Re: H-BRIDGE and DC-SERVO DRIVE improvement help!

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Asim Khan" <asimtec@y...>
wrote:

> Hi forum:
>
> I had build the H-Bridge by looking at this following
> design:
>
> http://cet.ssu.portsmouth.oh.us/~jlindeman/h-bridge/hbridge.html
>
> This above design uses PNP and NPN DARLINTON BJTS TIP12X series
> in its design. This design works ok as far as the switching
> frequesny applied is no more than 3~4Khz. I had put it in an
> application where the PWM freqyncy is 11.7KHz. at high frequenies
> this design fails, burning transistors.

I did a little searching, and the original page for
that design is at:

http://www.bobblick.com/techref/projects/hbridge/hbridge.html

There is considerably more info at that location, including
this comment from the circuit's designer:

"PWM operation over 3 khz will likely lead to high losses
and more heat dissipation, due to the simplicity of the
circuit and the construction of Darlington transistors.
You might get away with higher frequencies if you put a
1K resistor emitter-base on each TIP12x transistor. I
prefer to go with very low frequencies, 50 to 300Hz."

Darlingtons are not very good for high frequency switching,
especially with such simple drive circuits. Most high
frequency designs use MOSFETS.

> I had tested this desin in my lab and i found that when you
> drive the left PNP darlinton and the right NPN darlinton by
> appliying the PWM signal at left side then the right side PNP
> darlinton starts getting heat up, when the PWM frequency is
> 10~20Khz, and eventually the right side PNP darlinton blasts!!

> I had also tried MJ11032(NPN) AND MJ11033(PNP) darlinton high
> speed switch transistors(specially designed for hi amp
> inductive load swithching) with 50AMP continuos current
> capability in my circuit. but observations were the same.

Those transistors will only deliver their rated performance
with good base drivers. When switching high current at high
speed, you need to pull current out of the base during turn-off,
not just supply current during the on-state. The design that
you copied is optimized for low cost and simplicity, not
performance.

> I need to design a professional dc-servo drive which can
> operate upto 80V and can deleiver around 20-30AMP.

If you need a professionally designed drive, buy one that
was designed by a professional. I don't mean to sound harsh,
but power electronics is not simple. I expect that you could
design a good drive, given enough time and access to good
information. But by the time you have a good solid drive
design, you will have learned enough to pass as a professional
yourself, and will have quite a pile of smoking transistors
to show for it. It would be far cheaper and faster to buy
the drive.

> please suggest what changes should i made in this schematic
> so that i can use this H-BRIDGE design for HI frequency PWM
> signals and for hi amp loads !

If you are determined to build it yourself, you need a
solid understandig of basic electronics, and then you
need to do some reading. I will assume that you will
use MOSFETs...
International Rectifier has some app notes at:
http://www.irf.com/technical-info/appnotes.htm
The following ones should be particularly usefull:
http://www.irf.com/technical-info/appnotes/an-936.pdf
http://www.irf.com/technical-info/appnotes/an-937.pdf
http://www.irf.com/technical-info/appnotes/an-944.pdf
There are other technical docs at the IR site as well.
I learned this stuff back in the days when they still
published paper databooks, and the Applications sections
in the IR and other data books were very educational.

Another site that looks promising is:
http://www.powerdesigners.com/InfoWeb/index.shtml
The on-line textbook looks interesting, although it
seems to be based mostly on theory and simulations.
There are also a number useful articles under the
"Technical Articles" link. There are seven "Basics"
articles there that you should read.

Just to give you an idea of what you are getting into,
the book review secion of this site lists 104 books
in the field of power electronics.

Good luck!

John Kasunich