×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Electro-Magnet drop out time

Electro-Magnet drop out time

Electro-Magnet drop out time

(OP)

I'm trying to decrease the amount of time it takes for an electromagnet to actuate. Currently we're driving it with two MOSFET's, to drop out both sides of the magnet. The MOSFET's im using are 300V, N-CH, link below. I am familiar with the idea of letting the inductive spike peak to decrease coil drop-out times, hence the reason for going with 300V MOSFET's with a 12V magnet. I was hoping that the 300V MOSFET would not clamp, therefore allowing my spike to quickly dissipate, but in reality I am clamping at roughly 16V.

Any one have any insight?





http://www.fairchildsemi.com/ds/FQ/FQP22N30.pdf

RE: Electro-Magnet drop out time

Could it be that it's not the MOSFETs that are "clamping" but some other parasitic conduction path around the coil? I would suspect parasitic capacitance in the wiring, except that the dV/dt during discharge is quite low. Instead, could there be insulation breakdown initiated by the high dV/dt (and perhaps hidden overshoot) at the start but then sustained as a small arc for the 4.3ms? It's just that an arc has roughly Zener-like behaviour once it is established, which is kinda like what you're seeing. 16V is about a centimetre of arc - could this be occurring, perhaps within the electromagnet itself?

RE: Electro-Magnet drop out time

An electromagnet is essentially an inductor, so that means that it has a time constant associated with the series resistance in the circuit.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Electro-Magnet drop out time

Use an H-bridge and reverse the voltage to the magnet for some small period.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Electro-Magnet drop out time

Nice and smooooooooooth. The problem dear Brutus is not in our fets but in our circuits. Looks like open gate capacitance. If I had to guess it looks like it is driven by a microprocessor where the output pin goes fron HIGH to floating. Pop up a schematic.

RE: Electro-Magnet drop out time

(OP)
Thanks for all the replies, Im not sure we've hit on the solution yet.

LiteYear- To disprove this I decided to use a SPDT switch and drive it manually. The inductive spike I see is 800uS or so long, with an amplitude of about 350V.

IRStuff - I agree, but my experiment above has shown otherwise. Also, if this was an RC decay I dont think I would see the plateau at 16V. What do you think? The magnet is roughly 42mH.

itsmoked - That's a good idea, although I may not be able to from a fail safe stand point.

OperaHouse - The low side fet is driven by a JK flip flop (74HC109), and the high side is driven by an LTC1154 gate driver which is push pull. I will post the schematic in a few hours.

Thanks!



RE: Electro-Magnet drop out time

(OP)
Here is a portion of my schematic, the signals C1H, C1L, C2H and C2L are all generated by a JK flip flop.

RE: Electro-Magnet drop out time

That's even more creative than I thought.

RE: Electro-Magnet drop out time

(OP)
Thanks, but any ideas?

RE: Electro-Magnet drop out time

(OP)
Figured out my problem... The SP721 is a transient voltage suppressor,going against everything I want to do...

RE: Electro-Magnet drop out time

Can you get it in a higher-rated snubbing voltage?

RE: Electro-Magnet drop out time

Come on guys. Does anyone here really like that circuit?

RE: Electro-Magnet drop out time

One obvious problem is that the high-side driver should be a p-channel to eliminate the threshold and body bias drop from the source following n-channel.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Electro-Magnet drop out time

Why switch both high and low sides?

RE: Electro-Magnet drop out time

(OP)
IRStuff - I'm not sure i follow you completely. With the use of a gate driver why is a p-channel needed?

btrueblood - This is to prevent failure, in the case that either fet fails to drop out.

RE: Electro-Magnet drop out time

Gate drive = 12V
Typical 300v MOSFET threshold = 3V
Maximum possible source voltage = 9V
1A IDS required VGS = 6V --> maximum output voltage = 6V

This is freshman EE stuff here.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Electro-Magnet drop out time

What would cause the FET to fail? Use a higher-rated FET?

Not sure why you chose the SP721 to snub the magnet coils. I'd put something more mundane, like an opposed Zener pair, or Zener and standard diode if only switching one side. That at least gives you flexibility to pick the zener voltage as high as you are comfortable with, and speed the coil decay time.

RE: Electro-Magnet drop out time

(OP)
IRStuff - I am following what you say, but my gate drive is not 12V. It is roughly 30V, the LTC1154 is a charge pump driver.

I apologize for my lack of knowledge on the subject. Its been a long time since freshman year, especially when not designing mosfet circuits on a regular basis.

RE: Electro-Magnet drop out time

Just what are you trying to accomplish with this, speed,rep rate, wattage if not achieved by removing
suppressor. The drive voltage seems low if you are also looking for fast on actuation too.

A few years back I was involved with a glue application manufacturer. I was design a driver to
place evenly spaced dots of glue with a speed of 2ms on and 2ms off. That required quite a bit of
playing with pressures and sprong ernsions and was no were near a saleable product. Of course the
stroke length was also next to nothing and it would run hot. This grew out of an initial request for
a fairly fast driver that would not heat up the solenoid and thus change glue characteristics. We
drove the 6V coil with nearly 50V and then PWM to a low hold in current after the plunger had pulled
in. There was a solenoid driver chip that did all this LM1949 and you could glean som ideas from the
application notes. The problem is that you run into mechanical speed limitations of the solenoid.
If the solenoid actually has some stroke, the actuation time can be seenby monitoring coil current
and looking for a change in slope. This can be easiest be seen by driving a relay for practice.
The closing of a relay creates a major change in inductance and thus an easily seen slope change.
This will give you the true speed of your mechanical device. Unless the core is a permenant magnet,
there is little you can do in electronics to speed up besides letting the coil see an open circuit.
We put a bunch of time in this as I wanted to add high speed solenoid drivers to our product line.
We were not being paid for any of this development work. We met with LEDEX, the supplier of solenoids
to the glue applicator company. We were both in agreement that the mechanical design chosen was
not optimal for the application. They had also developed high speed electronic drivers at the
English division but could not find a market for them. With that the plug was pulled on that
project. It was interesting work while it lasted.

Speeding up the turn on time with a higher initial voltage may make the turn off time less
critical in short pulse applications. One definite way way to speed turnoff is to lower
current once actuator has traveled to the hold in current. THe lower the magnetic field,
the faster the field will collapse. The LT high side driver is certainly slower. The statement
that the high side is driven by a JK indicates that it is an active output and not just a power
enable. It also makes it likely this is a not a microprocessor design, limiting options.

I was surprised that the 1949 is still in production. The darlington out looks like it could
easily be replaced with a FET lowering voltage losses. This chip is kind old school and even back
then it was cheaper to do it with a transistor and some gates. With a microprocessor this is
easy to do with no added cost. Currents generally are predictable and there is no need to sample
coil current We should all be using this technology when driving a relay or solenoud regardless
of speed to reduce power requirements and board heating. Power savings is easily 50%.

RE: Electro-Magnet drop out time

Interesting stuff Opera! I had to build a glue dot controller that needed to pattern dots on a line that ran at 70MPH. The very next problem they had was getting the solenoids to run faster. This was for Grayco.

Keith Cress
kcress - http://www.flaminsystems.com

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources