I have two identical 48 V switched power supplies, both of which can deliver the same current of, say, 4 A.
Can I connect them in series to get 96 V at 4 A? Is this something possible / recommended?
Electronic – Connecting switch-mode power supplies in series to increase voltage
power supply
Related Solutions
The condition to consider for this to happen is a load beyond the rating of the supplies in question. Most severely, a short circuit. A short circuit need not be permanent and can even be a part of normal use: A motor, when starting, or a capacitor, when being charged from 0 V, looks like a short circuit, too.
To explain how a negative voltage can appear on one supply's output, let's use a diagram with some arrows. Supply A's output voltage shall be called VA, supply B's output VB, and the voltage across the load shall be called VL.
With KVL:
\$V_A + V_B = V_L\$
This is true for normal operation, when we want to use, for example, two supplies with VA = VB = 12 V for a load that needs VL = 24 V:
\$12V + 12V = 24V\$
For a short circuit, VL = 0 V, and KVL still applies. Thus, \$V_A + V_B = 0\$ or
\$V_A = -V_B\$
If we want to avoid negative voltages on any of the two supplies, the only possible solution is
\$V_A = V_B = 0\$
However, because of \$V = R \cdot I\$, and because any real power supply has a non-zero internal resistance, this also means that no current must flow. Now, a power supply will try to deliver current. Since the current or power limit of the two supplies will not be exactly equal, the "stronger" supply will be able to maintain a small, positive output voltage, and the only way to solve the KVL equation is by back-feeding the other supply, with a negative voltage across its output.
There are ICs available to do the whole thing, including the switching MOSFETs, full isolation between inputs, etc.
For example, the PS2115A from TI can autoswitch between two inputs, handle 2A, and is currently available for US$2.15 from DigiKey. Take a look at figure 14 for what you want.
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Best Answer
This can't be answered in general without knowing the details of your power supplies. Some will accept being connected in series without problems, some might not and start to become unstable. To be sure, you need a manual, ideally with a section like this:
(Source; Note that this same supply will not accept being operated in parallel with another one, which is o.k. for other supplies.)
Two common gotchas for series-connected supplies:
If you short the output, the two supplies are connected back-to-back, with a reversed polarity. Think of it this way: One supply is a bit stronger than the other one, and it will force its output voltage onto the other supply, upside down. Even some common conditions, like charging a capacitor connected as a load to the supplies, will look like a temporary short circuit.
48 V is considered a safe, low-voltage. The outputs of your supplies may be designed such that they rely on this fact. Connecting your supplies in series will yield a dangerously high voltage. Aside from posing a danger to the user, there may be components in your supplies that won't accept the increased voltage.