What does LLC converter stand for [Updated]

Last updated : Aug 18, 2022

Where are LLC converters used?

As an advantageous topology with electrically isolated features, a soft-switching function, high energy density, and high-frequency operation potential, the LLC resonant converters are widely used in EV charging stations, LED lighting drivers, photovoltaic systems, LCD TV power supplies, and other practical products.

How do LLC conversions work?

A regular LLC resonant half bridge converter works a combination mode of a voltage divider and amplifier of resonant inductor voltage of the resonant tank. At the resonant frequency, impedance of resonant tank is zero, which means the input voltage is 100% applied on the load.

What is an LLC resonant converter?

An LLC converter is made up of 4 blocks: the power switches, resonant tank, transformer, and diode rectifier (see Figure 1). First, the MOSFET power switches convert the input DC voltage into a high-frequency square wave.

What is an LLC transformer?

The LLC transformer is a key component in determining the efficiency of an LLC resonant converter. A LLC resonant power conversion topology minimizes switching losses by promoting zero voltage switching (ZVS), minimizing unnecessary power dissipation in switches.

What is LLC power supply?

An LLC current resonant power supply uses leakage inductance of a transformer for resonance and the voltage gain varies along with the switching frequency, which makes the design of a transformer more difficult than other control methods.

Is LLC bidirectional?

CLLLC, with its symmetric tank, is capable of bidirectional operation. The problem with using an LLC structure for bidirectional use is that the switching frequency, when operating in the reverse power flow mode, is governed by the transformer winding capacitance and the leakage inductance.

What is a PFC converter?

Power Factor Correction (PFC) shapes the input current of the power supply to be in synchronization with the mains voltage, in order to maximize the real power drawn from the mains.

What is ZVS and ZCS?

ZCS operates with constant on-time control, while ZVS operates with constant off-time control. With a wide input and load range, both techniques have to operate with a wide switching frequency range, making it not easy to design resonant converters optimally.

What is full bridge converter?

A full bridge converter is one of the commonly used configurations that offer isolation in addition to stepping up or down the input voltage. Other functions may include reversing the polarity and providing multiple output voltages simultaneously. Bridge converter has three main stages: The Square wave generator.

What is the need of resonant converter?

Circulating current in a resonant converter is used to charge or discharge the parasitic capacitors of the switching elements (e.g. MOSFET) during the switching dead-time, a period that all the switches are turned off.

What is soft switching?

Soft switching means that one or more power switches in a dc-dc converter have either the turn-on or turn-off switching losses eliminated. This is in contrast to hard switching, where both turn-on and turnoff of the power switches are done at high current and high voltage levels.

What is a resonance circuit?

Definition of resonant circuit An electric circuit which has very low impedance at a certain frequency. Resonant circuits are often built using an inductor, such as a coil, connected in parallel to a capacitor.

What is ZVS operation?

ZVS enables the voltage regulator to engage “soft switching”, avoiding the switching losses that are typically incurred during conventional PWM operation and timing.

What is the difference between active PFC and passive PFC?

Active PFC offers better THD and is significantly smaller and lighter than a passive PFC circuit (Figure 2). To reduce the size and cost of passive filter elements, an active PFC operates at a higher switching frequency than the 50 Hz/60 Hz line frequency.

What is the PFC circuit used for?

A power factor correction (PFC) circuit intentionally shapes the input current to be in phase with the instantaneous line voltage and minimizes the total apparent power consumed. While this is advantageous to utility companies, a PFC circuit also provides benefits in end applications.

How does PFC boost converter work?

The boost PFC converter uses a switching element to force the input AC current to be sinusoidal and in phase with the input voltage. The example used in this article showed that significant improvement to the power quality of a power supply can be obtained by using a boost PFC converter.

Why is ZVS preferred over Zcs?

In general, ZVS (half-wave mode) is more favorable than ZCS in DC-DC converters for high-frequency operation because the parasitic capacitance of the active switch and the diode will form a part of the resonant circuit.

How do you get Zcs?

To achieve ZVS, the COSS is tricked into discharging its energy before the gate signal is applied. Even a partial discharge is beneficial though ideally, all of the energy stored in COSS must be discharged into the load, bringing VDS to zero.

What is meant by hard switching?

Hard switching is a switching method that simply uses a device's own ability. Figure (a) shows a typical hard-switching current, voltage waveforms and its operating locus. During on-off transitions, both voltage and current are applied to the device.

What is a full converter?

20. What is mean by full converter? A fully controlled converter uses thyristors only and there is a wider control over the level of dc output voltage. It is also known as two quadrant converter.

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hello and welcome back to power electronics ee444 this is the last topic for the course and in this next set of videos we are going to cover resident dc to dc converters resident converters are a highly efficient type of dc to dc converter based on soft switching this first video is an introduction here's an overview for this video first i'm going to provide a block diagram of the resonant dc to dc converter structure then i'm going to show schematics of a series lc and a series llc resident converter we will look at what's called the equivalent load to the resonant circuit r sub e and then qualitatively i'll show how soft switching is accomplished in a series lc type of converter and that will also apply to series llc here are some really good references and i will provide this table in the description down below here's the block diagram of the generalized dc to dc resident converter there are three building blocks the first building block is a dc to ac inverter structure and we switch and have an output frequency of our inverter of f of switching fswx we feed this power waveform into a resonant circuit that has resonant frequency f sub zero f sub 0 and f sub switching are close in frequency but we will be controlling the switching frequency to control the output or the voltage output of our resonant circuit that signal will be applied to an ac to dc bridge rectifier we will be typically working at frequencies in the orders of tens of kilohertz hundreds of kilohertz even megahertz therefore the filter capacitor will be much smaller in size for our ac to dc bridge rectifier than what we typically used in our online type of rectifiers here's the schematic structure of a half bridge series lc resonant converter we see we have a half bridge on the left hand side of the circuit and the half bridge has two mosfets m1 and m2 switching in a complementary pair to create a square wave voltage at the node between them the height of the voltage is vdc and the lower side of the voltage is zero that square wave is fed into the resonant tank circuit that has a resonant frequency omega naught equal to one over the square root of lr times cr this tank circuit will smooth out the current flowing through the circuit that we feed into this transformer the transformer has a turns ratio of n to one to one it's center tapped and with n greater than one we will buck the system and create a lower output voltage and with n less than one we can boost the system here we show a full bridge rectifier with our center tap transformer we did study this in a previous homework problem finally we have a filter capacitors not cr that shouldn't be cr that's just a regular filter capacitor and an output load the llc resonant converter is similar to the series lc resonant converter with the addition of a shunt inductor l sub m typically we design or have the transformer designed to include magnetizing inductance uh and so it's not a separate component but part of how this transformer is designed and there's reasons for adding or having that l sub m that magnetizing inductance in the circuit which we'll talk about in later videos otherwise the topology is very similar to the series lc resonant converter when we do our analysis we would like to reflect the load back to an equivalent resistance the equivalent resistance r sub e is equal to 8 n squared divided by pi squared all times v out over i out in some publications it's written as eight n squared over pi squared times rl where rl is the output voltage divided by the output current and we will derive this equation in later videos the equivalent resistance allows us to further analyze our resonant circuit let's talk now about soft switching recall in past videos when we looked at switching the mosfets often times the voltage across the mosfet was held high by the load specifically an inductive load or the current flowing through the the switching mechanism was also high that's called hard switching and creates electrical stress on the device when we are trying to switch at at either full voltages or at full current loads in soft switching we switch when the voltage across the drain to source is nearly zero or at zero volts or when the current load is near zero so here we show our switching arrangement and this blue line is the output at this node uh we have a 20 volt source and we see we are switching between 20 volts and zero volts the red line is the current through our load and we see that that current i is sinusoidal in nature we also see that we are switching m1 to m2 when the current is nearly zero let's look at this in more detail let's specifically look at the case where m1 is closed and m2 is open and here we would have this switch is closed m1 is closed and m2 is open and that is right up here right before the switching moment right at this point we want to break before make and i've talked about this before where we're going to break m1 before making m2 when m1 opens up the current is still flowing in a positive direction but as m1 opens up the current will switch and flow through the body diode of m2 at this point in time right before the switching we start to turn off m1 m1 opens up the current is still flowing a little bit and starts to flow through the body diode of m2 when that happens the voltage across m2 becomes near zero it's basically a voltage a diode drop of about 0.5 volts to 0.7 volts depending on the body diode and at that point is when we turn m2 on so we see that we are at a near zero current condition and when we turn m2 on the voltage drain to source across m2 will also be near zero volts so we have zero voltage switching and zero current switching let's look at the case now where m1 is open m2 is closed this is the case where m2 is closed and m1 is open and right before the switching event right before this event of switching we see that the current is nearly zero through the load and the current is also negative meaning the current is flowing in this direction we are going to do a break before make with m2 so we're going to open up m2 before closing m1 the current has to flow somewhere and so the current that was flowing through m2 is now going to flow through the body diode of m1 just before this point before turning m1 on the current will switch directions and flow through the body diode of m1 when that happens the voltage drain to source across m1 will be approximately zero it'll be one diode drop across and at that point is when we turn m1 on again we will have a near zero voltage switching condition for m1 with a near zero current condition as well uh it that that is called soft switching and it it is extremely low power and easier on the switches on on the mosfets so let's cover the key points in this introductory video first we looked at the circuit for our resonant converters it was built on switching at a frequency that is near resonant with our dc to ac inverter and it had a tank circuit and it was followed by our bridge rectifier in the schematics we had we showed a transformer and that transformer in the llc configuration provides shunt inductance via its magnetizing inductanc

Thanks for your comment Clint Zills, have a nice day.
- Chi Domowicz, Staff Member

hi I'm Sabine Yaakov this presentations entitled resonant LLC converter power states design the intuitive approach let me start off with a basic resonant Network here we have a source resonant inductor capacitor and a load this is the expression for the V out to V in ratio V out to be in ratio which is represented here in sort of a normalized way this is the ratio and this is frequency or normalized frequency where the Q is defined as the resonant times L r over r ec being a series resonant network or as Z over RC when RAC when Z in the square root of the inductor divided by the capacitor now I series resonant converter which is based on the series resonant network we have a no Morea transformer than a rectifier a load and a a filter capacitor and of course we have a drive in this case I'm showing a half bridge drive this is a nonlinear circuit and it is rather difficult to get a relationship like V out to V in and therefore it is customary and it's very convenient to sort of represent this network in the series resonant network that we can analyze by phasor analysis that is we don't have to do time domain analysis as we have to do here because we cannot linearize this circuit being a highly nonlinear circuit now the way to do this X has two parts to it first of all we want to replace the square wave here by a sinusoidal waveform this is the half bridge this is a full bridge in the idea is that this resonant network is actually a filter so when you feed in a square wave you maybe see the first harmonic say this is the switching frequency this is the switching frequency of this square wave and so you are actually fitting here the square wave and the current would be pretty much like a sinusoidal current depending of course on the Accu for very low Q it will not be the case but usually or run these converter at very high Q no too high too though and therefore you will see the first harmonics the major harmonics but then they hire our mornings this is like the breakdown general representation of the breakdown of the harmonics of the square wave like 3rd 5th etc would be pretty far away so yeah they're not not going to see it so the first step would be to replace the square wave that we have here by the first component first harmonics now as it turns out by Fourier analysis they relationship between the peak of the first harmonics to the height of the maximum value of the square wave is 4 over pi so this is the first step this is for a full bridge and if we have a half bridge we have only half the value so we have to divide it by 2 so this takes care of this sinusoidal waveform that now we can replace the square wave by this source and this would be the relationship between the square wave and this the source now what about the resistor it was shown by Professor Steigerwald that one way to do it is really to equate power dissipation that is if we have a nonlinear circuit like this this would be like the resonant converter we assume that there is a sinusoidal current here coming from the converter for the reason I've already said this is rectified we got this rectified current and the average of this rectified current goes into the load now equating the power means that if I have this equivalent circuit I'd like the power dissipated like this linear resistor now the equivalent our AC register this part to be equal to the power dissipated by this DC resistor so we have the relationship between the DC current here and the peak value of the sinusoidal current at the input and since power is I swear over RL this is for the DC here and the same thing goes for the AC and we have already the relationship between the AC and DC it comes out that the RAC is 8 over pi square RL is about point eight that is you can replace this whole thing by a linear representation our AC being eight over PI square this resistor now there's also of course a relationship between the voltage that you'll find here the DC voltage and the voltage that you'll find here and here is the relationship again I'm doing it my equating the power the power here is v square of DC over RL the power here is V AC of square over RAC and by equating this to we get with this relationship that the voltage the AC voltage this will be RMS voltage here is related to the DC voltage here by about 0.9 now in many cases we would have a transformer so we have to reflect they are AC calculated safe secondary to the primary and of course the reflection is done by multiplying it by n square and being the ratio between primary and secondary so by this we can now modify the RAC to be to take into account the transformer and therefore we can have now a AC equivalent linear circuit that can be analyzed by phasor analysis and in theis pspice ltspice or any other circuit simulator it can be run by AC analysis this cannot be run by AC analysis because AC analysis is proper for linear circuits and this is a nonlinear circuit so now let's go back to the converter itself this is again a series resonant converter and one thing we have to worry about is the gain we can get and the reason we need again is that no money we'd like to have a constant output and therefore if the input is changing then the ratio between input and output has to be controlled and this is of course done by the feedback such that when the voltage is deviating from the required value it will change the switching frequency in this case so as to bring it back so the first thing we have to worry about is the gain that we can get and let's say that we need the gain between point four point six just arbitrary as an example this would mean that for the plots that I have shown here you need if you'd like to go from q1 to about q5 then you'd need this span of frequency which is very high so this is the reason why this series is a nun converter it's not very popular as a voltage regulator that when you have to regulate the voltage it's very good by by the way for current sourcing but this isn't beyond the subject I'm discussing here another way to do it is to use a multi resonant converter which is for example the LLC LLC means that you have two inductors rather than one any capacitor the idea here is as follows you have now two inductors this is again the RIC the equivalent circuit is done exactly the same way I've shown before and in this case we have two inductors if our AC here is smaller much smaller than the impedance of this LM at the operating point on either President you might take then the impedance of L sub M doesn't play any role and you have actually a serious resonant converter and this will be the resonant frequency and the cube for this circuit you can define it as the Omega R this Omega R by these two elements and our AC however if our AC is much larger than Omega L M then LM of course will pay a role here and if LM is larger than L R you can sort of neglect its first approximation L R and you see that you have now a parallel circuit here parallel resonant circuit here the frequency again will be one over two pi now LM CR and of course the this frequency turns out to be at a lower frequency than the frequency we have seen before

Thanks jdemolarj your participation is very much appreciated
- Chi Domowicz

About the author

Chi Domowicz

I've studied quantum technology at University of Maryland, Baltimore in Baltimore and I am an expert in social engineering. I usually feel drained. My previous job was food science technicians I held this position for 18 years, I love talking about watching movies and noodling. Huge fan of LeBron James I practice snow skiing and collect books.