Wide input range LLC resonant converters have drawbacks and limitations when used with a wide input voltage range from the mains. These include operating too close to the capacitive mode which can cause loss of zero voltage switching and reverse recovery of body diodes. It also results in a low attainable peak to nominal output power ratio or efficiency, especially at low loads. Other limitations include tighter tolerances required, lower duty cycle of the energy taking phase at low input voltages, and harder transformer design due to constraints. The presentation outlines these limitations and how to mitigate operating in the capacitive mode through frequency selection.

1. Wide input range LLC resonant converters,
analysis of limits and drawbacks
1Magnetica di Roberto Volpini.

2. Milan, June 6th 2012
Relator – Manuele Marconi
Presentation outline 2Magnetica di Roberto Volpini.

3. Wide-range LLC converters 3Magnetica 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.

4. Main topic:
Wide-range LLC converters
Wide-range LLC converters 4Magnetica di Roberto Volpini.

5. Wide-range LLC converters 5Magnetica di Roberto Volpini
Simplified circuit model of resonant LLC converter used in this discussion

6. Wide-range LLC converters 6Magnetica di Roberto Volpini.
Using LLC resonant converter with wide input mains range has drawbacks and
limitations
1. Working too close to the capacitive mode
2. Low attainable peak to nominal output power ratio, or must be traded off against efficiency
especially at low loads
3. 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 voltages
6. Harder to perform optimization and to choose the most appropriate power switches or drivers
7. Harder to design the LLC transformer, due to tighter constraints

7. Wide-range LLC converters 7Magnetica di Roberto Volpini.
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 LOAD
Q=∞ means SHORT CIRCUIT

8. Wide-range LLC converters 8Magnetica di Roberto Volpini.
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.

9. Wide-range LLC converters 9Magnetica di Roberto Volpini.
Calculation of the value of the Critical Resistance Rcrit
Zo0 Zo∞
Zo0 = output impedance with input short-circuited
Zo∞ = output impedance with input open-circuited
The value of the critical resistance is
found to be frequency dependent as
supposed

10. Wide-range LLC converters 10Magnetica di Roberto Volpini.
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.

11. Wide-range LLC converters 11Magnetica di Roberto Volpini.
Why is capacitive mode so bad?
What happens in capacitive mode is, moving down away from boundary line:
1. Losing ZVS
2. Eventually reversing the control law VS frequency

12. Wide-range LLC converters 12Magnetica di Roberto Volpini.
Loss of ZVS condition
1. Hard switching Q1 & Q2
2. Body diode of Q1&Q2
reverse-recovered
3. High level of energetic EMI
4. 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)

13. Wide-range LLC converters 13Magnetica di Roberto Volpini.
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.

14. Wide-range LLC converters 14Magnetica di Roberto Volpini.
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.

15. Wide-range LLC converters 15Magnetica di Roberto Volpini.
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.

16. Wide-range LLC converters 16Magnetica di Roberto Volpini.
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

17. Appendix 17Magnetica di Roberto Volpini.
Appendix

18. Appendix 18Magnetica di Roberto Volpini.
Differences between output rectifiers configurations
Center-tapped output with
full-wave rectification.
Suitable for low output
voltages with high currents
Vout=Vsec-VF
VRRM~= 2 Vout + VF
Single-ended output with
bridge rectification.
Suitable for high output
voltages with low currents
Vout= Vsec – 2 VF
VRRM~= Vout
In 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.

19. Appendix 19Magnetica di Roberto Volpini.
Correspondence of the physical model of the transformer to its APR model
Physical model
n=N1/N2, actual primary-to-secondary turn ratio
LM models the magnetizing flux linking all windings
LL1 models the primary flux not linked to secondary
LL2a and LL2b model the secondary flux not linked to
primary
APR model
a is not the actual primary-to-secondary turn ratio
Ls is the primary inductance measured with all
secondaries shorted out
Lp 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

20. Appendix 20Magnetica di Roberto Volpini.
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

21. Appendix 21Magnetica di Roberto Volpini.
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)

22. Follow us : 22Magnetica di Roberto Volpini.