Design considerations using multiple DC power rails 5v, 12v, 50v.

[Szewczykm]

I'm starting to put together a circuit that will be part of a pinball machine.  I will have a microcontroller and it will be switching 50v loads.

The power comes from a single multi-tap transformer which feeds a couple of power supplies in the game.  The 50v supply is on a different board than the 5v/12v supply but the power all comes from the same transformer and ultimately use the same ground.

I was considering using SSR's to keep the high voltage far away from my 5v components, but they are expensive (50v, 3 or 4 amps).  If I use properly sized mosfets, what else, if anything, do I need to worry about on this board if the 5v/12v/50v are all sharing the same ground?  (Other than the wires and traces being able to be sized to handle the current.)

Thanks!

Mike

[Simon]

you just said the 50v is from the same transformer. separate secondaries or taps ? you use an optoisolator if your bothered but a mosfet is fine

[Szewczykm]

you just said the 50v is from the same transformer. separate secondaries or taps ? you use an optoisolator if your bothered but a mosfet is fine

I've attached the schematic.  18vac goes to the 12/5 power supply where it is rectified and regulated.  The 48v goes to another power supply board where it's rectified and there's a large smoothing cap.

When wiring, if I connect my board to the 5v/Gnd on the power supply and just the 48v on the flipper power supply, is that a problem?  Or should I also grab ground on the 48v supply and tie it together with the ground from the 5v on my board?

[Simon]

if using mosfets you need a common ground connection, if you go with optoisolators you won't need to connect any common power

[Tandy]

As both share a common ground you have the potential to create a circuit between the 50V line and ground via your 5V circuit. You are probably best off using an opto-isolator to ensure this can not happen.

[Szewczykm]

Do you have any part recommendations for an opto-isolator?  I've used them before but they were for pretty low power applications.

Again I looked at SSR's (which look a lot like opto-isolators to me) and at 48v - 3A they were up in the $20 each range.  Is there anything closer to $2 - $5 or am I just dreaming here?

[Tandy]

The easiest thing is use a low power opto-isolator to switch a high power mosfet.

PS if you fill in your country it will be help with recommendations.

[Kjelt]

I think they mean you can use any cheap otocoupler and let the isolated output transistor switch the fet. So you have in rest a fet which gate is i active with a resistor to vcc or ground depending if p or n fet and the ouputtransistor of the optocoupler pull the gate high or low so the fet will conduct.
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[Simon]

Do you have any part recommendations for an opto-isolator?  I've used them before but they were for pretty low power applications.

Again I looked at SSR's (which look a lot like opto-isolators to me) and at 48v - 3A they were up in the $20 each range.  Is there anything closer to $2 - $5 or am I just dreaming here?

CNY17

[Simon]

As both share a common ground you have the potential to create a circuit between the 50V line and ground via your 5V circuit. You are probably best off using an opto-isolator to ensure this can not happen.

Depends on the use of the equipment, lots of higher voltage loads are driven from 5V logic directly via mosfets, opto's are not used every time you interface to a higher voltage and I'm guessing the user will not be touching the 5v logic

[Szewczykm]

A few more questions:

If using the opto-isolator to drive the fet.  I need to have an opto-isolator that can handle at least 50v through it and whatever current (nil?) will pass through it to the gate of the fet, correct?

Up to this point I've only worked with logic level mosfets driven directly from the microcontroller.  I'll have only one voltage coming in on the high power side, 48v.  I'm guessing I have the 48v connected to the drain of the mosfet and also to the collector side of the opto isolator. 

The Gate on the mosfet is connected to the emitter side of the opto-isolator.

The drain is connected to ground.

So when the microcontroller send 5v to the opto-isolator, it switches on the 48v to the mosfet gate, which then switches "on" the drain-source.

Do I have this right?  I get a little fuzzy on what is required on the gate to get the mosfet to be fully on.  I see Vgs on the data sheet and it says something like +/-20v.  Does that mean if the DS voltage is 48v, the mosfet will be completely on with a Gate voltage of 28v - 68v?

I attached a scribble - do I have that right?  (Edit: added a pulldown)

[cpt.charlie]

Be ware of the Vgs (around 48v in your case), the maximum allowed for your mosfet is 20v, you need to reduce the voltage at the gate with a zener.

[SeanB]

You are using AC loads, so TRIAC switches driven from the MCU direct will also work. Otherwise there are opto triacs that you can use which will also allow you to use a bigger triac to increase current handling capability, or if you are only using 100mA they can drive direct.

MOC3061 will work with your 48V secondary.

https://www.fairchildsemi.com/datasheets/MO/MOC3061M.pdf

[Szewczykm]

Ok, Vgs, why is it listed as +- 20v?

Also it's coming off the transformer as ac. It it's all rectified to dc.  There's no need for triacs.

[SeanB]

If DC then yes no triacs, was unsure, and I have seen a lot of pinball machines that just used the AC for things.

As to the +-20V gate rating, this is because the gate has a very thin oxide layer, which is part of the design and why it works. You limit the voltage to 20V WRT the sorce, as the mosfet will turn on with a positive voltage over the threshold voltage, and a negative voltage will help it turn off faster if needed. In you application this is not needed, but is used sometimes.

As well you get P channel devices, which need a negative voltage to switch. You also get depletion mode devices, which are normally on and where the voltage applied turns them off, but they are not common and are not really high power devices.

[Simon]

Ok, Vgs, why is it listed as +- 20v?

Also it's coming off the transformer as ac. It it's all rectified to dc.  There's no need for triacs.

most mosfets will withstand up to 20 or 30V of either polarity before they break, so the +/- spec is an absolute maximum. You want to look at the Vgs "test voltage" which usually is 10-16V, that is as much as you need.
 in the case of an N channel mosfet: source to ground, drain to negative of load, gate swithed to 10V. If you want to maintain isolation you need to step down your 48V to 10V-15V to drive the mosfet gate through the traic. You an use a zener or a voltage reg or in fact you may need a bit of both as most regulators don't like over 35-40V