FET Bed Heater Board

[Donziboy2]

So I noticed that the MakerFarm Pegasus 10" ships with a Bed Heater relay board and after doing some reading/thinking found it would not be very helpful in the long run and should be replaced with something better.

The design as I have it below has more then just one use and I can see it being used for many things. It is setup to allow any input voltage (change R3 as needed) 10mA device to control a much larger power device. The best Fets to use would have very low RDSon. The Surface Mounted Fet may not have enough cooling for +20A loads and the TO220 would need a heatsink of some kind.

To facilitate this I have started work on a replacement board design that will allow some customization of the parts.
Right now its set up to use 2 different Opto types and 2 different Fet packages. Slow Transistor type Opto's, Fast gate driver Opto's. Dpak(TO263) and TO220 Fets. I chose the packages for 2 reasons.
1. There is a large number of same pinout devices on the market.
2. They are off the shelf parts that are not hard to get.

HeadBed PCB1.jpgHeadBed SCH1.JPG

My Question for you guys is as follows.
1. Should it be the same size as the existing Relay Board and if so what are the dimensions since I dont have my Pegasus yet.
2. Should it be smaller?
3. Should I make it more specialized(like remove the opto and expect whoever uses it to have there own way of driving it)?
4. From my standpoint the standard slow optos should be more then enough, 1-100kHz switching frequency is more then enough to take care of our needs and it reduces losses due to switching(as long as the slow rise/fall does not increase on/off losses). Should I get ride of the Dip8 Gate Driver and just use the Dip4 Opto Transistors. Or should I do a limited run from Oshpark and pull out my trusty O-Scope and compare the Fast vs Slow opto's?
5. Did I make some huge mistake in this design. (considering I have used both circuits at home and at work they should be fine combined, as long as you dont mix resistors/misplace the dip4)
6. Should I replace the 2 large terminal blocks with screw terminals like these http://www.digikey.com/product-detai...798-ND/2171014

Not sure if this should be in one of the larger forums, but for me its first and foremost for the Pegasus and later on it may be used for other things.

Edit
Just noticed I put C1 wrong, should go to pin8 on U2 and GND.
I tried replacing the 2 terminal blocks with those screw terminals. They are kinda big lol. Might make good heatsinks.


Update......

HeadBed PCB8.jpgHeadBed PCB7.jpgHeadBed SCH2.JPG

[Donziboy2]

Got ride of the Pos Supply Post and just added a small jumper point for a wire.
Shank the board down to 2.77in square from 3.17in square. I can probably make it smaller but would like some feedback.....
HeadBed PCB3.JPG
I kinda like the screw posts, they are a little cheaper and they make good large secondary heat sinks.

[Mjolinor]

I don't see why it is so complex. If I needed to do this I would use a P channel and a TL431 to drive the gate of the MOSFET. Making sure -ve was common.

Can't see a need for anything more complex than that other than a couple of resistors to limit currents.

[Donziboy2]

Trying to isolate whatever your driving the load with from the higher voltages the load may have.

[Mjolinor]

Why? There is no need to do that at all.

[Donziboy2]

Fewer things go wrong that way. I would rather desolder a few parts from a sub board then replace a micro controller...

[Mjolinor]

Good design is one that fulfils the requirements but does not overdo it. Opto isolators will be the fail point in your circuit, they always are. They do fail safe usually but without it the circuit will not fail at all for many years. You are driving a mostly fixed resisitive load so the strain on any components is well within the design spec.

Belts and braces do not make trousers stay up any better.

[Donziboy2]

The only time I have seen Opto's fail is if to much current is drawn from them or to much voltage is pushed into them. We use FOD3182's at work to drive 3phase 3HP motors and when the IGBT's go they take everything between them and the FOD3182's out due to the 700V bus voltage.

This is more a generalized design then a specific one case setup, I want to give the end user some leeway in what they put on the board. I am considering removing the dip8 Gate driver opto's since I see very few cases in which you would need to switch at the very high speeds they can produce. The on/off losses with the slower 4Dip opto's is fairly small compared to the overall switching losses themselves.

I agree that the TL431 can be a simpler way in doing it (you could just drive the N Mosfet directly to) but I have preferences towards Opto's due to the protection they provide.

edit..
Added 10K pulldown on the fet gate to actually allow it to switch on and off....
HeadBed PCB4.jpg

[Mjolinor]

For sure if you want an opto then the 4 pin would surely suffice. 70p for the FOD3182 versus 3p for a generic 4 pin represents a huge saving.

I am also not surprised they don't fail at that price.

In SMTPs I would guess that the opto is the third most common failure point after the capacitors and the chopper transistor. I keep maybe 20 or so different types in my component drawers, I wouldn't do that if they didn't fail.

[Donziboy2]

We use 4pin opto's for things that don't need +2A to run. I considered the 8pins due to the high speeds they can achieve but I don't really see a need for them, also many have Under Voltage Lock Out built in so they may cause more headaches then help. In the 3 years I have been testing/fixing inverters I have never once replaced a 4pin style Opto. From experience I can say that generally things don't fail unless you use them outside there safe operating areas. I have to keep reminding the engineers where I work to make sure the electrics stay below ratings at all expected temperatures. Ive had to request changes to designs due to oversights like using a linear regulator inside an enclosed inverter that can have internal ambient over 60C. It was melting 200C rated plastics and destroying the 2 caps next to it.

I love having a job where I get paid to find ways to break things

Removing the 8pin gate driver style opto. (Removed 8Pin Gate Driver Opto, 1 Capacitor and 1 Resistor.)
HeadBed PCB5.JPG

edit.
I can probably shrink the board down more, not really sure yet. May play with the screw terminals, maybe make it possible to use the plastic blocks or the all metal screw terminals.