The link you posted is a good example for ... something else. When you read somebody using harsh language / tonality and looking down at other ppls, a warning sing should light up in your eyes. The guy did read a reprap page which explained some stuff, e.g. motivation for a freewheel diode, but understood what he wanted from it. Then took his view that to forum and used a superior attitude and harsh tone towards ramps developers. I consider this wrong on few different levels.
To pwm or not to pwm - that is the question. If you dont pwm, because heat loss vs hysteresis, you only get switching at like 1-2 seconds or so. So if its something >400Hz, then you are pwm-ing. Most part of losses in modern smps come from switching losses being 1-2 orders of magnitude bigger than conduction losses - this is how good current mosfets conduction modes get. And the number of 400Hz, its interesting for being kinda low, because the arduino will pwm way beyond that, but if you use a poor multimeter tool or perhaps if your low pwm state is low enough, then perhaps that 400Hz is the lowest value you can get, - but not the max. Check around these forums and see whats the pwm frequency range.
So if the fail is because of switching losses, its the pwm that causes it and its a different story. Looking at it differently, switching losses are because of pwm, and appear even if the load would be perfectly resistive. And it makes sense, since the arduino pwm is not terribly perfect waveform and pins have small power hence low dv/dt and longer transition in operating regions where losses are huge. Quick fix, stop trying to pwm the heated bed. Use bang-bang only, like 0%-100% = no pwm. And with a hysteresis like +-2 degrees, that will take seconds between on-off. The heat of mosfet transitions will have time to go away. Imo you will be fine with that. There is no need and no logic in trying to pwm the bed. I for one, use bang-bang even for extruder and im fine with it. Imho firmware defaults should be with heated bed on bang-bang and imo i personally would go that far to not even allow pwm on bed. And if the pwm is cause of fails through increasing switching losses, the freewheel diode wont really help. This is something for you to do, you can really test this. If you want a diode, the best place to put it is actually right on heated bed connector. Because thats where transient is generated then its better place there than on ramps, because ramps is further away. You can simply solder almost any diode across the heated bed connector, in reverse to normal flow, and try doing the same and try pwm the mosfet again, and i think it will fail when you pwm 12-13A in Khz range, with a pin with few mA current capacity and slow dv/dt. Even the fact that some mosfets do survive that its just a little short of miracle. Imo, pwm-ing the bed is not an electronic design fault, its more like user fault.
The isolation is only needed when there are different power supplies. The isolation adds delays to any signal, and thats no good, so isolation is a trade-off only to be used when necessary. If it would be only positive aspects to this, then all arduino boards would have optocouplers built in on all gpio. Also the guys behind ramps are guys which were in reprap, even admins, and did alot of things and while there is no need to emphasize that, they do have background and experience on their side, and i do respect and value that and their work and contributions. Generally i have nothing against criticism of anybody, but except with the part where some try to raise themselves this way. I dont mean to be a put down, but on some level, at least in my book, you have to do something first, and brag about it later. Even chickens do lay egg first, and start yakking after. Vice-versa doesnt quite work the same. If you want to learn from somebody, really take a good loook.Quote
In fact neither is there the slightest isolation between the power part and the control part (another design flaw of the important RAMPS but that would require a complete redesign) it's more than likely shit and engage the interference by mass to other areas of the circuit. All very cool going. , we think it's a pretty reasonable explanation for why they were going to take the ass MOSFETs. Honestly I find it rather unfortunate when a f**king diode cents worth. And some optocouplers not worth much more. Apparently seen I think I will review the entire design of the RAMPS to see what surprises you ... In order to solve the problem going to an electronics store and a couple ye pilláis certain power diode.
Ramps mosfet is a hexfet and if voltage gets too high it breaks down and conducts to ground. Check the datasheet symbol right side of that mosfet symbol and compare to the symbol of a the diodes, see which one matches, and read a bit about those. Their breakdown is mentioned in datasheet. It appears to coincide with the absolute max Vds value, but it may be a confusion as to which determines which, and what happens over the value. One would think the diode is there because it tries to protect the mosfet above its own max vds. I would rather have it the other way around. My pov is that the absolute max value is exactly that determined by the min breakdown, because above that point the mosfet simply no longer works because in breakdown mode the mosfet is simply entirely bypassed to ground. Absolute max in this case may not mean it immediately blows up. It doesnt mean it will explode, it means that diode creates a short to ground for voltages above that value, intended to sink transients to ground. If the transient is too big, temperatures too high, perhaps that diode will fail at some point, and will blow up eventually, but for ramps breakdown starts at 60v minimum, so perhaps wont blow up at 70v, will it. Not all mosfets work like so, but those do, and i am not against the freewheel diode, that is good, and the arguments for that are good, and its worth having, but its just not an universal panacea and neither lacking it is the root of all evil.Quote
The MOSFET ramps joint endure maximum 60V.
Ofc they do, they are meant to prove a point. That was exactly the point. But remember they are simulations. Read the datasheet of that mosfet: in practice, you wont get 70V transient forming across a body diode with 30V breakdown rating.Quote
Up in the simulations, ringing (oscillations) appear horrible.
Now thats the part which is interesting and which got me to post in the first place. Over 400 times per second. Priceless. Dunno where the numbers comes from, or which tool measured that, but that would be interesting to know.Quote
Over 400 times per second.
To pwm or not to pwm - that is the question. If you dont pwm, because heat loss vs hysteresis, you only get switching at like 1-2 seconds or so. So if its something >400Hz, then you are pwm-ing. Most part of losses in modern smps come from switching losses being 1-2 orders of magnitude bigger than conduction losses - this is how good current mosfets conduction modes get. And the number of 400Hz, its interesting for being kinda low, because the arduino will pwm way beyond that, but if you use a poor multimeter tool or perhaps if your low pwm state is low enough, then perhaps that 400Hz is the lowest value you can get, - but not the max. Check around these forums and see whats the pwm frequency range.
So if the fail is because of switching losses, its the pwm that causes it and its a different story. Looking at it differently, switching losses are because of pwm, and appear even if the load would be perfectly resistive. And it makes sense, since the arduino pwm is not terribly perfect waveform and pins have small power hence low dv/dt and longer transition in operating regions where losses are huge. Quick fix, stop trying to pwm the heated bed. Use bang-bang only, like 0%-100% = no pwm. And with a hysteresis like +-2 degrees, that will take seconds between on-off. The heat of mosfet transitions will have time to go away. Imo you will be fine with that. There is no need and no logic in trying to pwm the bed. I for one, use bang-bang even for extruder and im fine with it. Imho firmware defaults should be with heated bed on bang-bang and imo i personally would go that far to not even allow pwm on bed. And if the pwm is cause of fails through increasing switching losses, the freewheel diode wont really help. This is something for you to do, you can really test this. If you want a diode, the best place to put it is actually right on heated bed connector. Because thats where transient is generated then its better place there than on ramps, because ramps is further away. You can simply solder almost any diode across the heated bed connector, in reverse to normal flow, and try doing the same and try pwm the mosfet again, and i think it will fail when you pwm 12-13A in Khz range, with a pin with few mA current capacity and slow dv/dt. Even the fact that some mosfets do survive that its just a little short of miracle. Imo, pwm-ing the bed is not an electronic design fault, its more like user fault.