Presentazione standard1

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Wide input range LLC resonant converters, analysis of limits and drawbacksWide input range LLC resonant converters, analysis of limits and drawbacks

Magnetica di Roberto Volpini.

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Milan, June 6th 2012Relator – Manuele MarconiMilan, June 6th 2012Relator – Manuele Marconi

Presentation outlinePresentation outline Magnetica di Roberto Volpini.

3Wide-range LLC convertersWide-range LLC converters Magnetica di Roberto Volpini.

Presentation outline

Resonant converter are best used with narrow (and high) range of input voltages, thus often requiring a PFC pre-regulator when operated from the mains. This reduces the switching frequency span required to deal with all situations.

Nonetheless, PFC is a cost. Operation from a wide (and relatively low) range of input voltage is still possible, but has drawbacks and limitations which the designer must account for.

These slides are meant to show what these limitations are, where they come from and how to face them.

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Main topic:Wide-range LLC convertersMain topic:Wide-range LLC converters

Wide-range LLC convertersWide-range LLC converters Magnetica di Roberto Volpini.

5Wide-range LLC convertersWide-range LLC converters Magnetica di Roberto Volpini

Simplified circuit model of resonant LLC converter used in this discussion


Using LLC resonant converter with wide input mains range has drawbacks and limitations

1. Working too close to the capacitive mode2. Low attainable peak to nominal output power ratio, or must be traded off against efficiency

especially at low loads3. Wasting precious dynamics towards the high range of output powers.4. Controlled tolerances on Cr, Ls and Lp or on the primary current sensing components 5. Lower duty-cycle of the “energy taking phase” when compared to high input voltages6. Harder to perform optimization and to choose the most appropriate power switches or drivers7. Harder to design the LLC transformer, due to tighter constraints


Possible to work close to the potentially destructive capacitive mode.

The Q curves vary with the load (as a rule of thumb, Q is proportional to Iout):

Q=0 means OPEN LOADQ=∞ means SHORT CIRCUIT


The boundary between capacitive and inductive modes can be better understood by analysis of Input impedance of the resonant tank versus frequency

In the white zone the input impedance Zin can be either inductive or capacitive, depending on the load.

We can see this looking at the slope of the input impedance curve. Positive slope means inductive, negative slope means capacitive.

This also means that, for each frequency x between fno and fnr there is a max load (Qm(x)) that, if crossed, makes the input impedance of the tank capacitive.


Calculation of the value of the Critical Resistance Rcrit

Zo0 Zo∞

Zo0 = output impedance with input short-circuitedZo∞ = output impedance with input open-circuited

The value of the critical resistance is found to be frequency dependent as supposed


Calculation example

Parameter Value

Ls (primary referred leakage) 75uH

Lp (magnetizing inductance) 470uH

a (ideal turn ratio) 6

Cr (resonant capacitance) 15nF

Pout, max (nominal output power)

150W

Pout, max peak power +10%

Vout (nominal output voltage) 28V

Fr1 150kHz

Fr2 56kHz

For even higher loads, capacitive mode is unavoidable at that frequency anyhow!

Solution: sensing (and limiting) of primary resonant current

To avoid capacitive mode at peak output power , the minimum switching frequency must be set to 89kHz.


Why is capacitive mode so bad?

What happens in capacitive mode is, moving down away from boundary line:1. Losing ZVS2. Eventually reversing the control law VS frequency


Loss of ZVS condition

1. Hard switching Q1 & Q22. Body diode of Q1&Q2

reverse-recovered3. High level of energetic EMI4. Large and energetic

negative voltage spikes in the HB

5. High current capability of gate drivers and low driving impedance required

6. higher demands on Cr’s peak voltage specification (+50% of what is normally reached when in inductive mode)


Tolerances and imprecision in the current limiting circuitry

If the design requires a very high value of λ (we are in the case of λ=0.9), the programmed fmin should be very precise, or should account for tolerances in the Lp, Ls and Cr value themselves.

If not, a very slight frequency decrease in response to a sudden load or line change, and when already under resonance, could bring the converter in the capacitive region.


Low duty-cycle of the “energy taking phase” (1)

Shown in yellow, from t0 to t1, is the duration of the period during which energy is taken from the input

At resonance.


Low duty-cycle of the “energy taking phase” (2)

Frequency is pushed down, the duty-cycle is shorter and the peak current value is higher.

high input currents at low line, stress on MOSFETs, input and resonant caps.

Below resonance, at lower input voltage.


Split of the resonant capacitor in two caps with halved value

Reduced AC current requirements on both Cin and the Cr/2

Reduction of the differential mode noise caused by d(Iin)/dt.

Best suited for high power levels and also in PFC-less LLC converters

17AppendixAppendix Magnetica di Roberto Volpini.

AppendixAppendix


Differences between output rectifiers configurations

Center-tapped output with full-wave rectification.Suitable for low output voltages with high currentsVout=Vsec-VFVRRM~= 2 Vout + VF

Single-ended output with bridge rectification.Suitable for high output voltages with low currentsVout= Vsec – 2 VFVRRM~= VoutIn the single ended configuration, LL2 doesn’t add up reverse voltage on reverse biased diodes

For both:

The differential term across LL2 diminishes voltage available on secondary winding, and this is taken into account even when we refer LL2 at primary side by multiplying by turn ratio squared.

What we lose in the APR model is the insight on behavior in cross regulation between multiple secondary windings. In this situation, the presence of LL2 creates a decoupling effect local to secondary side.


Correspondence of the physical model of the transformer to its APR model

Physical modeln=N1/N2, actual primary-to-secondary turn ratio LM models the magnetizing flux linking all windingsLL1 models the primary flux not linked to secondaryLL2a and LL2b model the secondary flux not linked to primary

APR modela is not the actual primary-to-secondary turn ratio

Ls is the primary inductance measured with all secondaries shorted outLp is the difference between the primary inductance measured with secondaries open and Ls

NOTE: LL1 +LM = Ls + Lp = L1 primary winding inductance, but LS≠LL1 and LP≠LM


Capacitive parasitics in transformer model

Above a certain frequency the gain slope versus frequency reverses again because of parasitics!

Solution: limit fmax under the red circled frequency, minimize Cp and Cj


References

1. ST Microelectronics AN2644 (An introduction to LLC resonant half-bridge converter)2. ST Microelectronics AN2450 (LLC resonant half-bridge converter design guideline)3. International Rectifier AN-1160 (Design of resonant half-bridge converter using IRS2795(1,2) Control IC)

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