3 Basics of power supplies

1. Linear power supplies
Linear regulated power supplies excel by their highly constant output voltage, low ripple and fast regulation, even under high line and load transients. Good power supplies feature a ripple of less than 1 mVrms which is mostly neglegible. Further they are free from EMI emission in contrast to SMPS. 

A conventional mains transformer isolates the line from the secondary which is rectified and supplies an unregulated voltage to a series pass transistor. Capacitors at the input and output of the regulator serve as buffers and decrease the ripple. A high precision reference voltage is fed to one input of an amplifier, the second input is connected mostly to a fraction of the output voltage, the output of this amplifier controls the series pass transistor. This analog amplifier is generally quite fast and is able to keep the output voltage within tight limits.

2. Switched-mode power supplies (SMPS)
SMPS operate with very much higher efficiencies than linear regulated power supplies. The DC voltage to be converted is chopped at a high frequency rate thus requiring only comparatively tiny and light ferrite chokes or transformers with low losses, also, the switching transistor is switched fully on and off hence switching losses are low. In principle regulation of the output voltage is achieved by changing the duty cycle of the switch driving waveform.

1st Off-line SMPS
The line voltage is rectified, the buffer capacitor required is of fairly small capacitance value because the energy stored is proportional to the voltage squared (E = 1/2 x C x U2).

2nd Secondary SMPS
These still require a 50 or 60 Hz mains transformer, the secondary output voltage is rectified, smoothed and then chopped. The capacitance values needed here for filtering the 100 resp. 120 Hz ripple are higher due to the lower voltage. All SMPS feature a very much higher efficiency from appr. 70 up to over 95 % compared to any linear supply. They are lighter, smaller. The capacitors on the output(s) of a SMPS may be quite small due to the high frequency, but the choice depends also on other factors like energy required for buffering or ac ripple from the load (e.g. motors).

In principle the size of the major components decreases with increasing operating frequency, however, the efficiency drops apppreciably above appr. 250 kHz as the losses in all components rise sharply.

3. Parallel and series operation
It is mandatory that the power supplies used are definitely specified for these operating modes. This is the case with all Rohde & Schwarz supplies. As a rule, the output voltages to be combined are independent of each other, hence, it is allowed to connect the outputs of one supply with those of another or more.

In this mode the output voltages add, the output current is the same for all supplies. As the sum of all voltages may well surpass the 42 V limit touching of live parts may be fatal! Only qualified and well instructed personnel is allowed to operate such installations.

The current limit of the outputs in series should be adjusted to the same value. If one output reaches the current limit the total voltage will break down.