Paul McGowan writes:
Continuing our thoughts on power supplies and the differences between a classic linear supply and a SMPS, I thought it might be helpful to list some generalized plus and minus’s of both. For this post I will keep the supplies focused on power amplifiers – I do this because there lies the biggest use/advantage of a SMPS.
I am going to emphasize that in each case, linear vs. SMPS, we are discussing only well designed versions – not everything else out there (lots and lots of bad designs – especially SMPS). Let’s start with a typical classic power amplifier linear supply and list what’s wrong with it:Poor power factor on the line
- Switching noise from its diodes added to the power line and internal amplifier circuits
- High ripple levels on its output
- Large magnetic field
- Heavy, bulky and large
- Simple to design and build
- Not a lot of radiated energy inside the chassis
- Low output impedance
So the first on our list is power factor – and before I get into this tough subject let me add that a classic power supply has a power factor of about 0.7 and a traditional SMPS is much worse: sometimes as low as 0.2. But again, we’re going to focus only on well designed and modern approaches where the classic supplies still have 0.7 but modern SMPS have a stunning 0.9 to 1. But more on SMPS later.
What is power factor? It is the ability of a device to accept power in synch – synchronized between voltage and current.
To deliver power you need both voltage and current – voltage being the “pressure” or level and current being the motive force. If we use a water analogy, voltage might be the level of water in a tank (the potential) and current might be the pressure a pump (or gravity) exerts to send that water out of the tank.
If you think about it, a tank full of water is nice but rather useless if we wish to harness its potential energy. An empty tank has no potential and a full tank is, well, full of potential. But potential only matters when we do something with it – so this is where current comes in.
If we release a stream of water from the tank, that stream now has the power to do something – turn a wheel, wash your driveway, transfer the water from one place to another. So it is the combination of potential energy (voltage) and motive force (current) that gives us the power to do something.
In a perfect world the voltage and the current work together to deliver the power. This is referred to as a Power Factor (PF) of 1 – meaning the voltage and the current work together in synchronous harmony. A power factor of something less than 1 means the voltage is doing its thing but the current isn’t in synch. For our water analogy it would be as if the output of the tank came in spurts rather than a steady flow.
If we were to look at both current and voltage with a PF of 1, on an oscilloscope, we would see two identical movements. In the case of power coming out of our wall, that movement would look like two sine waves – one for voltage and one for current – both moving in harmony.
But when the PF is less than 1 then the voltage sine wave looks perfect but the current sine wave doesn’t. The current sine wave does nothing for a little bit and then shoots up and all kinds of current flows and then it stops flowing. Here’s a picture of what the current and voltage looks like when they are not tracking each other:
The blue trace is the current and the yellow trace is the voltage. With a PF of 1 (the ideal) the blue trace would look like the yellow trace.
Conventional power supplies all have this effect on power delivery. Why do we care? Well, we care for a couple of reasons: what it does to our AC line quality and noise.
If you have enough current being drawn at the same time with a low PF, as is typical with any piece of equipment with a PF of less than 1, after a while the top of the pretty voltage sine wave begins to collapse – something we refer to as flat topping. When this happens every piece of gear connected suffers performance because it begins to get starved for enough power right at the moment in time it’s most needed.
When you flatten out the sine wave in your wall something else happens – noise. That flattened area produces harmonics and extra unwanted “stuff”. Not good for our systems. The PS Power Plant adds energy back into the sine wave and fixes the flat topped sine wave problem for good. A power conditioner cannot help at all. This isn’t a shameless plug for a Power Plant (well maybe a little) but I wanted to simply point out a problem and a cure. 🙂
Lastly, because we share our power with our neighbors and everyone in our city, the effects of this flat topping are cumulative – because all sine waves coming into our home are in synch – so products that are attached to the AC power are all synchronized. The holy grail in power is a PF of 1, and then this flat topping never happens. A classic supply cannot do this: but a really tricked out SMPS can (although most don’t).