a while and in reviewing the Psubs messages I came 
across the references to the IGBTs. I have some comments about the use of 
IGBTs in low voltage motor controllers.

1) The IGBT (Insulated Gate Bipolar Transistor) is a combination of a bipolar 
transistor and a MOSFET. The collector of the device is actually the emitter 
of the bipolar device which is confusing. Since the base of the bipolar 
transistor is driven by the MOSFET the device has a fairly high offset 
voltage caused by the emitter base voltage of the bipolar and the voltage 
drop of the MOSFET. This offset voltage generally restricts the use of IGBT 
to high voltage circuits.

2) An example is a 12 volt trolling motor drawing 40 amperes from a set of 
batteries providing a nominal 24 volts. Due to switching spikes a voltage 
rating of 200 volts will be used. A switching frequency of 20 kHz will also 
be used. Initially a IRGP450U IGBT from International Rectifier is selected. 
This device has a Vces voltage rating of 500 volts which is nice and is more 
tolerant of voltage spikes. The current rating is 33 amperes so two devices 
will be required. This means that each device will switch 20 amperes. The 
duty cycle is 54% for the 24 volt source and 12 volts out. The voltage offset 
or Collector-to-Emitter Saturation voltage is about 1.8 volts at 20 amperes. 
This means that each device will be required to dissipate 19.44 watts or 38.9 
watts total for both devices. This neglects switching losses and whatever 
losses are involved in making the devices current share. Making the devices 
current share will require active circuits or magnetics that add losses. 
Paralleling more than two devices is a pain.

3) Using an IXYS IXTH 68N20 instead of the IGBT the voltage rating is 200 
volts with a current rating of 74 amperes. Again using two devices the power 
dissipation is calculated as 7.21 watts per device or 14.4 watts total. The 
drop across the switches is calculated as 0.7 volts. If three devices are 
used which is easy with MOSFETs the total dissipation goes down to 9.6 watts. 
Switching losses with the MOSFETs are almost negligible and paralleling the 
devices is also easy simply requiring separate gate resistors.

4) Losses with the IGBTs will be almost 40 watts, the MOSFETs will dissipate 
14.4 or 9.6 watts. The MOSFETs will clearly provide a more efficient simpler 
motor controller. This efficiency is important since it equates directly to 
battery capacity requirements ( a bigger battery or less cruising time).

5) The above discussion applies only to low voltage motor controllers. When 
the motor and battery voltages get above or near 100 volts the device 
selection gets more complex. At higher voltages the IGBTs probably have an 
advantage due to lower cost and higher current ratings.

6) The commercial low voltage motor controllers almost always use MOSFET 
primarily for the above reasons.

7) Due to silicon utilization the IGBT will continue to be lower cost but 
will be less efficient in low voltage circuits.

8) Gate drive power is small for both devices. Switching losses and offset 
voltages are higher in IGBTs. This is due to removing base charge from the 
bipolar. Turn off losses are much higher than MOSFETs in conventional 
circuits.



For what it's worth I designed and built my motor controller for my Sub's 
motor which is a 24 volt motor drawing up to 65 amperes from a 36 volt 
battery bank. I used 4 parallel MOSFETs. I also recommend buying a commercial 
motor controller for your use unless you have a background in power 
electronics. The cost of developing your own will be more expensive 
generally. 

Ken Martindale

In a message dated 5/15/99 12:30:22 PM Eastern Daylight Time, 
rmorrisson@unidial.com writes:

<< Thanks Dave,
 I went to the site and saw all the nice regress pictures (p-n junctions),
 read about half of the text and remembered why I am an ME.  Actually, I
 didn't see what the significance of the difference was between a MOSFET
 and an IGBT was in actual practice.  Does this device handle more
 current?, voltage?, use less power to turn on?, or what?  I purchased a
 couple of PWM controllers recently to use in variable speed diver
 propulsion units.  They were surplus and therefore not expensive.  If I
 start making these for sale I will be looking for low cost units which I
 can get on a regular basis.  I would even consider making the units if the
 design was robust and the parts could be obtained cheaply.  Could these
 IGBT's be used in a 12vdc, 30 amp PWM cost effectively?  A simple answer
 will suffice for now, details could follow later if appropriate.
 Thanks for your inputs, >>