2 19VDC laptop chargers in series

[jason701802]

Originally Posted by Ether

Even though they're isolated (so there's no leakage path), I still wonder:

1) These chargers are designed to source current, not sink it.

2) They're probably switching technology.

3) If one's switched "on" while the other's switched "off", then the "off" one sees the "on" one trying to push current through it. That's where I think Eric's earlier comment may come into play. Wish I knew more about this kind of stuff.

Isolated switching power supplies (which A/C supplied switching power supplies usually are) can easily be and are regularly hooked in series. The supply will never be sinking current, current will always be going out the (+) terminal and in the (-) terminal. To be safe, reverse biased diodes should be connected to each output so that if one power supply comes on more quickly, it will not apply reverse voltage to the other. See here: http://www.acopian.com/acopianPowerS...y.aspx?nsId=17

Originally Posted by jwallace15

Adding them in parallel will double the amperage.

While this is true for linear power supplies, it is not true for switching power supplies unless they are specifically designed for it. Because the output voltage of a switching power supply is actively controlled, any minute differences in output voltage will result in the supplies fighting each other. It will work for supplies that are well matched, but because they will use some current fighting each other, the available current will be less than combined rating and should only be done temporarily when there are no other options.

[Daniel_LaFleur]

Originally Posted by jason701802

While this is true for linear power supplies, it is not true for switching power supplies unless they are specifically designed for it. Because the output voltage of a switching power supply is actively controlled, any minute differences in output voltage will result in the supplies fighting each other. It will work for supplies that are well matched, but because they will use some current fighting each other, the available current will be less than combined rating and should only be done temporarily when there are no other options.

Jason,
While this is true, a simple fix for parallel switchers is 'OR'ing diodes. This will cause the 2 supplies to droop share.


Ether,
Traditional switching supplies are isolated so running them in series is usually fine. Adding a steering diode should give you protection from reverse biasing the supply. I'd also suggest powering the downstream supply first (for the same reverse bias reason ... although it should not matter).

[Ether]

Originally Posted by jason701802

Isolated switching power supplies (which A/C supplied switching power supplies usually are) can easily be and are regularly hooked in series. The supply will never be sinking current, current will always be going out the (+) terminal and in the (-) terminal. To be safe, reverse biased diodes should be connected to each output so that if one power supply comes on more quickly, it will not apply reverse voltage to the other. See here: http://www.acopian.com/acopianPowerS...y.aspx?nsId=17

In other words, the diodes are there to prevent Supply A from trying to push forward current through Supply B (via the load), when Supply B is not ready.

That's essentially what I said in my earlier post, except I expressed it in terms of Supply B being in the "off" switching state, rather than not having powered up yet.

Can someone explain what aspect of the supply's output circuitry does not like having a voltage applied across it... said voltage being of opposite polarity so that said voltage is attempting to push current through the supply in the forward direction (as would be the case we are discussing with the two supplies in series)?

[jason701802]

Originally Posted by Ether

Can someone explain what aspect of the supply's output circuitry does not like having a voltage applied across it... said voltage being of opposite polarity so that said voltage is attempting to push current through the supply in the forward direction (as would be the case we are discussing with the two supplies in series)?

There may be more, but the first thing that comes to mind is the electrolytic capacitors. They will fail even under small reverse voltages, and if they aren't properly vented, they can fail quite catastrophically: http://www.youtube.com/watch?v=D-RZ5RTAdSg.