What defines the “quality” of a PSU?
There are literally hundreds of specifications we can look at. In my opinion these are the most important:
1. Load regulation: How well the PSU is able to maintain a steady output voltage with changes to the load applied to it.
2. Line regulation: How well the PSU is able to maintain a steady output voltage with changes to the input voltage.
3. Ripple & noise rejection: How well the PSU can filter out ripple and noise.
4. Stability: How well the PSU is able to maintain a steady output voltage in the face of different types of load.
5. Thermal drift: How well the PSU is able to deal with temperature changes.
6. Output impedance: This is added for completeness but is covered under point 1 – load regulation.
What is power supply voltage ripple?
Voltage ripple, or simply referred to as “ripple” is a side-effect of converting AC to DC. This phenomenon occurs in both linear and switch-mode (SMPS) power supplies.
AC Mains power is “pulsed” as a sine wave between the LIVE and NEUTRAL lines at a frequency of between 50Hz and 60Hz (country dependent). The LIVE line switches between positive voltage and negative voltage (with respect to NEUTRAL) at each cycle, thus if you live in a country with 50Hz mains, we will have 50 cycles of positive potential and 50 cycles of negative potential each second.
The transformer receives AC voltage and current from the mains. It then “transforms” this input voltage to an output voltage which is normally lower (but can also be higher) than the mains voltage. As an example, if the mains voltage is 230V RMS, and we have a transformer with 10:1 ratio between input and output, then we will see 23V RMS on the output.
The transformer output is still an AC waveform which “pulses” at the same frequency as the AC Mains. Because the output is still AC, we then feed this transformer output into a full wave bridge rectifier. The bridge rectifier doesn’t do anything to the shape and size of the pulses – what it does do is ensure that all the pulses are of the same polarity. After the bridge rectifier we now have pulsed DC.
If you look at the graph above you will see that the output voltage pulses are all of the same polarity (DC), but the output voltage is hardly steady. It pulses between 0 V and peak which isn’t really useful to us yet. The next thing we need to do is find a way to “filter” that pulsating power into something that’s a bit more steady. This is done using a capacitor.
The capacitor acts like a very fast battery – the incoming pulse charges up the capacitor to full voltage. As the transformer pulse voltage starts to decline down to zero, the stored energy inside the capacitor will do its best to maintain the output voltage.
Now: The reason we have a power supply in the first place is because we have some kind of circuit that needs power. This circuit will have a certain current requirement which needs to be met by the power supply. This current draw we are referring to is called the “load”.
So let’s look at what happens when we have a power supply connected to a load: The incoming power pulse (via the transformer and bridge rectifier) charges up the capacitor so we now have full power available. As the transformer pulse starts going down to 0, the power supply voltage is maintained by the capacitor alone. In the meantime the load on the power supply is drawing current from the supply. Seeing as the energy is provided solely by the capacitor, the available energy inside the capacitor is diminishing as the load consumes it, therefore the power supply output voltage is dropping. This keeps happening until the next transformer pulse comes along and recharges the capacitor to full, after which the cycle repeats. This can be seen graphically in the figure below.
The grey line represents the transformer pulses (via the bridge rectifier) and the red line represents the output voltage of the power supply. It clearly shows the recharging and diminishing of the capacitor as discussed above.
Looking closely at the red line in the graph above, you will notice it has a specific amount by which it varies along the Y axis (Voltage). This variation in voltage is called the ripple voltage of the power supply.
The amount of ripple (ie – the total variation in output voltage) is determined by 2 things: the total amount of capacitance used and the current draw of the load.
The more capacitance we use, the more energy “storage” we have available, thus the less the load will diminish the voltage between pulses, thus more capacitance = less voltage ripple.
Conversely, the higher the current draw from our load, the faster the energy stored in our capacitor will diminish, thus higher load current = more voltage ripple.