LLC Resonant Converter Calculator
Compute LLC converter resonant frequency, gain curve, and Q factor. Popular topology for server PSUs, LED drivers, and AC/DC adapters for ZVS operation + high efficiency.
How to Use
- Enter Lr (resonant inductor), Lm (magnetizing inductor), and Cr (resonant cap).
- Enter load Rac (reflected from output).
- Tool computes fr1 (series resonant), fr2 (parallel), and normalized gain at f_sw.
Show Work
Formulas
History of LLC Resonant Converters
The LLC resonant topology evolved from the series-parallel resonant converter research of the 1980s (Fred Lee at Virginia Tech, Dragan Maksimovic). Commercial deployment accelerated in the mid-2000s as data-center PSU vendors chased 80 PLUS Gold/Platinum efficiency targets. Bob Mammano and Laszlo Balogh's TI Application Note SLUP263 (2001) became the industry reference for LLC design. Modern server PSUs (Titanium rating, 96%+ peak efficiency) universally use LLC stage 2 after PFC stage 1.
About This Calculator
Enter Lr (resonant inductor — often transformer leakage), Lm (magnetizing inductance), Cr (resonant cap), and R_ac (load reflected to primary: Rac = (8/π²)·N²·Rload). The tool computes the two resonant frequencies, loaded Q, and ratio m.
LLC design typically targets fr1 at nominal operating point, Q ≈ 0.3-0.5, m ≈ 5-7. For full design including gain curves across line/load, use ST AN2450 or Infineon AN_201512_PL52_005 reference notes, or LLC design tools from major chip vendors. Everything runs client-side.
Frequently Asked Questions
Why LLC?
Provides zero-voltage switching (ZVS) across wide load range, which nearly eliminates switching losses in the primary MOSFETs. Efficiencies > 96% achievable at full load. Standard topology for 500W+ server PSU, high-end LED drivers, and AC/DC adapters.
Two resonances?
fr1 = 1/(2π·√(Lr·Cr)) is the series resonance — converter operates at or near this for optimal ZVS. fr2 = 1/(2π·√((Lr+Lm)·Cr)) is the parallel resonance — at light load, gain peaks here causing ZVS loss if you drop below fr2.
ZVS boundary?
ZVS requires capacitive impedance seen by the half-bridge — i.e., operating above fr1 (or between fr2 and fr1 at light load). Below fr2 the tank is inductive and ZVS is lost; hard-switching causes EMI and efficiency drops.
Common Use Cases
Server PSU
1500W Titanium-efficiency server PSU: LLC primary with PFC front-end, 96-97% peak efficiency.
LED Driver
200W LED driver at 380 V bus: LLC half-bridge for dimmable high-efficiency output.
Laptop/TV Adapter
90-240 W AC adapters: LLC after PFC for efficiency targets under EPA guidelines.
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