Datasheet

OVER CURRENT PROTECTION
The evaluation board is not configured with over current pro-
tection and will be in continuous current limit condition. There-
fore, 200 CFM of airflow is a must during the over current
condition.
If the customer desires to configure the evaluation board with
the hiccup mode enabled, a 4700pF capacitor needs to be
connected from RES pin to AGND. In the event of an output
overload (approximately 35A) the unit will discharge the soft
start capacitor, which disables the power stage. After a delay
the softstart is released. The shutdown, delay and slow
recharge time of the soft start capacitor reduces the average
power consumption of the unit in an overload condition.
Average Current Limit
The major drawback of the half-bridge topology is that during
current limit condition, the center-point of the capacitor divider
tends to runaway either towards the input voltage rail or to-
wards the ground. This phenomenon saturates the trans-
former and requires the capacitors in the divider to be rated
to at least the input voltage. In an overload condition, the
PWM cycle is terminated by the current sense comparator
instead of the PWM comparator. This is similar to peak current
mode control, which inherently results in an on-time between
both the phases of the half-bridge topology. Any such imbal-
ance, for an extended period, will cause the voltage at the
center-point of the capacitor divider to drift either towards the
input voltage or the ground. However, in an average current
limit scheme, the PWM cycle is terminated through the PWM
comparator, by pulling down the PWM control input. Because
of its averaging nature, the PWM control input voltage is slow
moving and is essentially held at a constant dc voltage. This
results in the on-time between the both the phases to be equal
and thus balances the center-point of the capacitor divider.
Figure 2 shows the current limit waveforms in a soft-short
condition and Figure 3 shows the current limit waveforms in
a hard-short condition.
It can be observed from the Figures 2 and 3 that the center-
point of the half-bridge capacitor divider is balanced in both
soft-short and hard-short conditions. The response of aver-
age current limit circuit is same whether the short is soft or
hard. During an overload event, the average current limit
scheme converts the power supply from a constant voltage
source to a constant current source. This scheme is often
known as “brickwall current limiting.” A V
OUT
vs I
OUT
curve,
shown in Figure 4, illustrates the brickwall current limiting.
30112703
Trace 1 (C1) Output Voltage
Trace 2(C2): Voltage on the ACL Capacitor
Trace 3 (C3): Output current
Trace 4 (C4): Voltage at the center-point of the half-bridge capacitor divider
FIGURE 2.
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