Electronics > Beginners
Solenoid driver MOSFET flyback protection
HwAoRrDk:
I want to drive a 12V vacuum solenoid with a MOSFET, and to drive it I had in mind to use a nice component I came across a while back, the ON Semi NUD3124 - an N-channel MOSFET, protection diodes and bias resistors all packaged conveniently in SOT-23. However, the solenoid I have draws more current (>420mA) than the NUD3124 can handle, so that is unfortunately out. What I thought to do instead was replicate the integrated parts of it with discrete components - a bigger MOSFET, diodes, resistors, etc.
But, I am not quite 100% on understanding why some of the NUD3124's integrated component parts are like how they are, and I was hoping someone could offer some explanation.
The diagram of the integrated components, from an associated app note, is as follows:
As I understand it, this is an active clamping arrangement; when the solenoid's turn-off flyback voltage spike occurs, this passes through the 28V clamping zener and turns the gate back on, thus routing transient current through the MOSFET.
My questions about this are:
- Why are there two pairs of 14V gate zener diodes? Obviously, they are not exactly in parallel - there is a 10K resistor between - but it still seems to me the left-most pair is redundant. :-//
- Also, why are those zeners 14V when the datasheet says the Vgs has an absolute maximum of 12V?
- What's stopping all the transient current flowing through the 28V and 14V zeners to ground, instead of partially through the FET as described?
Also, if I want to reproduce this arrangement with discrete components, does it particularly matter what value of zener diodes I use on the gate, so long as it is above my gate drive voltage (5V) and below the maximum Vgs? I already have some 5.1V zeners, so was thinking of using those. Also an 18V bi-directional TVS diode for the clamping between drain and gate.
Benta:
The NUD3124 probably has a lot more protection than you need, it's designed for automotive applications where a load dump carries a lot of power.
In the simplest application (like your solenoid), the upper 28 V Zener is likely all that's needed.
The leftmost 14 V Zeners are not there to protect the device, but the circuit driving it. The next set of 14 V Zeners are for protecting the MOSFET gate. And the lower 28 V Zener is for clamping a negative spike during a load dump.
You decide, it depends on your application.
HwAoRrDk:
--- Quote from: Benta on November 26, 2018, 05:33:48 pm ---The NUD3124 probably has a lot more protection than you need, it's designed for automotive applications where a load dump carries a lot of power.
--- End quote ---
Ah, well, see, it will actually be used in an automotive application. :)
--- Quote from: Benta on November 26, 2018, 05:33:48 pm ---The leftmost 14 V Zeners are not there to protect the device, but the circuit driving it.
--- End quote ---
Okay. I still don't understand why one set of 14V zeners doesn't serve as double duty for protecting both the gate and the driving device (i.e. MCU). Or this just a belt-and-braces approach?
Ian.M:
The left-most Zeners protect the 10K and 100K resistors against flash-over during ESD events which could erode the (presumably thin film) resistance element and eventually cause them to go open circuit.
5.1V zeners are a *bit* too close to 5V CMOS logic levels. Consider what happens if they are 5% under their nominal zener voltage and also what happens if the5V rail is slightly higher than expected.
A TVS diode, unless very low energy handling capacity, is likely to have excessive junction capacitance, resulting in significant extra dissipation in the MOSFET during switching due to the Miller effect.
HwAoRrDk:
--- Quote from: Ian.M on November 26, 2018, 11:36:09 pm ---The left-most Zeners protect the 10K and 100K resistors against flash-over during ESD events which could erode the (presumably thin film) resistance element and eventually cause them to go open circuit.
--- End quote ---
I presume the resistors are susceptible to that because they are on-die and so are physically very small? So I guess that pair aren't at all necessary in a discrete component circuit, because there is little chance of such happening to an SMD or THT resistor.
--- Quote from: Ian.M on November 26, 2018, 11:36:09 pm ---A TVS diode, unless very low energy handling capacity, is likely to have excessive junction capacitance, resulting in significant extra dissipation in the MOSFET during switching due to the Miller effect.
--- End quote ---
Not familiar with that phenomenon, will have to look it up.
As a wild stab in the dark, I would guess capacitance in the diode causes turn-on and -off to be too slow, making the FET spend too much time in the linear region?
I plan to PWM the solenoid to reduce hold current, so such a side-effect would probably be exaggerated under that condition?
I have some regular 18V zeners, though - just thought a bi-directional TVS would be convenient as it's both diodes in one component.
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