LLC converter operating in capacitive region - diyAudio
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Old 2nd September 2012, 10:41 AM   #1
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Default LLC converter operating in capacitive region

Hello,

The below is a basic (LTSpiceIV) simulation for a LLC converter which is to supply a 175W , full-bridge class D amplifier.

(LTspice IV is a free download from linear.com)

The upper resonant frequency of this LLC stage is 128KHz.

However, in the simulation attached, the fets are switching at 80KHz.
-in other words, the LLC converter is being operated deeply in the “capacitive region”.
In the capacitive region, the antiparallel diodes in the fets should be being severely reverse recovered……….creating very large switching losses and lots of emc noise.

However, the simulation shows perfectly acceptable operation…..without the reverse recovery and with no great dissipation in the fets

[it is page 5, RHS of this which tells of the woes of the “capacitive region” in an LLC resonant converter.

http://www.fairchildsemi.com/an/AN/AN-4151.pdf
]

Howcome this LLC simulation, operating at too low a frequency, in the “capacitive region”, is operating OK?
Attached Files
File Type: asc LLC cap region GO6.asc (3.2 KB, 16 views)
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