AN1673 Datasheet, PDF (21/24 Pages) STMicroelectronics – L6598 OFF-LINE CONTROLLER FOR RESONANT CONVERTERS

Figure 23. Output V/I Characteristics.
TWICE
FBACK
R_S
V_LOOP
I_LOOP
AN1673 APPLICATION NOTE
VO
18V
V_Out/2
D03IN1461
I_Out 3.8A
Miscellaneous.
Few critical points to be considered in design and debugging phases are:
– The frequency accuracy; the actual frequency can be affected by external components (e.g. the preci-
sion of Cosc. and its thermal variation). Moreover, when the Cosc is much low, the stray effects must
be considered (the pin and PCBs capacitance). The oscillator accuracy can be affected if the current
drown from the references (Pin2_Rfstar and pin4-Rfmin) exceeds 100µA.
– The layout of Pcb and coupled noises: a good criterion for checking the noises is to test the signal at
the Cf(pin3) that should be a symmetric triangular waveform in order to ensure the correct drivers duty.
Another point to be tested is the voltage at Out(pin14) that, during the freewheeling, should not be too
much below ground (few volt).
Power Factor section.
In spite of the PFC stage should not be strictly necessary for the resonant application, in our design it has
been introduced for the following reasons.
The front-end stage of conventional off-line converters, consists of a full wave rectifier bridge with a ca-
pacitor filter, gets an unregulated DC bus from the AC mains. Therefore the instantaneous line voltage is
below the voltage on the capacitor most of the time, thus the rectifiers conduct only for a small portion of
each line half-cycle. The current drawn from the mains is then a series of narrow pulses whose amplitude
is 5-10 times higher than the resulting DC value.
Lots of drawbacks result from that: much higher peak and RMS current drawn from the line, distortion of
the AC line voltage, over currents in the neutral line of the three-phase systems and, after all, a poor utili-
zation of the power system's energy capability.
This can be measured in terms of either Total Harmonic Distortion (THD), as norms provides for, or Power
Factor (PF), intended as the ratio between the real power (the one transferred to the output) and the ap-
parent power (RMS line voltage times RMS line current) drawn from the mains, which is more immediate.
A traditional input stage with capacitive filter has a low PF (0.5-0.7) and a high THD (> 100%).
The International norms and standard requirements have spurred the design of high power factor.
For these reasons power factor corrector (PFC) is now diffusing in off-line power management. With a high
power factor switching pre regulator, interposed between the input rectifier bridge and the bulk filter ca-
pacitor, the power factor will be improved (up to 0.99). The current capability is increased; the bulk capac-
itor peak current and the harmonic disturbances are reduced.
Moreover, the PFC with its pre regulated high voltage bus, provides an important advantage, because op-
erating at fixed DC input, it makes much easier the resonant converter to be controlled.
The L6561 is an integrated controller dedicated to PFC stage by using the transition mode technique and
is optimized for low to medium power applications.
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