STCMB1 datasheet(46/59 Pages) STMICROELECTRONICS

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Application information

STCMB1

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DocID028958 Rev 4

Differently, the proposed circuit shown in Figure 28b operates as a capacitive current

divider; Cs will be typically selected equal to Cr/100 or less and will be a low-loss type, and

the sense resistor Rs will be selected as:

Equation 29

With this solution the associated power dissipation will be reduced by a factor (1 + Cr / Cs).

This circuit is then recommended when the efficiency target is very high.

For proper operation of the LLC, the STCMB1 must sense the instantaneous tank

current for the proper operation of the hard switching prevention function. As shown

in Figure 25, an RC filter close to the ISEN_HB pin is recommended to reduce the

noise level on the pin, but the delay it introduces has to be negligible with respect to

the operating frequency of the LLC. As a rule of the thumb, the following can be

considered: time constant R • C should be no more than few hundred ns (400 ns)

when the LLC frequency is about 100 kHz.

Because the RC filter on the pin introduces a phase delay of the signal on the ISEN_HB pin

with respect to the current flowing in the resonant tank, it might happen that the anti-

capacitive mode protection do not reveal the incorrect situation because the protection does

not see the real phase among the voltage and current in the resonant tank. Therefore, with

longer time constants it is recommended that the converter operation close to the capacitive

mode boundary and during the short-circuit should be checked for possible hard switching

cycles.

The ISEN_HB pin, which is able to withstand also negative voltages, is internally connected

to the input of a first comparator, referenced to the VISENx (first level protection threshold for

frequency-shift, set at 0.8 V typ.), and to that of a second comparator referenced to the

VISENdis (second level protection threshold for immediate stop, set to 1.5 V typ.).

7.24.1

First OCP protection

If the voltage at the ISEN_HB pin exceeds the VISEN_HB_x (frequency shift threshold)

referenced at 0.8 V (typ.), a comparator is tripped causing the activation of the CSS

discharging the internal switch for 5 µs (OCP1 event). The discharge of the soft-start

capacitor quickly increases the oscillator frequency, limiting the energy transfer to the output

and reducing the peak resonant current. After the discharge time (5

s), if the overload is

still there, the switching frequency will decrease again and the resonant current will

increase. If the ISEN_HB pin voltage reaches the VISEN_HB_x again the operation described

above will be repeated. Under the overload condition, this operation results in a peak

primary current that periodically oscillates around the maximum value allowed by the sense

resistor on the ISEN_HB pin, the RS, above calculated in Equation 29.

This overcurrent protection is effective in limiting the primary-to-secondary energy flow in

case of overload. In spite of this, the output current through the secondary winding and

rectifiers under these conditions might be high enough to jeopardize converter's safety if

continuously flowing.

In order to prevent the converter damaging, the operation during the overload described

above will have limited duration; afterward the converter will have to be forced to work

intermittently, which brings the average output current to values such that the thermal stress

of the transformer and the rectifier is within safe limits.

0.76V

IRES pk

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