ISL6144_07 Datasheet, PDF (23/29 Pages) Intersil Corporation – High Voltage ORing MOSFET Controller

ISL6144
TABLE 2. INPUT VOLTAGE = 12V TO 24V
I_{Load_Max}
MOSFET PART NUMBER
N
75A
IRF1503S
3
SUM110N03-03P
3
STB100NF03L-03
3
NOTES:
11. V_DSS = 30V; I_D = 190A; r_DS(ON) = 3.3mΩ
12. V_DSS = 30V; I_D = 110A; r_DS(ON) = 2.6mΩ
13. V_DSS = 30V; I_D = 100A; r_DS(ON) = 3.2mΩ
14. All Above listed r_DS(ON) values are at V_GS = 10V
Another important consideration when choosing the ORing
MOSFET is the forward voltage drop across the drain-
source. If this drop approaches the 0.41V limit, (which is
used in the V_OUT fault monitoring mechanism), this will
result in a permanent fault indication. Normally, this voltage
drop is chosen to be less than 100mV.
Setting the External HS Comparator
Threshold Voltage
Typically, DC/DC modules used in redundant power systems
have some form of active current sharing to realize the full
benefit of this scheme including lower operating
temperatures, lower system failure rate, as well as better
transient response when load step is shared. Current
sharing is realized using different techniques; all of these
techniques will lead to similar modules operating under
similar conditions in terms of switching frequency, duty cycle,
output voltage and current. When paralleled modules are
current sharing, their individual output ripple will be similar in
amplitude and frequency and the common bus will have the
same ripple as these individual modules and will not cause
any of the turn-off mechanisms to be activated as the same
ripple will be present on both sensing nodes (V_IN and
V_OUT). This would allow setting the high speed comparator
threshold (V_TH(HS)) to a very low value. As a starting point, a
V_TH(HS) of 55mV could be used, the final value of this TH
will be system dependant and has to be finalized in the
system prototype stage. If the gate experiences false turn-off
due to system noise, the V_TH(HS) has to be increased.
The reverse current peak can be estimated as:
I_REVERSEP
=
V_{-----T---H-----(--H----S----)---+-----}V_{----S----D----}±_----V_{----O-----S---(---H----S----)}
r_DS(ON)
;
where
(EQ. 16)
V_SD is the MOSFET forward voltage drop
V_OS(HS) is the voltage offset of HS Comparator
The duration of the reverse current pulse is in the order of a
few hundred nanoseconds and is normally kept well below
the current rating of the ORing MOSFET.
Reducing the value of V_TH(HS) results in lower reverse
current amplitude and reduces transients on the common
bus voltage.
Just a reminder, this is not an operating scenario, but it is
rather a fault scenario and should not occur frequently. As
explained above, different power supplies have different
noise spectrum and might need adjustment of the V_TH(HS).
The following procedure can be used for V_TH(HS) selection:
Choose a value of V_TH(HS) such that the net HS comparator
threshold voltage is positive to allow turn-off only when V_IN
is lower than COMP. Take into account the worst-case of the
HS Comp offset (V_OS(HS) = +25mV to -40mV). A good
starting value is 55mV. The sum of R₁ and R₂ is not to
exceed 50kΩ. It is suggested to choose R₂ = 47.5kΩ and
calculate R₁ according to Equation 17:
R₁
=
---------------------^V----^T----^H----⁽--^H----^S----⁾---------------------
V_REF(VSET) – V_TH(HS)
R₂
(EQ. 17)
R₁ = 499Ω
R₁ resistor connected between VOUT and COMP pins
R₂ resistor connected between COMP and VSET pins
V_REF(VSET) = 5.3V
1. Operate all parallel feeds in current sharing mode (either
by using current sharing techniques or by simply
adjusting the voltages very close to each other for natural
current sharing).
2. Vary the load current from 1A to its maximum value,
monitor the gate voltages, and make sure all gates are on
(note that at very light loads the current sharing scheme
might stop functioning and only one feed carries this light
current, at this point gate voltages will be just above the
gate threshold, and even maybe one gate will be on while
the others are not).
3. If at medium to maximum load currents all feeds have
their gates on, then the chosen V_TH(HS) is suitable.
4. If only one feed has its gate on, the threshold value is too
low and the gate is turning off due to power supply noise
and needs to be increased. Also, the feeds may not be
sharing the load current due to discrepancy in output
voltages and current sharing failure.
5. Verify the current sharing scheme and output voltages. If
the output voltages and currents of each feed are equal
but one or more of the gates is still off, increase V_TH(HS)
by increasing R₁ in 250Ω to 500Ω increments (this
increases V_TH(HS) by 25mV to 50mV) until all feeds have
their FETs turned on.
23
FN9131.3
February 15, 2007