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I am designing a driver circuit to drive a hydraulic proportional valve from an IC's PWM output. (IC is ATmega 2560). The coils on the valve draw approx 3Amps at %100 duty cycle with a power supply of 28v.

MOSFET's seem to be the way to accomplish this, thou I am struggling to find a 'safe' solution. The two primary methods I have so far come up with are;

A logic level MOSFET: (Please note R7 and R10 are the coil)

enter image description here

Or using an op-amp to boost the PWM signal and use a higher Vth MOSFET:

enter image description here

It seems the logic level version has a ~1v drop across Q3 when driving the coil, and I am concerned about that causing damage (heat, power loss etc)

Which of these (or any other way) would be the best way to provide control of an inductive load? (Or have I totally ended up with an incorrect design?)

(EDIT: Please note that protecting the IC from any single driver failure is paramount)

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  • \$\begingroup\$ You can probably use the first configuration. You just need to find a logic level FET with lower Rds(on). If you want to protect the IC, you will need to put some kind of robust over-voltage protection on the gate. This is just in case the FET fails, in which case the gate could become energized at an elevated voltage. You can also use a FET gate driver IC. It may need a 10V-15V rail, but you can create that with some crude method from the 28V. Like a zener follower or something. \$\endgroup\$
    – user57037
    Feb 25, 2018 at 4:37
  • \$\begingroup\$ Not sure what the purpose of D3/D5 is. \$\endgroup\$
    – user57037
    Feb 25, 2018 at 4:41
  • \$\begingroup\$ The FET failure is my primary concern,(if one driver fails; the physical world 1 door fails, thats ok, but taking out the IC and failing all 3 would be bad.) I will look into FET gate drivers, thanks. D2/D3 are collapsing field protection for the FET. (R7 and R10 are solenoid coils, used R's for sim performance) \$\endgroup\$
    – Josh Cronin
    Feb 25, 2018 at 4:53
  • \$\begingroup\$ D3 and D5 do not make sense to me. When the FET turns off, the drain voltage will be clamped at around 30V by D2 or D4 (depending on which circuit you are looking at). D3 and D5 are in parallel with the intrinsic diode in the FET. \$\endgroup\$
    – user57037
    Feb 25, 2018 at 5:04
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    \$\begingroup\$ No reason for R6 to be that high, you will just reduce Vgs and increase Rds(on), especially if it's a 3V3 logic output. Drop it to 100 Ohms. \$\endgroup\$
    – Finbarr
    Feb 25, 2018 at 5:40

1 Answer 1

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The first circuit is close to what you need.

If you are expecting to use PWM for linear positioning, you really need to dissipate the energy in the proportional coil very rapidly. The best way to do that is to allow the voltage across the coil to rise to a controlled value on turn-off.

In the schematic below when the PWM turn off the voltage rises to about 34 V which quickly dissipates the energy from the valve coil.

The MPT10N10EL gets exposed to about 60 V just after turnoff, well within its rating.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Oh fantastic. I haven't done much with MOSFETs so choosing one was daunting. I see your point about allowing the voltage to rise, i had not thought of that. Thanks. \$\endgroup\$
    – Josh Cronin
    Feb 26, 2018 at 2:14
  • \$\begingroup\$ What did your draw that in? I keep seeing these style schematics around, they look good. (Sorry: off topic, just curious) \$\endgroup\$
    – Josh Cronin
    Feb 26, 2018 at 2:15
  • \$\begingroup\$ @JoshCronin ...when you provide an answer there are tools to insert links, images and make a schematic (it even has simulation). \$\endgroup\$
    – Jack Creasey
    Feb 26, 2018 at 4:44
  • \$\begingroup\$ the MPT10N10EL is obsolete now, any good (read: easy) way to find comparable items without crawling through data sheets all day :) \$\endgroup\$
    – Josh Cronin
    Feb 26, 2018 at 12:12
  • \$\begingroup\$ Choose from the Digikey menu: digikey.com/products/en/discrete-semiconductor-products/… Perhaps something like the FQD13N10L: fairchildsemi.com/datasheets/FQ/FQD13N10L.pdf \$\endgroup\$
    – Jack Creasey
    Feb 26, 2018 at 17:12

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