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

Asim,

There is no problem with the bridge circuit as designed. The problem
is the circuit will not function at frequencies above a kHz or so
without overheating.

The cause for the heating is due to the saturation delay time of the
Darlington transistors; as designed this time is around 20uS or more.
During this time, the "off" transistor continues to conduct while the
other transistor in the half-bridge is turned "on". That results in a
tremendous "shoot-thru" current, which overheats both transistors.

Darlingtons are used for their large gain; you cannot replace them
with faster standard transistors in this circuit because the base
drive cannot provide enough current. At 30A Icc a standard transistor
would require a 6A base current. That is about a 1,000 times more
than the base drive circuit can deliver. The transistors will burn up
because they cannot saturate.

Mariss

--- 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 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!!
> It seems the the right side PNP darlinton recieves the driving

pulses

> when it should not get driven. since in an hbridge at a single

moment

> there only two transistors should conduct, and in my case 3
> transitors starts conducting which causes so much high current
> through the transitor.
> same happens when i apply the PWM signal at right hand side, but

this

> time the left side PNP darlinton starts getting heatup
>
> 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.
> I need to design a professional dc-servo drive which can operate

upto

> 80V and can deleiver around 20-30AMP.
>
> 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 !
> i request everyone to please suggest me any designs and or links
> best regards
> @sim