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Solenoid driver design

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mbless:
I have an old peak-and-hold PWM driver for injector solenoids that died on me. For those that don’t know, the peak-and-hold method uses an initially greater current (100% duty cycle) to actuate the solenoid faster and then a smaller current (lower duty cycle) to hold the solenoid open and reduce power and heat generation.

I opened it up to see if anything let out the magic smoke. I expected to see a low-side N-channel mosfet to switch the solenoid on and off, but instead it uses a high- and low-side N-channel. The attached "high and low-side driver.png" schematic shows the driver design. Note the high-side mosfet requires a bootstrap driver to function properly.

I’ve never seen this design before, so doing a search on it I found little information. The only article I found with info is https://www.edn.com/. In it is Figure 5 showing a high- and low-side N-channel mosfet. The article didn’t address the design, but user Howman said there are three states:
1.   Drive mode: both FETs on
2.   Coast mode: one FET on and one FET off. Coil current freewheels through the other FET and diode.
3.   Brake mode: both FETs off. Clamps coil voltage to bus and is higher, thus current decays more rapidly yielding faster turn off.

So my questions for EEVBlog are:
1.   Is there merit to this design and what Howman explained for the three states?
2.   Is this quantifiably better (faster turn off) than a single low-side mosfet? I tried simulating this in LTspice. The attached current trace image shows the solenoid current using single low-side mosfet (green curve) and the high- and low-side mosfet (blue curve) are right on top of each other. I see a difference in the low-side mosfet drain voltage, where the single FET design (green) has significantly more ringing. I do have my doubts that this can be simulated accurately given LTspice doesn’t capture armature movement effects on the solenoid current.
3.   Is there an even better solenoid driver design than either one of these?

langwadt:
with a single lowside mosfet you have the choice between using a flyback diode resulting in a slow turn off,or not using flyback diode and then every pwm cycle dumps energy into the mosfet

chuckb:
I don't know what it will do to the simulation but, D1 and D3 are 150 ma diodes used to pass or clamp 2 amps of circulating current.

How quickly current changes in the inductor is based on the voltage across it. With just a MOSFET or D3 clamping the inductive energy there will be 1-2 V across the inductor so the current decays slowly. With the clamp MOSFET turned off (and diode isolated) the Transorb at 30 V will allow the current to decay 10-20 times quicker. A speedup of 10-20 time is about the limit I have seen. Winding capacitance may play a part in limiting this also.

Another way to look at it is, the 0.11 Joules of energy in the inductor is dissipated quicker in the transorb (60W peak) verses the diode and the MOSFET (4 watts peak).

In 28 V 1 amp solenoid power relays I have seen core movement generate back EMF that causes a 10% change in the current rise waveform. 

mbless:

--- Quote from: chuckb on November 24, 2019, 02:19:37 pm ---I don't know what it will do to the simulation but, D1 and D3 are 150 ma diodes used to pass or clamp 2 amps of circulating current.

--- End quote ---

Good catch. I forgot to change it back to 1N5624 (3A) that's used in the driver. It doesn't affect the voltage or current traces, unfortunately.


--- Quote from: chuckb on November 24, 2019, 02:19:37 pm ---How quickly current changes in the inductor is based on the voltage across it. With just a MOSFET or D3 clamping the inductive energy there will be 1-2 V across the inductor so the current decays slowly. With the clamp MOSFET turned off (and diode isolated) the Transorb at 30 V will allow the current to decay 10-20 times quicker. A speedup of 10-20 time is about the limit I have seen. Winding capacitance may play a part in limiting this also.

--- End quote ---

That makes sense, thanks for the info. I wonder how M1 is driven that would makes D3 useful then.

chuckb:
When M1 is turned on it connects R2 to D3 allowing the inductor current to flow through D3. Otherwise the M1 internal diode would block the current flow.

What is the rise and fall time of the pulse source in the simulation? If it's to fast it may excite parasitic oscillations.

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