A fly back diode is just a diode that will conduct when the power to inductive load is turned off. My question is, how does one decide on the parameters for the diode so we can choose the correct part? The fly back diode shall be used with DC motor. Is it possible to just choose any diode and it will do?
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asked Apr 13, 2023 at 1:15
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1\$\begingroup\$ I would expect current rating and reverse voltage rating \$\endgroup\$– saiCommented Apr 13, 2023 at 1:22
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1\$\begingroup\$ Technically, you need to know the worst case current decay waveform that will result when you switch off the motor in the presence of the diode. This will determine how much power the diode needs to dissipate. However, you usually do not exactly know or bother to measure this waveform and the diode datasheets will not give you a direct parameter to compare this waveform's power against. \$\endgroup\$– DKNguyenCommented Apr 13, 2023 at 2:37
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\$\begingroup\$ Does this answer your question? How to choose a flyback diode for a relay? \$\endgroup\$– jonathanjoCommented Apr 13, 2023 at 6:20
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1 Answer
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To be very objective: Reverse Voltage, Max Power and Switching Speed.
1 - It must withstand your load voltage - so diode reverse voltage must be higher than your load's nominal voltage.
2 - It will briefly get all load current - but it happens so quick, so you dont need a giant diode with huge current capability. You must figure out for how long the transient lasts and how much power it have, and then chose the diode based on power dissipation needs, not just load current (simulations are great for this).
3 - The faster it responds the better - you trying to get rid of the voltage peak on your switch and surrounding circuit so the faster the diode starts conducting current, the lower the voltage peak. It matters a lot when using PWM, in this case you would need a Schottky instead of a regular Rectifier.
Thats basically it, other stuff get really specific depending on your circuit.
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\$\begingroup\$ I won't be able to do simulation if I am using a motor right? I mean, I would need to know the inductance of it and only then can I run a SPICE simulation. \$\endgroup\$ Commented Apr 13, 2023 at 4:00
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1\$\begingroup\$ @quantum231 do you have the manufacturer's datasheet for the DC motor? Taking the example of Permanent Magnet DC Brush Motors Technical Data Sheet from MOOG there is a Terminal Inductance specified for the motors. \$\endgroup\$ Commented Apr 13, 2023 at 9:28
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1\$\begingroup\$ If you have access to a scope, you can measure the spike directly on your switch. Assuming you're using a MOSFETs on DC, you can measure its Vgs, and find out instantly how high and how long lasts your transient, and then you simulate. Way faster, cheaper and hands on. My preferred way to go. \$\endgroup\$ Commented Apr 13, 2023 at 18:42
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\$\begingroup\$ I cannot name a specific motor at this time but I will keep both of your inputs in mind. \$\endgroup\$ Commented Apr 14, 2023 at 4:01