This video talks about switching inductive loads with a low-side nFET. It accompanies §32.3 of Applied Analog Electronics (leanpub.com/applied_analog_el....
Heh, im glad my colleague told me during my internship to use a flyback diode for an electromagnet connected to a PLC. I didn't fully understand what it did, nor did I find the explanation on Wiki very clear. Great explanation, you filled that knowledge gap for me :)
You've just articulated the reason why I have stopped using semiconductors to switch inductors - the need to add extra components just to protect your switch. For example, I have a high-current 12V solenoid that I want to switch briefly with energy from a capacitor bank, using an adjustable timer circuit to dump the load into the solenoid coil. My system works perfectly when replacing the solenoid coil with an incandescent lamp, but when I try to sub in the solenoid, the back-emf fries even my strongest MOSFETS, IGBTs etc, no matter how high their supposed voltage ratings. It got too expensive in semiconductors, and too much trouble to solder in diodes... so I am back to using electromechanical relays to switch the current to the solenoid. Luckily the back-EMF from the relay coil doesn't seem to worry my smaller and cheaper MOSFET acting between it and the timer. Sometimes old tech is more trustworthy and resilient than newer in my experience.
Electromechanical relays are still used when you want a simple on-off function, especially when you are switching AC currents, but if you are frying semiconductors in the application, then you probably will have only limited life from the contacts in the relay. Snubbing or free-wheeling diodes are essential, though, so if you've been omitting them, then it is no wonder that you've been frying your semiconductors.
Hello, great video in detail, thanks for it. By the way, that snubber diode can be a regular 1n4007 diode or it has to be a fast recovery diode such as 1n4148? thanks
I suppose that a lot depends on how big the inductive load and how much current you are switching. I've only switched fairly small currents, and so a regular diode was fine.
You can also add a Zener Diode (in the reverse direction) in series with the normal diode. The Zener diode will allow the voltage to rise higher than Vdd+0.7, and the spike will dissipate faster. Use a diode with as high voltage as possible that your drive transistor can handle (leave some safety margin). For example, if your Vdd is 12V and your drive transistor can tolerate 35V Vds, choose a Zener voltage of 12-18 V.
thank you for describing, it's very clearly. the example takes mosfet as low side switch. if mosfet at high switch, when the mosfet turn off, the energy storing in the inductor will discharge to the parasitic capacitor of mosfet through Vdd and cause spike at drain as well? thank you!
Yes, the situation is symmetric if you switch on the high side instead of the low side. The basic concept-that current through the inductor continues to flow-still applies. The voltage spike will have the opposite polarity from the spike from low-side switching.
Great video, very clear presentation! I have 3 follow-up questions: 1. what is the physical nature of the parasitic capacitance ("where is it") ? 2. Does the voltage spike when switchig off cause electromagnetic noise? 3.What if one adds a capacitor on purpose parallel to the parasitic capasitance,,would that decrease the voltage spike?
1. The gate is a conductor, the drain and source are conducting semiconductors (and the channel, when the FET is on). There is an insulating oxide material between the gate and source/drain/channel. Hence, a capacitor (the Miller capacitance). But the capacitance being discussed here is mainly wiring capacitance-any pair of conductors that are not connected together are insulated from each other and so have some capacitance dependent on their size, position, and insulating materials. 2. The voltage spike and associated current are electrical noise-when the current flows through a wire, a magnetic field is created, so there is electromagnetic noise created. 3. Putting in a larger capacitance lowers the frequency of the LC tank, which can spread out the spike making it somewhat lower voltage but ringing longer.
You can comment here or on my blog gasstationwithoutpumps.wordpress.com. If it is about the book, there is a forum at community.leanpub.com/c/applied_analog_electronics (which was disabled for a while, because of a leanpub change, but should be enabled again).
@@gasstationwithoutpumps thx for the reply, it would be rally helpful if u could help me with u r email, bcz i need to send u some schematics for further discussion.. The book link is not working.
Heh, im glad my colleague told me during my internship to use a flyback diode for an electromagnet connected to a PLC. I didn't fully understand what it did, nor did I find the explanation on Wiki very clear.
Great explanation, you filled that knowledge gap for me :)
Glad to have been of use!
You've just articulated the reason why I have stopped using semiconductors to switch inductors - the need to add extra components just to protect your switch.
For example, I have a high-current 12V solenoid that I want to switch briefly with energy from a capacitor bank, using an adjustable timer circuit to dump the load into the solenoid coil. My system works perfectly when replacing the solenoid coil with an incandescent lamp, but when I try to sub in the solenoid, the back-emf fries even my strongest MOSFETS, IGBTs etc, no matter how high their supposed voltage ratings.
It got too expensive in semiconductors, and too much trouble to solder in diodes... so I am back to using electromechanical relays to switch the current to the solenoid. Luckily the back-EMF from the relay coil doesn't seem to worry my smaller and cheaper MOSFET acting between it and the timer.
Sometimes old tech is more trustworthy and resilient than newer in my experience.
Electromechanical relays are still used when you want a simple on-off function, especially when you are switching AC currents, but if you are frying semiconductors in the application, then you probably will have only limited life from the contacts in the relay. Snubbing or free-wheeling diodes are essential, though, so if you've been omitting them, then it is no wonder that you've been frying your semiconductors.
Hello, great video in detail, thanks for it. By the way, that snubber diode can be a regular 1n4007 diode or it has to be a fast recovery diode such as 1n4148? thanks
I suppose that a lot depends on how big the inductive load and how much current you are switching. I've only switched fairly small currents, and so a regular diode was fine.
You can also add a Zener Diode (in the reverse direction) in series with the normal diode. The Zener diode will allow the voltage to rise higher than Vdd+0.7, and the spike will dissipate faster. Use a diode with as high voltage as possible that your drive transistor can handle (leave some safety margin). For example, if your Vdd is 12V and your drive transistor can tolerate 35V Vds, choose a Zener voltage of 12-18 V.
thank you for describing, it's very clearly. the example takes mosfet as low side switch. if mosfet at high switch, when the mosfet turn off, the energy storing in the inductor will discharge to the parasitic capacitor of mosfet through Vdd and cause spike at drain as well? thank you!
Yes, the situation is symmetric if you switch on the high side instead of the low side. The basic concept-that current through the inductor continues to flow-still applies. The voltage spike will have the opposite polarity from the spike from low-side switching.
Great video, very clear presentation! I have 3 follow-up questions: 1. what is the physical nature of the parasitic capacitance ("where is it") ?
2. Does the voltage spike when switchig off cause electromagnetic noise?
3.What if one adds a capacitor on purpose parallel to the parasitic capasitance,,would that decrease the voltage spike?
1. The gate is a conductor, the drain and source are conducting semiconductors (and the channel, when the FET is on). There is an insulating oxide material between the gate and source/drain/channel. Hence, a capacitor (the Miller capacitance). But the capacitance being discussed here is mainly wiring capacitance-any pair of conductors that are not connected together are insulated from each other and so have some capacitance dependent on their size, position, and insulating materials.
2. The voltage spike and associated current are electrical noise-when the current flows through a wire, a magnetic field is created, so there is electromagnetic noise created.
3. Putting in a larger capacitance lowers the frequency of the LC tank, which can spread out the spike making it somewhat lower voltage but ringing longer.
@@gasstationwithoutpumps thanks for your time! It all fits together.
👍👏
Hey gr8 video, how do I contact you...
You can comment here or on my blog gasstationwithoutpumps.wordpress.com. If it is about the book, there is a forum at community.leanpub.com/c/applied_analog_electronics (which was disabled for a while, because of a leanpub change, but should be enabled again).
@@gasstationwithoutpumps thx for the reply, it would be rally helpful if u could help me with u r email, bcz i need to send u some schematics for further discussion..
The book link is not working.
@@pragatmudra8372 OK, try emailing karplus@soe.ucsc.edu
@@gasstationwithoutpumps i just emailed you...
I tought the voltage spikes are due to the magnetic flux collapse of the coil! 😂
It is-that is completely consistent with the explanation here.
Its false spike is because of mmf fallout