Re: Duet 3.3V supply quality January 01, 2014 06:04AM |
Registered: 10 years ago Posts: 14,672 |
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kwikius
So my proposal is to hang some hefty capacitance off the input leads where they enter the enclosure
I ignore the existaing capacitance at the ATX PSU, which I believe is stated by Rory166 to be 2200 uF somewhere, but its 2 ft away down the leads. I assume the ATX PSU can handle the large extra capacitiive load I plan to add! I also ignore the hot end heater and concentrate on the Bed Heter Lets keep it simple.
So some numbers. Lets say we have a current of 20 A with bed heater on.
Assume we want a max voltage drop of 6 V, when the Bed heater switches on (So 5v reg has enough headroom) Lets also arbitrarily assume we want to supply that for 1 ms.
My math gives a required Capacitance of around 3000 uF.
The max ESR of the Cap allowed is 6 V / 20 A == 0.3 R which I think is achieveable. Just to be sure I'll get 3 1000 uF caps and put them in parallel. ( Maplin dont specify ESR)
Re: Duet 3.3V supply quality January 01, 2014 06:30AM |
Registered: 10 years ago Posts: 14,672 |
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Radian
If those mods include adding a few thousand uF across C1/C3 to keep the ATX 12V from browning-out then I mostly agree. I've found this to be absolutely crucial to minimise sudden lock-ups during printing. Unfortunately it hasn't made it 100% reliable but I report that on the basis of only a couple of lock-ups since I did the mod.
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Radian
I'm still suspecting another vector to PSU related lock-ups because with the additioanl input capacitance in place, the Duet will tick-over all day and night without crashing so long as it's not actually printing. Slowing the high-current switching edges may help with this. I'm now investigating the big flat ribbon cable carrying the 10A to the bed heater to see what kind of EM field it creates. The high flyback voltage, fast edges and large surface area may make it a candidate for capacitive coupling.
Re: Duet 3.3V supply quality January 01, 2014 06:33AM |
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Re: Duet 3.3V supply quality January 01, 2014 06:51AM |
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Re: Duet 3.3V supply quality January 01, 2014 07:11AM |
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Re: Duet 3.3V supply quality January 01, 2014 07:44AM |
Registered: 10 years ago Posts: 256 |
There is the drop when the heater bed turns on but I believe the current through the bed heater circuit is 20 A , so when that switch turns off abruptly ( as we know it does!) there is no where for the lead inductance and bed inductance to go, so I guess that is what I am trying to address. (hmm Did I say that before .. no ) The existing capacitance on Vin consists of the stepper driver capacitors, but doesnt look like they are close to the ground point (which basically is the screw terminals where the supply enters AFAICS)Quote
dc42
Quote
kwikius
So my proposal is to hang some hefty capacitance off the input leads where they enter the enclosure
I ignore the existaing capacitance at the ATX PSU, which I believe is stated by Rory166 to be 2200 uF somewhere, but its 2 ft away down the leads. I assume the ATX PSU can handle the large extra capacitiive load I plan to add! I also ignore the hot end heater and concentrate on the Bed Heter Lets keep it simple.
So some numbers. Lets say we have a current of 20 A with bed heater on.
Assume we want a max voltage drop of 6 V, when the Bed heater switches on (So 5v reg has enough headroom) Lets also arbitrarily assume we want to supply that for 1 ms.
My math gives a required Capacitance of around 3000 uF.
The max ESR of the Cap allowed is 6 V / 20 A == 0.3 R which I think is achieveable. Just to be sure I'll get 3 1000 uF caps and put them in parallel. ( Maplin dont specify ESR)
Hi Andy,
Your maths looks OK to me, except that if you are worried only about the transient caused by the bed heater turning on, then it is only the bed heater current you need to worry about, not the total 12V current.
Re: Duet 3.3V supply quality January 01, 2014 09:35AM |
Registered: 10 years ago Posts: 14,672 |
Re: Duet 3.3V supply quality January 01, 2014 10:44AM |
Registered: 10 years ago Posts: 256 |
Quote
dc42
AFAIR, Ian said the bed current was 9A by design, and someone else on this forum reported 11A. I've just measured the resistance to be 1 ohm at the bed, and there will be some resistance in the ribbon cable. So 9 to 11A is believable.
I think the board has a ground plane as one of the inner layers, in which case the stepper driver decoupling capacitors will help suppress the transient when the bed heater turns off, except at very high frequencies - which is why I added a 1uF ceramic capacitor to my board. The stored energy in the lead and bed inductance appears to be dissipated mostly as avalanche energy in the mosfet - which is another reason to add a ceramic decoupling cap between the bed +ve output and a ground point close to the mosfet source terminal.
There will be some shunt capacitance and reverse recovery time in D1, but I believe C1/C3 should be more than adequate to take care of that.
Re: Duet 3.3V supply quality January 01, 2014 10:50AM |
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Re: Duet 3.3V supply quality January 02, 2014 03:10AM |
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Re: Duet 3.3V supply quality January 02, 2014 04:09AM |
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Re: Duet 3.3V supply quality January 02, 2014 05:17AM |
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Re: Duet 3.3V supply quality January 02, 2014 05:34AM |
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Re: Duet 3.3V supply quality January 02, 2014 06:20AM |
Registered: 10 years ago Posts: 256 |
Quote
dc42
Andy, the body diode of a mosfet is in the wrong place in the circuit to act as a flyback diode in a single-mosfet circuit like this. It's different in a mosfet H-bridge, where the body diode of the upper mosfet acts as a flyback diode when the lower mosfet turns off, and vice versa.
Re: Duet 3.3V supply quality January 02, 2014 08:24AM |
Registered: 10 years ago Posts: 14,672 |
Quote
kwikius
The flyback diode would be connected with anode to TR2 drain and cathode to Vin, basically across the bed heater inductance. On switching off TR2 the diode starts to conduct with 10 A running through it into Vin.
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kwikius
Alternatively without the diode, on switching off TR2, then the drain of TR2 reaches @50 V and then acts basically like a 50 V Zenner. In this case at switch off ( get a bit hazy here) Vin continues to ( smoothly?) supply 10 A and the energy in the inductor is dissipated in the Mosfet.. so ideally a gentler solution, although maybe not working so great in practise?
Re: Duet 3.3V supply quality January 02, 2014 05:56PM |
Registered: 10 years ago Posts: 300 |
Re: Duet 3.3V supply quality January 02, 2014 06:22PM |
Registered: 10 years ago Posts: 14,672 |
Quote
Without knowing exactly what the present (or future) firmware is going to do with the gate drive, keeping the MOSFET in it's semi-conducting region with 10A flowing could soon turn into excessive dissipation. What I mean is, it depends on the number of on/off cycles in a given amount of time and we don't necessarily have any control over this.
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So a 1uF suppression type capacitor between Source and Drain will have 12V across it when the MOSFET is turned on but the charge it's holding is insignificant given the characteristics of the MOSFET.
Re: Duet 3.3V supply quality January 03, 2014 04:58AM |
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Re: Duet 3.3V supply quality January 03, 2014 05:09AM |
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Re: Duet 3.3V supply quality January 03, 2014 06:06AM |
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Quote
dc42
So I'm thinking that a capacitor of 0.22uF would probably be enough to prevent avalanching, saving 40uJ dissipation at turn-off, but only add 16uJ of dissipation at turn-on.
Re: Duet 3.3V supply quality January 03, 2014 06:12AM |
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Re: Duet 3.3V supply quality January 03, 2014 06:31AM |
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Re: Duet 3.3V supply quality January 04, 2014 05:31AM |
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