Hello, I'm a first time poster that has been lurking here for a while. I'm a mechanical engineer that has been finding himself doing more and more electrical work and embedded programming over the last year or two. I have hit a roadblock on one of my projects and am hoping that someone can help me better understand what is happening.
Some of our previous engineers were notorious for cutting corners, making unsafe circuits, and leaving little to no documentation on projects. I am currently trying to revise one of the boards that they worked on in the past. I have managed to fix the pots for the amplifier and AtoD circuits, have removed the excess capacitors from the rectifier circuit (because why wouldn't you store more than 5x what you need?), and have added optical isolation between the AC and DC signals (for some reason, they thought tying the DC common into the AC neutral with caps storing over 2000uF was a good idea), but am stuck on the main driver circuit.
I am trying to drive a fairly large electromagnet (half transformer) that moves a vibratory assembly (the electromagnet has springs opposing it to reset the assembly's position). The vibration is controlled from an IC that receives feedback from an accelerometer. Prior to working on this project, the electromagnet was driven by an IRS21864 chip with four schottky diodes and two IGBTs (~$70 worth of components for a simple on-off action that doesn't require polarity switching). After adding in the optical isolator, I have been trying to drive the circuit using a single N-channel Mosfet and a flywheel diode.
Unfortunately, I have been left with no datasheet on the electromagnet and am waiting to hear back from the manufacturer on if they can provide one since apparently it was a custom order from a small company. I have attached images of both the electromagnet and my current driver circuit. The electromagnet is powered by rectified mains (170V here in the US) through PWM. The older unit in the lab is currently drawing under 100W under load and we have a 3A fuse in the unit and I plan to limit the maximum PWM to keep the driver under 200W. The PWM frequency is set to 60Hz to allow the springs in the unit time to recover and produce vibration.
When I first ran the current circuit, I noticed that the temperatures on the Mosfet (Q1) and Flywheel Diode (D2) were both increasing. After adding the ferrite bead (FB1), the Mosfet temperature appeared to be well under control. While the Mosfet's temperature is fine, the temperature across the flywheel diode continues to rise and I am completely unsure as to why. When the duty cycle of the PWM is increased, the Flywheel Diode overheats faster; I have been turning the circuit off when the outer temperature of the diode and heatsink read 60C on my IR thermometer. To my understanding, when the Mosfet turns off, the current in the electromagnet (P6) is redirected through the Flywheel Diode and back into the electromagnet coil to safely discharge. The diode that I started with had Vf = 1.6V @ 10, and Io = 10A, so I figured I might just need to make it bigger and tried other diodes with Vf = 1.8V @ 10, and Io = 36A and then one with Vf = 1.8V @ 30, and Io = 99A (I have listed digikey links with the info on each and the datasheets below).
1st: https://www.digikey.com/product-detail/en/rohm-semiconductor/SCS210KGC/SCS210KGC-ND/3902856
2nd: https://www.digikey.com/product-detail/en/microsemi-corporation/APT10SCD120B/APT10SCD120B-ND/3770182
3rd: https://www.digikey.com/products/en?keywords=APT30SCD120B
To my surprise, there was no detectable performance difference between these three diodes and they all three would reach the same temperatures at approximately the same time when tested at the same duty cycle (with or without the ferrite bead). I feel like I have to be missing something very obvious, but am unsure what is causing them to get this hot in the first place, let alone why increasing their size had no effect.
After this, I tried adding an RC snubber circuit on the return path between the Flywheel Diode and the HV circuit. As expected, the resistors bled off a large portion of the energy and greatly reduced the turnoff time of the circuit, but built up heat very quickly. I tried this with a spare 1uF cap and five 3W 5k resistors in parallel (this should equal a 1k 15W resistor) and again with five 100k 3W resistors in parallel (equal to a 20k 15W resistor)
I am currently considering ordering a unidirectional TVS diode and placing it between the Flywheel Diode and HV circuit (with the cathode towards the Flywheel Diode) to act as a snubber. Since Vds on the Mosfet is 600V, and I would like to be able to use this circuit globally, I am going to assume that the maximum rectified input voltage would be 355V, so I would want the breakdown voltage of the TVS diode to be .8*600-355 = 125V at the minimum. Should one of the below TVS diodes help, or am I on the wrong track?
1st: https://www.digikey.com/product-detail/en/littelfuse-inc/5KP120A/1294-5KP120A-CHP/4864269
2nd: https://www.digikey.com/product-detail/en/micro-commercial-co/15KP120A-TP/15KP120A-TPCT-ND/2776339
Thanks for taking the time to go through this. If anyone is able to shed some light on why this is happening and where I should go from here, I would be very appreciative. I am running out of time on this project and am to the point where I am just getting frustrated with it and not making progress.
TL;DR: Flywheel diode (D2) is overheating under load. I have tried an RC snubber without success and am considering a TVS snubber. The load is an electromagnet on rectified mains.
Some of our previous engineers were notorious for cutting corners, making unsafe circuits, and leaving little to no documentation on projects. I am currently trying to revise one of the boards that they worked on in the past. I have managed to fix the pots for the amplifier and AtoD circuits, have removed the excess capacitors from the rectifier circuit (because why wouldn't you store more than 5x what you need?), and have added optical isolation between the AC and DC signals (for some reason, they thought tying the DC common into the AC neutral with caps storing over 2000uF was a good idea), but am stuck on the main driver circuit.
I am trying to drive a fairly large electromagnet (half transformer) that moves a vibratory assembly (the electromagnet has springs opposing it to reset the assembly's position). The vibration is controlled from an IC that receives feedback from an accelerometer. Prior to working on this project, the electromagnet was driven by an IRS21864 chip with four schottky diodes and two IGBTs (~$70 worth of components for a simple on-off action that doesn't require polarity switching). After adding in the optical isolator, I have been trying to drive the circuit using a single N-channel Mosfet and a flywheel diode.
Unfortunately, I have been left with no datasheet on the electromagnet and am waiting to hear back from the manufacturer on if they can provide one since apparently it was a custom order from a small company. I have attached images of both the electromagnet and my current driver circuit. The electromagnet is powered by rectified mains (170V here in the US) through PWM. The older unit in the lab is currently drawing under 100W under load and we have a 3A fuse in the unit and I plan to limit the maximum PWM to keep the driver under 200W. The PWM frequency is set to 60Hz to allow the springs in the unit time to recover and produce vibration.
When I first ran the current circuit, I noticed that the temperatures on the Mosfet (Q1) and Flywheel Diode (D2) were both increasing. After adding the ferrite bead (FB1), the Mosfet temperature appeared to be well under control. While the Mosfet's temperature is fine, the temperature across the flywheel diode continues to rise and I am completely unsure as to why. When the duty cycle of the PWM is increased, the Flywheel Diode overheats faster; I have been turning the circuit off when the outer temperature of the diode and heatsink read 60C on my IR thermometer. To my understanding, when the Mosfet turns off, the current in the electromagnet (P6) is redirected through the Flywheel Diode and back into the electromagnet coil to safely discharge. The diode that I started with had Vf = 1.6V @ 10, and Io = 10A, so I figured I might just need to make it bigger and tried other diodes with Vf = 1.8V @ 10, and Io = 36A and then one with Vf = 1.8V @ 30, and Io = 99A (I have listed digikey links with the info on each and the datasheets below).
1st: https://www.digikey.com/product-detail/en/rohm-semiconductor/SCS210KGC/SCS210KGC-ND/3902856
2nd: https://www.digikey.com/product-detail/en/microsemi-corporation/APT10SCD120B/APT10SCD120B-ND/3770182
3rd: https://www.digikey.com/products/en?keywords=APT30SCD120B
To my surprise, there was no detectable performance difference between these three diodes and they all three would reach the same temperatures at approximately the same time when tested at the same duty cycle (with or without the ferrite bead). I feel like I have to be missing something very obvious, but am unsure what is causing them to get this hot in the first place, let alone why increasing their size had no effect.
After this, I tried adding an RC snubber circuit on the return path between the Flywheel Diode and the HV circuit. As expected, the resistors bled off a large portion of the energy and greatly reduced the turnoff time of the circuit, but built up heat very quickly. I tried this with a spare 1uF cap and five 3W 5k resistors in parallel (this should equal a 1k 15W resistor) and again with five 100k 3W resistors in parallel (equal to a 20k 15W resistor)
I am currently considering ordering a unidirectional TVS diode and placing it between the Flywheel Diode and HV circuit (with the cathode towards the Flywheel Diode) to act as a snubber. Since Vds on the Mosfet is 600V, and I would like to be able to use this circuit globally, I am going to assume that the maximum rectified input voltage would be 355V, so I would want the breakdown voltage of the TVS diode to be .8*600-355 = 125V at the minimum. Should one of the below TVS diodes help, or am I on the wrong track?
1st: https://www.digikey.com/product-detail/en/littelfuse-inc/5KP120A/1294-5KP120A-CHP/4864269
2nd: https://www.digikey.com/product-detail/en/micro-commercial-co/15KP120A-TP/15KP120A-TPCT-ND/2776339
Thanks for taking the time to go through this. If anyone is able to shed some light on why this is happening and where I should go from here, I would be very appreciative. I am running out of time on this project and am to the point where I am just getting frustrated with it and not making progress.
TL;DR: Flywheel diode (D2) is overheating under load. I have tried an RC snubber without success and am considering a TVS snubber. The load is an electromagnet on rectified mains.