Application information
L6599A
The circuit shown in Figure 28b can be operated in two different ways. If the resistor RA in
series to CA is small (not above some hundred Ω, just to limit current spiking), the circuit
operates like a capacitive current divider; CA is typically selected equal to Cr/100 or less and
is a low-loss type, the sense resistor RB is selected as:
Equation 7
⎛
⎜
⎜
⎝
⎞
⎟
⎟
⎠
0.8π
ICrpkx
Cr
RB
=
1+
CA
and CB is such that RB·CB is in the range of 10 /fmin
.
If the resistor RA in series to CA is not small (in this case it is typically selected in the ten kΩ),
the circuit operates like a divider of the ripple voltage across the resonant capacitor Cr,
which, in turn, is related to its current through the reactance of Cr. Again, CA is typically
selected equal to Cr/100 or less, not necessarily a low-loss type this time, while RB
(provided it is << RA) according to:
Equation 8
R2A + XC2
0.8π
ICrpkx
A
RB =
XCr
where the reactance of CA (XCA) and Cr (XCr) should be calculated at the frequency where
Crpk = ICrpkx. Again, CB is such that RB·CB is in the range of 10 /fmin
I
.
Whichever circuit is used, the calculated values of Rs or RB should be considered just a first
cut value that needs to be adjusted after experimental verification.
OCP is effective in limiting primary-to-secondary energy flow in case of an overload or an
output short-circuit, but the output current through the secondary winding and rectifiers
under these conditions might be so high as to endanger converter safety if continuously
flowing. To prevent any damage during these conditions, it is customary to force the
converter’s intermittent operation, in order to bring the average output current to values such
that the thermal stress for the transformer and the rectifiers can be easily handled.
With the L6599A the designer can externally program the maximum time TSH that the
converter is allowed to run overloaded or under short-circuit conditions. Overloads or short-
circuits lasting less than TSH do not cause any other action, therefore providing the system
with immunity to short duration phenomena. If, instead, TSH is exceeded, an overload
protection (OLP) procedure is activated that shuts down the L6599A and, in the case of
continuous overload/short-circuit, results in continuous intermittent operation with a user-
defined duty cycle.
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Doc ID 15308 Rev 7
STMICROELECTRONICS [ ST ]
