Power factor correction is always a topic in power electronics or in electronics general, but often its quite unclear what it means. In the following I try to explain it a little bit and give some practical tips and hints for your design or future design.
First of all what is this thing called power factor. A detailed description can be found here at Wikipedia but what does that mean in practice. When talking about power factor you always talk about AC (or mains) powered devices. For the energy provider it is highly important that the load, that is drawn from its power grid and power plants is a resistive load – that means power factor is 1 or close to 1.
What does that mean in practice? Lets draw some circuits to show …
The upper circuit you see is just an AC source with an resistor. Pure resistive load. The second schematic you see is a AC source, with rectifier, ideal cap and resistor. Again, just a resistor load. But, show both the same ohmic behaviour to the AC source?
The simulation plot will tell us:
First plot is the resistor only schematic and the second plot is the rectifying circuit, and no there is nothing wrong with the simulation, but why is it? In the first circuit you have power factor 1 – ideal world example. The second one is a more real world like example. In your application you need a certain DC voltage, so AC is going to be rectified and then transformed in the desired DC voltage rail for your application – in this example then replaced by a resistor for simplicity. When you look at the second plot and the current waveform, this waveform isnt looking like a sine wave at all like in the first plot – so the power factor turned very poor. This comes from the cap in the circuit which keeps the voltage stable for the output but it also needs to be charged from the input and so the input current is very high at a very short amount of time to get the energy into the cap that needs to be provided to the output.
What does poor power factor for the power provider mean? In the first plot with the good power factor you see the current is very low and in the second one the peak current is very high – 22mA vs ~1.2A . The load is similar in both cases but in the second case the provider has to use much bigger cables to provide the same power which is just uneconomically and waste of material. So we’ve discovered the root cause why power factor correction is necessary.
When is power factor correction needed? Well, there are different limits in standards. For industrial electronics the standards say that everything at 75W and above needs active power factor correction and with lighting there is the limit at 25W, every LED driver at and above 25W needs active power factor correction.
So what can be done for power factor correction? There are two ways: active and passive. Passive power factor correction can be done with input filters but with 50Hz frequency and the goal of power factor of 1 the filters are just huge, in size and cost. Active power factor correction is the approach to have smaller input filters but an active circuit that keeps the power factor at a high level.
In practice these circuits are either boost or flyback converters – it depends on your application. For output powers lower than 100W a flyback solution is often a very charming solution and for higher output powers there is no way around boost converters (multiple phases, bridgless rect. and so on).
These circuits put an inductor on the input side and switch it very fast. With this switching they are modulating the input current in a way, that the current seen by the AC source is looking like a sine wave. Of course there are some input filters needed to filter harmonics but that are much smaller filters than with purely passive PFC.
Lets talk about the practical hints: There are plenty of IC’s in the market that promise a good power factor correction and at nominal load this fact is true but thats not the complete truth. When you do a power supply design you design it around a maximum load that can occur. This means you almost never achieve the nominal output power in real life for this power supply. And there comes the downside. A lot of these parts, especially the cheap ones, are optimized for a maximum point of load where the power factor is good. This makes sense for simple and cheap lighting applications without dimming but when it comes to part-loads or something they quickly loose power factor and you’ll end up with a power factor of 0.4 or lower at 50% of your nominal load which is indeed not the way a circuit should perform.
When it comes to topology it again is dependent on your application. If your application needs lower than 100W a flyback maybe is a good fit – but be aware at the output of the flyback there then a 100Hz ripple that comes from the AC side so I recommend to do an AC/DC which is isolating and then a DC/DC which then takes care of your output voltage. This is also a good approach for engineers which do want to do a plattform for their power supplies. Design it once and then change the output voltage with the DC/DC in a very easy way. Also makes a lot less headache at the EMC testing.
If you need more power then go for a boost pfc stage and then an isolating DC/DC which meets your application. It can be a push pull or fullbridge for example but be cautious with LLC topologies after your boost stage. The reason is simple, Boost-PFC-stages are in general fixed frequency and variable duty cycle circuits, LLC are fixed duty cycle and variable frequency this leads to harmonics on the charge and discharge currents of the electrolytic cap between this stages and damages it which results in a short or almost no lifetime of your power supply.
My favorite part for flyback-pfc solutions below 100W is the LT3798 and this is not because I’m a Linear Tech fanboy – its because this part shows good power factor behaviour over a wide load range. I’ve tested a lot of such parts and there was just one part that was as good as this but the manufacturer made it obsolete two years ago.
At least here is a plot I’ve made some time ago about the power factor behaviour of the LT3798. As you can see at 40% of the max. load the power factor is still at 0,9, which is pretty awesome analog engineering and makes the PFC curve almost ideal.
on the x-axis there is the percentage of the output power and on the y-axis there is the power factor.
Hope this article helped a little for getting some insights into PFC. If there is the need of further informations or you’ve spotted mistakes – just comment.
Happy engineering !
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