PFC for linear power supplies, hybrids and test equipment - Page 1
Forum section: electronics / projects

author: b_force
author_rank: Super Contributor
timestamp: August 02, 2016, 10:28:13 am
content: I have been searching around for a while now, but I can't find any information about PFC and linear power supplies.All information is talking about PFC in combination with a SMPS design.Is there any difference with linear power supplies?I have seen many tear-downs from Lab bench power supplies, but in many cases it is obvious that there is no PFC added to the circuit.Is there an exception for test equipment?And how about hybrids? (so using a conventional linear power supply with a buck converter after the rectifier and capacitors)

author: MosherIV
author_rank: Super Contributor
timestamp: August 02, 2016, 10:37:07 am
content: You only really need to do PFC when the power level goes above 150W, hence most linear bench PSU do not have PFC.Most bench PSUs will only go upto 4A (maybe 5A) and usually no more than 30V, that is only a max of 120W (150W)PFC is really about minimizing current draw near zero crossing of AC, since there is no voltage to drive current. So PFC tries to maximise the current draw at peak voltage of the AC.

author: b_force
author_rank: Super Contributor
timestamp: August 02, 2016, 10:48:36 am
content: I thought PFC is needed for everything that goes above 75W?Or does this not fall into the Class A category, or can this be seen as professional equipment?

author: MosherIV
author_rank: Super Contributor
timestamp: August 02, 2016, 11:10:20 am
content: QuoteI am not aware of any regulations, here in the UK, that require equipment that low power to be fitted with PFC.(Correct me if I am wrong)QuoteI am not aware of what equipment classification you are referring to.QuoteIf the power is in excess of 200W then it is a good idea to add PFC.eg many early PC SMPS PSUs (250-300W) did not have PFC. Now PC SMPS are a minimum of 350W (more likely 450W) they all have PFC.Most Linear PSUs are unlikely to need PFC because the transformer itself tends to stop current surges around the zero volt crossing.True, you can get a very large switch on surge but that is different to current draw near the zero volt crossing.Again, PFC is about minimising current draw when there is little/no voltage available to push current.

author: oldway
author_rank: Super Contributor
timestamp: August 02, 2016, 11:36:38 am
content: QuoteYes, there is a good reason for this: to work correctly, PFC must charge a capacitor at a voltage higher than the peak value of ac voltage, that's (230V + 10%) 253V x 1.4142 = 357.8VOutput voltage of PFC is choosen between 390V and 400V.This is too high for a linear power supply but very good for SMPS.

author: b_force
author_rank: Super Contributor
timestamp: August 02, 2016, 11:36:44 am
content: @MosherIVI thought PFC was all about correcting the power factor, or in other words the mismatch between the phase of the voltage and current?So, in other words, you say that there is no or little phase shift when using a linear power supply?It's also a little confusing, because most guidelines speak about EMC and PFC in one single breath.I guess with SMPS you trow a lot of 'garbage' (= EMC) back into the net.I was referring to EN61000-3-2https://en.wikipedia.org/wiki/IEC_61000-3-2(I have an official hard copy myself as well)

author: bktemp
author_rank: Super Contributor
timestamp: August 02, 2016, 11:50:00 am
content: Quote from: b_force on August 02, 2016, 11:36:44 amPower factor is not only about the phase shift, a rectifier produces almost no phase shift, because most current flows at the peak voltage, but it has no perfect power factor, because the current flows only at the peak voltage.power factor = active power / apparent powerThe stray inductance of a transformer and its copper resistance improve the power factor of a secondary side rectifier compared to the rectifier in a SMPS. That's why PFCs are only used in a SMPS.

author: MosherIV
author_rank: Super Contributor
timestamp: August 02, 2016, 11:58:24 am
content: QuoteYes, that is true for large inductive motors. Inductive motors skew the phase of current verses voltage.So for factories that have lots of machine tools with inductive motors, PFC was about correcting the phase of current v Voltage.Factories would be charged more for electricity if they were drawing more current off phase hence it was worth spending the money to correct the phase.I would guess that this is less of an issue in today's industrial environment because most machine tools use VFD to drive the motors now.The VFDs probably have PFC built into them.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 02, 2016, 12:07:37 pm
content: AFAIK the 75 W (or VA ?) power limit applies to the typical power used. For a typical lab supply and Hifi amplifier this is essentially the idle state or low power. So usually you are below that limit, except maybe class A amps - but those are usually a little crazy and don't care much about energy saving anyway.Pure linear power supplies are often made because of lower noise. So you usually don't want the switching noise of the modern active PFC as used with SMPS. There is still the option to use old style passive PFC with an series inductor. The inductor can be on either side of the transformer / rectifier. It's usually not as good as active PFC and might not be enough to meet limits for higher power supplies. The passive PFC inductor might be nearly as heavy as the transformer - so it's not that easy. Also the output impedance goes up - which can lead to higher losses. So not many fans of PFC with linear supplies.Stray inductance and resistance of the transformer helps a little, but this gets relatively small with larger transformers. It's more like you should add a little inductance / resistance with larger units, not to get to high current peaks.

author: b_force
author_rank: Super Contributor
timestamp: August 02, 2016, 12:13:37 pm
content: Ok, but still, even a linear power supply can have a PF as bad as 0.6-0.7 (sometimes even lower).Which is, to my knowledge, pretty much the same as a SMPS.The main difference I think is that a SMPS kicks back a ton of (high) harmonic garbage back into the net.Am I right that the conclusion must be that the PFC stage in a SMPS has more to do with that harmonic garbage (= EMC) than actually correcting the PF?Which is nothing more than a nice extra when correcting the harmonic content?Most people refer to EN61000, (hence the 75W) but that standard is clearly ONLY about the emissions going back into the net, which doesn't apply to a linear power supply.It doesn't say anything about when to correct the PF on to which level.Are there any standards about when a certain amount of correction is needed for a certain application and power?

author: MosherIV
author_rank: Super Contributor
timestamp: August 02, 2016, 12:21:45 pm
content: QuoteIn my opinion - NO.This is basic CE compliance. In Europe, all electronic equipment must comply with a common set of electrical standards for radiated and conducted emissions.Simply, they state that no equipment is allowed to emit any significant level of noise and also not be affected by noise.The PFC may help with that but it is NOT because of it.SMPS switching frequencies are asynchronous to the mains frequency, so SMPS may try to draw current at any point in the mains cycle.If the SMPS tries to draw current when there is little/no voltage to push current, this can cause the voltage to further sag - hence the need for SMPS to have PFC.

author: madires
author_rank: Super Contributor
timestamp: August 02, 2016, 01:09:50 pm
content: You might want to readhttp://www.onsemi.com/pub_link/Collateral/HBD853-D.PDF.

author: oldway
author_rank: Super Contributor
timestamp: August 02, 2016, 01:54:41 pm
content: A lot of people are wrong because of the name PFC.The true function of a PFC is not to correct the power factor but to modify the current to be perfectly sine.A rectifier with capacitive filtering produce high peak currents during a very small angle of conduction.Such a current is very bad for the mains ...1) rms value is very high , this make the cables warm and there is a loss of energy.2) harmonics content is very bad, this can induce oscillations and overvoltages.This problem of high peak current occur as well with SMPS (without PFC) than with linear power supply, even if the leakage inductance of the transformer reduces a little this peak current.

author: b_force
author_rank: Super Contributor
timestamp: August 02, 2016, 02:15:39 pm
content: Quote from: madires on August 02, 2016, 01:09:50 pmI am familiar with that handbook, but it doesn't give an answer to my questions.Once again, this document only goes into IEC61000 and the harmonics coming from a certain power supply, but only for SMPS designs and not for linear power supplies.It also doesn't give an answer when PFC is needed from a phase shift point of view.

author: MosherIV
author_rank: Super Contributor
timestamp: August 02, 2016, 02:32:59 pm
content: QuoteI think the general consensus from all the posts is that you do not need PFC for linear PSUs, for all the reasons stated.When do you need PFC?When it becomes a problem.....at low currents it is not going to cause a big problem drawing current from mains voltage when the voltage is not at it's peak.As the load increases and the current draw increases, the voltage may not be able to drive the current needed : either the voltage of the mains will sag or the load will fail to get the current it needs.Most engineers would not let it get that far, they would engineer in a safety margin. That is why PFC is considered when the power output reaches 120/150W

author: b_force
author_rank: Super Contributor
timestamp: August 02, 2016, 02:44:23 pm
content: Ok, but even from linear power supplies you can expect very ugly current behavior?Especially with some switching buck convertors behind the mains transformer/rectifier.(kind of a hybrid solution)You're saying 150W is a reasonable limit. Where is this based on? (not on IEC61000) When does it becomes a problem?There must be some kind of limit somewhere, otherwise they wouldn't add extra capacitors in fluorescent lamps and other equipment.Besides, I have seen power supplies and audio amplifiers easily going over 300-500W.

author: bktemp
author_rank: Super Contributor
timestamp: August 02, 2016, 02:45:50 pm
content: Quote from: MosherIV on August 02, 2016, 02:32:59 pmSorry, but that sounds wrong.You can build SMPS in kW range without any PFC without much problems. You just need to increase the capacitors to store enough energy while the voltage is high enough, but this will also increase the peak current and make the power factor very low.If you look at the typical mains voltage, it is not a sinewave anymore, because all the SMPS without PFC draw current only during the peak, the voltage gets loaded only during this period, distorting the sinewave. I can measure 2-3% THD and can easily see the distortions on the scope.The main purpose of a PFC is to reduce the harmonics on the current.

author: MosherIV
author_rank: Super Contributor
timestamp: August 02, 2016, 03:00:10 pm
content: QuoteNo, linear PSU do not overly distort the current phase in relation to the voltage phase.A linear PSU can only top up the capacitors (which is the largest power/current draw) on each mains voltage peak - therefore they do not cause much current phase distortion.QuoteThe switching buck after the transformer will draw power mainly from the capacitors of the linear thereby not distorting the current/voltage relationship.QuoteI do not have any specific laws or regulation to base this on. I seem to remember reading a Microchip application note about one of their PFC devices and they seem to imply the 150W.QuoteWhen the current draw combined with the PF is such that too much current is being pulled at the wrong part of the mains waveform.As someone pointed out : cables can get hot.The PSU may not be able to maintain output correctlyWorst case, mains voltage is distorted to the point where other electrical equipment stop working correctly, or the electricity supplier starts to notice.

author: b_force
author_rank: Super Contributor
timestamp: August 02, 2016, 03:16:25 pm
content: Yes, I think everybody already can imagine what can go wrong, that's also not the point.The point is, what IS acceptable?Or is that purely based on subjective ideas?There must be some kind of standard/regulations.Only thing I can find is reducing the harmonics, like bktemp said. (the 61000 standard)But that doesn't tell me anything about the rest of the story.

author: madires
author_rank: Super Contributor
timestamp: August 02, 2016, 04:09:37 pm
content: AFAIK, there are no standards for the power factor, at least not in Germany. But the electricity suppliers require some minimal power factor in their contracts. I've just check one and it states, that the customer has to stay above 0.9 (inductive loads), should avoid any capacitive overcompensation, and also avoid any load-independed fixed compensation.

author: b_force
author_rank: Super Contributor
timestamp: August 02, 2016, 04:48:46 pm
content: Quote from: madires on August 02, 2016, 04:09:37 pmWell if Germany doesn't have any standards, I don't think anyone else hasA power factor of 0.9 is pretty high.You don't reach that with a standard linear supply.It actually amazes me that this isn't regulated by laws.At this point it's all about subjective ideas for what is reasonable.I guess developers need to check every contract from the electricity suppliers.

author: C
author_rank: Super Contributor
timestamp: August 02, 2016, 05:21:08 pm
content: b_forceIf you are the one creating the AC power source, then PFC(a resistive load) is more important. You have two choices put in a larger AC power source or do better at creating a resistive load for that power source.It comes down to a question of cost and where to spend that money.When you look at only PFC then the costs are higher. If PFC can make your circuit better while doing PFC then the costs are lower.Rules are trying to force the extra cost where it will do the most good.For the PC's, size and weight first made a switching supply look better, but a switching supply can still have the diode bridge/capacitor of a linear supply. The rules are a force to spend a little more so that the AC Source can be better.Large capacitors after a diode bridge are bad in that they create larger current spikes to charge those capacitors. The large capacitors would let you to continue to function for a while with loss of AC Power. Here you are talking the cost difference of reducing current spikes and inrush with the possible gains of smaller transformer.The keep it low cost changes what is looked at. At some point an AC input to an H-Bridge becomes cheaper. Some of the higher cost methods also allow bidirectional AC power flow.

author: MagicSmoker
author_rank: Super Contributor
timestamp: August 02, 2016, 06:51:40 pm
content: Quote from: AcHmed99 on August 02, 2016, 02:02:00 pmA common misunderstanding, this... Power factor correction is one or more of the following:1. Bring the phase angle of the current waveform in sync with the voltage waveform (whatever that voltage waveform may look like!).2. Reduce the harmonic content of the current waveform - usually regulations state the % magnitude of the current allowed for each harmonic. Note that you can have rather high THD and still have good power factor. See, for example, the block-shaped current when a large inductance, small capacitance LC filter is used after rectification.3. Reduce the crest factor of the current waveform (which is inextricably linked to #2). That is to say, make the peak-to-average of the current waveform match the voltage waveform.As for the motivation behind PFC regulations, the reasons given depend on whom you ask, it seems, but the two that make sense to me are that low PF results in increased I²R heating of all the conductors in the supply network and that on common 3ph distribution networks the high crest factor/THD waveform of the straight capacitor input mains rectifier/filter results in a lot of 3rd harmonic in the neutral conductor of Wye networks, and the 3rd harmonic currents from each phase-to-neutral branch always sum together in the common neutral which is not supposed to be fused or otherwise protected against overcurrent.As for putting PFC in a linear supply, you can, you just have to do it on the secondary side of the mains transformer. Seems kinda pointless to me, because the only compelling reasons for using a linear power supply these days is to minimize noise and/or increase robustness/reliability and a PFC stage is usually a switcher, so noisy, and usually a boost topology, so has some interesting (read: spectacular) failure modes (though to be fair, well designed and loop compensated boost PFC stages are highly reliable).

author: Kleinstein
author_rank: Super Contributor
timestamp: August 02, 2016, 07:29:44 pm
content: For the power factor there are two effects: one is a phase shift between voltage and phase. This is the more classical part that applies to AC motors, fluorescent lamps and similar. The second part is harmonic distortion in the current drawn - this mainly applies to rectifier and filter cap combinations. Here there is no real difference between a SMPS (without extra PFC) and a linear stabilized supply as the source is the rectifier and not the regulation. This gets a problem if large such loads are present as it started with power hungry PC and TVs. The high frequency EMI part is a different part and usually not a big deal with a linear PSU.There are legal limits for new (already quite some time) units in the EU, and they should apply no matter if it is build as a SMPS or a linear one. However the limits are really strict only for higher power devices. AFAIK there are also some exceptions for handheld devices: e.g. to allow a handheld hairdryer to use a simple diode to reduce the power.It's just that linear supplies are usually small and AFAIK the important case is the typical power drawn, not the maximum power. Because of this Hifi amplifiers and linear lab supplies usually don't have a problem with this because in this sense they are usually low power devices.If you want to build something like a 150 W class A audio amplifier (thus with a high power as it's normal mode of operation), you will need some extra provisions for PFC to legally have the CE marking. However a more normal 300 W class AB amplifier will not have a problem as is can still be low power under normal use (e.g. < 10 W output level).

author: seriouscoinage
author_rank: Contributor
timestamp: August 02, 2016, 11:29:03 pm
content: I think the point that MagicSmoker is trying to make is that a distorted current waveform can cause a low PF, even if it is in phase with the voltage waveform. For example, if your load draws its current in a narrow pulse at the peak of the voltage waveform (e.g. a rectifier/capacitor power supply) you will have a low power factor even though the current waveform is in phase with the voltage. The total power delivered to the load (the average of the instantaneous voltage times instantaneous current) will be much less than the volt-amps/apparent power delivered to the load, which is the RMS line voltage times the RMS current.

author: Someone
author_rank: Super Contributor
timestamp: August 02, 2016, 11:54:25 pm
content: Quote from: AcHmed99 on August 02, 2016, 08:16:30 pmNot completely, Power Factor is a combination of many things and you geometrically sum the contributions of harmonics to the traditional lead-lag component. You cannot have a poor THD (harmonic content) and a good Power Factor, although many PFC systems rely on calculating the harmonics of the long term average rather of individual cycles on the assumption there will be many of them in operation cancelling each other out.Quote from: Kleinstein on August 02, 2016, 07:29:44 pmThis is the crux of it, its how the DC is first created that is causing power quality issues. The intermediate DC voltage could then be used for any arbitrary purpose. Although highly pulsed loads could have problems with active PFC as typical loads rely on the averaging above to achieve their ideal figures.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 03, 2016, 07:11:52 am
content: To get a good power factor the waveform of the current has to be the same as the waveform of the voltage. So if the voltage is in crazy shape, the best power factor is having that same crazy waveform as well. However normally the waveform in the grid is still reasonably close to a sine, maybe sometimes going towards a rectangular shape.The PF combines the effects of harmonics and phase shift in one number.

author: b_force
author_rank: Super Contributor
timestamp: August 03, 2016, 11:52:47 am
content: Quote from: AcHmed99 on August 03, 2016, 12:29:48 amThat blog makes it very understandable.It's only not completely clear to me what the difference is between PF and DPF?(doesn't say anywhere)Another thing I don't completely get, is why a small but higher current spike is worse.At the end we are talking about energy (or kWh).The total energy of a small spike can be a whole lot less than a low, but longer perfect sine wave shape.

author: SeanB
author_rank: Super Contributor
timestamp: August 03, 2016, 11:55:59 am
content: PFC for anything with a SMPS inside is more to reduce harmonics in the neutral in the 3 phase distribution system. As the neutral typically is a thinner cable ( as it only has to handle the difference currents in the system, so can be smaller than the phase conductors in a supply) the extra harmonics tend to add up and overload it. Thus the need to reduce harmonics of the mains in a SMPS.With inductive loads like transformers and motors you can add either a simple capacitive correction , or for larger transformers with a diode rectifier you add a series LC circuit tuned to 3 times mains frequency, to reduce those harmonics. Alternatively you use a ferroresonant transformer, which provides enough extra leakage inductance to isolate the power factor.The power companies ( and large industrial users) have to add power factor correction to the supply side so this is not reflected down the power transmission lines, so as to reduce RF emission from the lines. Thus they also have power factor compensation capacitor banks, harmonic trap filters and synchronous capacitors in the power yards so as to keep the power factor over 0.9 lagging with only low levels of harmonics on the incoming feeds. This is a non billable power consumption, so they ideally want to keep this loss as low as possible.

author: oldway
author_rank: Super Contributor
timestamp: August 03, 2016, 01:40:44 pm
content: A very good explanation:https://us.tdk-lambda.com/ftp/other/pfc_switchmode_powersupplies.pdf