Design and analysis of contactor coil control circuit

As a typical device of small current control and large current, contactor is widely used in the field of automation design and intelligent control , Here is a brief introduction to my understanding of contactor control circuit .
1. Typical control circuit

2. working principle
K1 For contactor ,D1 Schottky diode ,Q1 by mos tube , When the control signal RELAY For high voltage ,Q1 Conduction ,B The point is low , The coil generates magnetic force , Contactor pull . here IQ=200MA,ID=0MA.
When the control signal RELAY Low power level ,Q1 end ,B The point is high level and will cause B The point voltage is greater than 24.55V, be D1 Conduction B The point is clamped at 24.55V, Not quite right MOS Tube damage .ID=200MA, Formation of current storage , When ID Less than 80MA The contactor is disconnected .
3. Disconnection time analysis
The above circuits have no problem in common application scenarios , But when there is a requirement for the breaking time of the contactor , May not meet the requirements , The following analysis .

about K1:
------------------------------①
be △ID=200-80=120MA, Voltage across the inductor U=0.55V, The measured equivalent inductance of the coil is 35mh, be △t≈8MS. Plus the inherent breaking time of the contactor 10-15ms, Then the total disconnection time will be greater than 20ms, Unacceptable in many application scenarios .
4. Rectification direction
from ① Type it , If you want to reduce △t, Can pass , Reduce L and △ID, increase U, because L and △ID It is the inherent property of contactor , You cannot change , Therefore, our adjustable means are only limited to increasing the voltage at both ends of the coil when it is turned off .
1： Available in diode D1 Series resistance , Increase the voltage .
2： A slightly higher threshold can be used TVS Tube as clamping device .
Note the following ：
1,MOS Pipe selection needs to be improved VDS Breakdown voltage ;
2,LAYOUT when , Contactors need to be handled carefully ,MOS tube , Connection between diodes , Do not get too close to the signal line , Not too long , It will affect radiation emission, etc EMC experiment .

5. Experiment related waveforms

chart 1 No diode, no TVS Disconnection time

chart 2 There is reverse diode disconnection time

chart 3 Add resistance to the diode