HV111
Circuit Breaker/Fast Return to
Limit (Diode ‘OR’ing)
V
PWRGD
-V
NN
I
DS
1A/div
V
DRAIN
-V
NN
Fast return to limit also acts as a circuit breaker with delay,
in that large currents will be limited and then the system shut
down if it does not recover. Keep in mind, however, that
severe faults which bring V
DRAIN
within ~1V of V
PP
will be
limited with the PWM Ilimit function. Fast return to limit also
overcomes the problem of “second inrush” or “diode ‘OR’ing
which is illustrated in Fig. 12 above. To understand this
phenomenon, consider what happens when the system has
been operating from the battery from a length of time. The
battery voltage has dropped perhaps ten volts or more.
Suddenly power is restored and the voltage suddenly jumps
from the battery voltage to the fully regulated input voltage.
This voltage jump puts a dv/dt upon the filter capacitances
that creates a second inrush. This second inrush can cause
damage if not limited, which is the purpose of the second
inrush fast return to limit.
Test Setup
V
IN
Figure 11 – Fast Return to Limit
A current monitor continually examines the current level
(waveform 4 in Fig. 10 & 11) and if it exceeds 2A at any
time, the MOSFET changes to a fixed 1.2A current source.
The MOSFET will remain in this state until the expiration of
an over-current timer (t
OC
= 75ms), whereafter the MOSFET
gate will be turned off and 9sec auto-retry interval will begin
(see DRAIN waveform 2 in Fig. 11, waveform 3 is V
PP
). In
the event that the over-current is cleared before the over-
current timer has expired, then the over-current timer will be
reset.
The 2A breaker will react and begin to limit current in under
10µs, however, it may take up to 100µs for the system to
stabilize at the limit level (1.2A).
optional
GND
16" Long
3" Long
12" Long
100k
Cload
100µf
Resistive
Load Box
+
48V
–
C_Source
470µf
PWRGD
Drain
V_PWRGD
DC Source
Vds
GND
I
Iin
Current Probe
DC/DC
V
PP
V
O
PWRGD
UVLO/
Enable
ENABLE
GND
-48V
BAT
V
NN
DRAIN
HV111
Figure 12 – Diode ‘OR’ing
8
SUPERTEX [ Supertex, Inc ]
