Rainbow Electronics MAX5079 - Manual (Page 11)

MAX5079
ORing MOSFET Controller with
Ultra-Fast 200ns Turn-Off
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c. During the hot insertion, a voltage spike can occur
at N1 and N2 and cause the (V OUT1 to V BUS ) or
(V OUT2 to V BUS ) voltage to go negative. If the
reverse voltage is below the fast-comparator
reverse voltage threshold (V
FTH
) but above the
programmed slow-comparator reverse voltage
threshold (V
STH ), the spike is ignored for the pro-
grammed blanking time (t STH ). If the spike is
longer than 50ns (the fast-comparator internal
blanking time, t
FBL ) and larger than V FTH , then U1
and U2 will turn off N1 and N2 quickly. If the mag-
nitude of the voltage spike is above V
STH but less
than V FTH , and longer than the slow-comparator
blanking time (t STH ), U1 and U2 will turn off their
respective ORing MOSFETs (N1 and N2) by dis-
charging their GATE pins to PGND. The external
load-sharing circuit of PS1 and PS2 will force
V
OUT1 , V OUT2 above V BUS and N1, N2 will turn
back on through the 2mA current sourcing from
the GATE pins of U1 and U2. To avoid this situa-
tion the user can set the slow-comparator thresh-
old and blanking time depending on the
magnitude and duration of the voltage spikes.
d. PS3 fails to start. V UVLO (U3) threshold is not
crossed and U3 keeps N3 off.
e. PS3 goes into an overvoltage condition (no feed-
back). This causes V BUS to go into an overvoltage
condition increasing the loading on PS3 (provided
PS3 is able to supply all the required BUS cur-
rent). The current-sharing circuit will force the out-
puts of PS1 and PS2 to increase and eventually
saturate at their current-sharing voltage range.
Eventually only PS3 will have a positive voltage at
IN (U3) with respect to BUS. PS1 and PS2 will
have a negative voltage at V
OUT1 and V OUT2 with
respect to BUS. All overvoltage inputs OVI (U1),
OVI (U2), and OVI (U3) sense the overvoltage, but
only OVP (U3) is asserted and latched low. GATE
(U3) is pulled to PGND and remains low as long
as V
OVI
≥ 0.6V. When V
OVI drops below 0.6V,
OVP remains low. However, U3 tries to turn on N3
unless V
OUT3
is actively kept below the undervolt-
age lockout. Use OVP (U3) to either drive the
cathode of the optocoupler to shutdown PS3 from
the primary side or use OVP (U3) to fire an SCR
connected between V
OUT3 and PGND.
3) PS1, PS2, PS3 are turned on with a shorted BUS.
Body diodes of N1, N2, and N3 conduct and short the
outputs of PS1, PS2, and PS3 to PGND. The power
supplies go into current limit (either in foldback or in
hiccup mode). The MAX5079s remain in undervoltage
lockout and keep all ORing MOSFETs off. The average
current sourced by PS1, PS2, or PS3 must be low
enough so as not to exceed the MOSFETs power dissi-
pation (P
D = V F x I SHORT ).
a. Use additional n-channel MOSFETs in series with
N1, N2, and N3 in the reverse direction to isolate
the power supplies from a shorted bus (Figure 3).
When power is turned on with a shorted bus, V
IN_
(U1, U2, U3) increases and V
UVLO
rises above
the UVLO threshold. The MAX5079’s GATE out-
puts start charging the back-to-back ORing
MOSFET gates. The short-circuit condition at BUS
collapses V
IN (U1), V IN (U2), and V IN (U3) send-
ing the MAX5079s into undervoltage lockout. This
turns off the MAX5079s entirely, including dis-
charging of the charge-pump storage capacitors.
The IN voltages come back up again crossing
UVLO (UVLO has 60mV hysteresis). A new cycle
starts and the time required to charge the charge-
pump capacitor and the turn-on time of the device
serves as a dead time. However, the dead time
may not be enough to reduce the dissipation in
the MOSFETs to an acceptable level. We advise
in keeping the short-circuit current low and pro-
viding hiccup current-limit protection to the power
supplies (PS1, PS2, and PS3).
b. Any other overload condition that would sustain the
IN voltage above UVLO, will keep the MOSFETs ON
continuously. Ensure the MOSFETs’ current
rating is higher than the maximum short-circuit
source current of the power supplies (PS1, PS2,
and PS3) to avoid damage to the ORing MOSFETs.
4) PS1, PS2, and PS3 are present and PS1 is short-
ed to GND.
V OUT1 drops below V BUS . The negative potential from
V IN (U1) to V BUS increases above the fast-comparator
threshold and lasts longer than the 50ns blanking time.
The MAX5079 (U1) takes its power from the voltage at
BUS (U1). Connect BUS close to C
BUS , away from N1
so that U1 can receive power from BUS for a few
microseconds until N1 isolates BUS from IN. N1 is dis-
charged with 2A pulldown current, turning off N1 and
isolating PS1 from the BUS. The load-sharing circuit of
PS2 and PS3 will increase PS2 and PS3’s load current
until the total bus current requirement is satisfied.
For V IN (U1) < 2.75V, V AUXIN (U1) must come from an
independent source or remain on for some time (a few
microseconds) after V
IN
(U1) has failed. This minimum
on-time is needed to discharge the gate of the ORing
MOSFET and isolate PS1 from the BUS.