[Diy_efi] Measuring Pulse Width/Duty Cycle of an Injector?
niche at iinet.net.au
Tue May 24 04:49:09 CDT 2005
At 03:47 PM 24/05/05, you wrote:
>--- Steve Ravet <Steve.Ravet at arm.com> wrote:
>> I haven't see all driver circuits out there but I
>> think most of them don't duty cycle the injector,
>> they just reduce the voltage applied to the
>The only designs I have seen that do this are Bosch
>systems. The advantage is that the output transistor
>is always driven at 100%, when driven (duty cycle is a
>straight square wave), so heat is greatly reduced over
>a regular P&H setup. The drawback is broadband EMF. :(
Not necessarily, well rather it might be the case that
designers who dont care about EMC switch at the highest
slew rates because its an easy design and probably careless
as it can cause parasitic oscillations.
EMC issues can be easily addressed by handling the slew
rate effectively, turn off as well as turn on time - yes this does
increase the die heating but not significantly, in my experience
depending on the duty cycle it only adds 2 to 5% to device
ie. You can easily see several MHz emissions from FETs
with a slew rate drive of less an 1uS when operating with a
PWM of around 50 to 100Hz. A proper commercial design
handles and provides proper RF decoupling at the FET gate
and hopefully at the source to drain capacitive coupling,
so slew rate might be increased to say 100-500uS and the MHz+
emissions are completely eliminated.
Having said that vehicle EMC allowances are almost as
wide as mobile phone concessions - which allow the
mobile to walk over almost all the available VHF and UHF
spectra provided its for a "short period" (tm).
Typical case of huge commercial pressures from the mobile
phone makers trampling over interest in minimising
cross spectral noise... :(
My mobile and that of visitors often couples into the infra
red (yes I did say infra red) headphones across the room
and the PC amp and the monitor embedded speaker amp
when the mobile does its ping/pong with the local cell,
which is from 1 in 10 to 1 in 200 seconds.
Hence most GSM/CDMA mobiles are prohibited from
use near medical and aviation equipment and quite rightly so !
Perth, Western Australia
VL Commodore Fuse Rail that wont warp or melt !
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