High Voltage DC Power Supply Ripple
When comparing power supplies, carefully check the ripple specifications. There are different ways to express this, and the actual ripple could be very different, even though it initially appears that two power supplies have the same specifications. Ripple is commonly expressed in absolute terms, e.g., 100mV or relative, e.g., 0.1%. Second, it can be expressed in peak-to-peak (the best way, in our opinion), peak, or rms.

The ripple is a function of output voltage, output current, control circuits, components, and grounding issues. In low voltage power supplies, the dependency on output current is generally dominant. However, in HV power supplies, dependency on output voltage is often the dominant parameter, especially with lower power models. Parasitic capacitance between the ac multiplying section (HV transformer plus voltage multiplier) and the output section results in ripple being picked up or injected into the output.

Ripple of switching high voltage power supplies, especially, tend to include spikes. Thus, rms is not a good way to express ripple. Peak-to-peak specifications include the spikes.

Difficulties in grounding, noisy components, and ripple on low voltage power supplies can result in a ripple floor, i.e., a minimum ripple no matter what the output voltage and current.

Output Safe Operating Area
While it is easy to assume that a high voltage power supply will operate at any combination of voltage and current, up to its ratings, that is not always the case. Some linear types will overheat when the output is at low voltage and high current. Some switching types will oscillate at low output voltage and low output current.

Output Current Limiting
Not all high voltage power supplies have well defined output current limiting. As a result, at some operating points, it is possible for the power converter to overheat and fail.