A question about multiple voltage switching power supplies.

Discussion in 'The Projects Forum' started by John Schulien, Apr 14, 2015.

  1. John Schulien

    Thread Starter New Member

    Apr 13, 2015
    Hello. First time poster.

    I have a question about power sequencing in switching power supplies. I want to use a modern switching power supply to power a vintage Intel 8080 microprocessor circuit. The target circuit requires three voltages:

    +12V @ 350 mA
    +5V @ 1.3 A
    -10V @ 200 mA (which is converted on-board to the -5V substrate bias voltage needed by the 8080.)

    Each of these three voltage inputs is buffered on-board with a 22uF capacitor to ground.

    I would like to power this board with a multiple voltage switching power supply (Mean Well RT-50B) with the following specs:

    +12V @ 2.0 A
    +5V @ 4.0 A
    -12V @ 0.5 A (The 79M05 -5V voltage regulator on the 8080 board has a wide operating range with a maximum of -35V, so the higher voltage should be safe.)

    From the standpoint of providing adaquate power, this supply appears to fit the bill. However, I found a warning on the 8080 Wikipedia page that I did not find in any of the Intel datasheets or literature:

    "The -5 V power supply ... must be the first power source connected and the last disconnected, otherwise the processor will be damaged ... The +12 V power supply ... must be the last connected and first disconnected power source."

    In researching this requirement, I found this in Steve Ciarcia's "Ciarcia's Circuit Cellar, Volume 3" (available for free in Google Books), p. 109, with regard to 4116 RAM:

    "The most important thing to remember when designing any computer that uses 4116s is that the power-supply voltages have to be turned on and off in sequence. To keep from blowing the 4116 on power-up, the -5V supply must be turned on before the +5V and +12V supplies. On power-down, the -5V has to remain on while the +5 and +12 are removed. In lower-current power supplies (such as you will probably use), the sequencing can be accomplished through the time-constants of the power supply itself. This technique is used in the TRS-80. By giving the -5V section a very fast time constant compared to the other two supplies, it appears to come on first. On power-down, the sequence is reversed. Because the -5V has such a low-current draw on it, it will stay up long after the other voltages have dropped."

    and on page 103:

    "Power-supply sequencing is important because many power supplies overshoot their rated voltages when they are turned on. If VSS (-5V) is not turned on and VDD (+12V) over-shoots to more than +15V, the [4116] chip will blow. Applying VSS first provides an extra margin to prevent device destruction."

    Now back to the power supply I want to use:

    Although I have no schematic of the power supply, after spending an evening reading about the secondaries of switching power supplies, it seems intuitive that in order for the +12V and +5V outputs to source 24W and 20W respectively, they would have to have larger L and/or C values in their output sections than the -12V output, rated for 6W, and thus should have longer time constants.

    Thus I hypothesize that this power supply will be safe for this circuit because the -12V output is expected to come up faster than the +5V and +12V supplies, thus applying the proper power-on sequence, and the low negative voltage power consumption will make the -5V source outlast the +5V/+12V sources on power-off.

    But I ask before I test, out of caution. I don't want to let out the 40 year old magic smoke. I'd appreciate any comments and tips from those with a better understanding of switching power supplies.


    John Schulien

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  2. #12


    Nov 30, 2010
    Summary: How fast does each voltage become available when you switch on a Meanwell RT-50B power supply?
    When you turn it off, in what order do the supplies drop out?
    I need -12V to come on first and quench last.
    I need the +12V supply to come on last and quench first.
    The +5V supply is always in the middle, time wise.
    The victim is an Intel 8080

  3. John Schulien

    Thread Starter New Member

    Apr 13, 2015
    Right. Much shorter. I am also interested in the theory but that's the practical question.