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

Asim Khan 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.
>

Why are you using Bipolars at all, and Darlingtons, even worse? I have
a full-bridge
PWM driver running at 100 KHz, using IR Hexfets. They run cool except
when delivering
large currents to the motor.

>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
>
>

Well, there must be some kind of crosstalk that is causing one of the
drivers to turn on
due to other signals (most likely the switching transients from one of
the Darlingtons)
on the board. layout is extremely critical, and control signals have to
be carefully
routed away from high dV/dT and dI/dT power signals.

>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.
>
>

The output transistors are not at fault. The driver circuit is. I note
you have made
no provisions for removing the stored charge on the Darlington
transistors when you
want to turn them off. This will lead to extremely long turn-off times,
easily tens
of uS, which is obviously way too long for a 10+ Khz PWM drive. A quick fix
might be to place resistors across the B-E terminals of the PNP
Darlingtons to
remove the stored charge. Something around a few K Ohms should work, the
lower the resistance, the better, as long as it doesn't prevent the
Darlingtons from
saturating. You could also put small caps, perhaps 470 pF, across R3, 4
7 and 8, to
improve the response to the input signal. Since the drive for the NPN
Darlingtons
is cross-connected to prevent them from being turned on until no current is
flowing in the DRIVE transistors for the PNP Darlingtons, the drive circuit
is probably not responsible for the trouble just because it is too slow
to respond to
the input. So, that pretty much points toward stored charge.

Also, know that bipolar transistors get more sensitive when they get
hot, and
suffer from a condition called secondary breakdown. When hot, and
delivering
a lot of current to the load, especially under short-circuit conditions,
(as when
both PNP and NPN transistors are conducting at the same time) so that the
C-E voltage is high, the transistor cannot be turned off! The ultimate
failure
you see is secondary breakdown, but the cause is not draining the stored
charge
from the PNP transistor.

Jon