Driving a proportional solenoid valve with no flyback diode

crutschow

Joined Mar 14, 2008
34,281
crutschow, the details aren't necessarily proprietary but its just a small component of a larger system. It has to do with reverse polarity protection. I don't want to sway from the discussion but I'm doing reverse polarity protection for a maximum of 50 A at a system level. These flyback diodes on low-side drivers cause a lot of grief when it comes to this protection under some fault conditions.
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Is the normal operating current also 50A? There are ways to protect against reverse polarity using a series MOSFET.
 

Sensacell

Joined Jun 19, 2012
3,432
The whole point of driving a magnetic actuator with PWM is to use the energy storage property of the coil to efficiently control the average current, minimizing loss.

Not having a recirculating path (back-EMF diode) means you are dumping this stored energy into the switch (avalanche mode) in a wasteful and potentially damaging manner. The only reason to not use a diode is:

1) To have the fastest current decay (response time) when turning it off.

2) If you need bipolar operation- even here, the drive electronics must provide a recirculation path.
 

THE_RB

Joined Feb 11, 2008
5,438
Agreed! If not maintaining proper recirculating current (ie with no diode) the entire magnetic field is dumped each cycle, causing big EMI issues and big heating of something (snubber) that needs to dissipate all that wasted energy.

Then every PWM cycle there will be big current through the drive FET to build that magnetic field back up again.

Making things worse is that the average magnetic field is lower because there is no field in the PWM LO period. So the PWM HI period needs to have much higher current to give the correct average current in the coil.

It's a really dumb way to do it when the solenoid device requires a steady current.

Use a diode, and deal with the "reverse polarity protection" in some other way.
 
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