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NCP1239KD65R2G

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NCP1239
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22
LATCHED SHORT CIRCUIT PROTECTION WITH PRE-SHORT
In some applications, the controller must be fully latched
in case of an output short circuit presence. When the error
flag is asserted, meaning the controller is asked to deliver its
full peak current, upon timer completion, the controller
latches off: all pulses are immediately stopped and V
CC
hiccups between the two levels, V
CC(on)
and V
CC(min)
.
However, in presence of a small V
CC
capacitor, it can very
well be the case where the stored energy does not give
enough time to let the timer elapse before V
CC
touches the
V
CC(off)
. When this happens, the latch is not acknowledged
since the timer countdown has been prematurely aborted. To
avoid this problem, NCP1239 combines the error flag
assertion together with the UVLO flag: upon start up, as
maximum power is asked to increase V
OUT
, the error flag is
temporarily raised until regulation is met. If during the time
the flag is raised an UVLO event is detected, the part latches
off immediately. When latched, V
CC
hiccups between the
two levels, V
CC(on)
and V
CC(min)
until a reset occurs
(Brown-out event or V
CC
cycled down below V
CC(reset)
). In
normal operation, if a UVLO event is detected for any
reason while the error flag is not asserted, the controller will
naturally resume operations. Please also note that this
pre-short protection is activated only during start-up
sequence. In normal operation, even if an UVLO event
occurs while the error flag is asserted, the controller will
enters in auto-recovery mode. Details of this behavior are
given in Figure 41.
Figure 41. UVLO Event during Start-Up Sequence and in Normal Operation
New sequence
UVLO
AND
OCP flag
at start−up
latched
resumed
Glitch
or
overloa
d
O
CP flag
0
1
reset
Fb
OK
t
t
t
V
cc(on)
V
cc(off)
V
cc(min)
v
cc
(t)
v
DRV
(t)
LATCHING OR AUTO-RECOVERY MODE
The B, C, D and E versions are auto-recovery. When an
overload fault is detected, they stop generating drive pulses
and V
CC
hiccups between V
CC(min)
and V
CC(on)
during the
auto-recovery timer before initiate a fresh start-up sequence
with soft-start.
The A, F, G, H, I and J versions latch off when they detect
an overload situation. In this condition, the circuit stops
generating drive pulses and let V
CC
drop down. When V
CC
has reached 10-V
CC(min)
level, the circuit charged up V
CC
to V
CC(on)
. The controller enters in an endless hiccup mode.
The device cannot recover operation until V
CC
drops below
V
CC(reset)
or brownout recovery signal is applied.
Practically, the power supply must be unplugged to be reset
(V
CC
<V
CC(reset)
). Please note that the controller always
enters in auto-recovery mode when the UVLO event occurs
without internal error flag signal (ie: without overload).
NCP1239
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23
FREQUENCY FOLDBACK
The reduction of no-load standby power associated with
the need for improving the efficiency, requires to change the
traditional fixed-frequency type of operation. This
controller implements a switching frequency folback when
the feedback voltage passes below a certain level, V
FOLD
,
set at 1.9 V. At this point, the oscillator turns into
a Voltage-Controlled Oscillator (VCO) and reduces
switching frequency down to a feedback voltage of 1.5 V
where switching frequency is 26 kHz typically. Below
1.5 V, the frequency is fixed and cannot go further down.
The peak current setpoint is free to follow the feedback
voltage from 3.2 V (full power) down to 1 V. At 1 V, as both
frequency and peak current are frozen (250 mV or ≈31% of
the maximum 0.8-V setpoint) the only way to further reduce
the transmitted power is to enter skip cycle. This is what
happens when the feedback voltage drops below 0.8 V
typically. Figure 42 depicts the adopted scheme for the part.
Figure 42. By Observing the Voltage on the Feedback Pin, the Controller Reduces its Switching Frequency for an
Improved Performance at Light Load
F
SW
V
FB
V
CS
V
FB
65 kHz
26 kHz
0.8 V 3.2 V
V
fold
3.2 V
0.8 V
0.47 V
FB
V
freeze
[
0.25 V
1.0 V 1.9 V
1.9 V
max
max
min
Frequency Peak current setpoint
V
fold
[
min
V
skip
V
skip
0.8 V
1.5 V
V
fold(end)
skip
SLOPE COMPENSATION
Slope compensation is a known means to fight
sub-harmonic oscillations in peak-current mode controlled
power converters (flyback in our case). By adding an
artificial ramp to the current sense information or
subtracting it from the feedback voltage, you implement
slope compensation. How much compensation do you need?
The simplest way is to consider the primary-side inductor
downslope and apply 50% of its value for slope
compensation. For instance, assume a 65-kHz/19-V output
flyback converter whose transformer turns ratio 1:N is
1:0.25. The primary inductor is 600 mH. As such, assuming
a 1-V forward drop of the output rectifier, the downslope is
evaluated to:
S
OFF
+
V
OUT
) V
f
NL
p
+
19 ) 1
0.25 @ 600 u
+
(eq. 14)
+ 133 kAńsor133mAńms
If we have a 0.33-W sense resistor, then the current
downslope turns into a voltage downslope whose value is
simply:
S
Ȁ
OFF
+ S
OFF
@ R
SENSE
+
(eq. 15)
+ 133 m @ 0.33 [ 44 mVńms
50% of this value is 22 mV/ms. The internal slope
compensation level is typically 29 mV/ms (for the 65-kHz
version) so it will nicely compensate this design example.
What if my converter is under compensated? You can still
add compensation ramp via a simple RC arrangement
showed in Figure 43. Please look at AND8029 available
from www.onsemi.com
regarding calculation details of this
configuration.
NCP1239
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24
Figure 43. An Easy Means to Add Slope Compensation is by Using an Extra RC Network Building a Ramp
from the Drive Signal
R1
C1
D1
1N4148
Rsense
R3
DRV
CS
R4
A 2
ND
OVER-CURRENT COMPARATOR FOR ABNORMAL OVER-CURRENT FAULT DETECTION
A severe fault like a winding short-circuit can cause the
switch current to increase very rapidly during the on-time.
The current sense signal significantly exceeds V
ILIM1
. But,
because the current sense signal is blanked by the LEB
circuit during the switch turn on, the power switch current
can become huge causing system damage.
The NCP1239 protects against this fault by adding an
additional comparator for abnormal over-current fault
detection. The current sense signal is blanked with a shorter
LEB duration, t
LEB2
, typically 120 ns, before applying it to
the abnormal over-current fault comparator. The voltage
threshold of the comparator, V
ILIM2
, typically 1.2 V, is set
50 % higher than V
LIMIT1
, to avoid interference with normal
operation. Four consecutive abnormal over-current faults
cause the controller to enter latch mode. The count to 4
provides noise immunity during surge testing. The counter
is reset each time a DRV pulse occurs without activating the
Fault Over-Current Comparator.
Please note that like timer-based short-circuit protection,
A, F, G, H, I and J versions are latching off compared to B,
C, D and E versions that are auto-recovery.
OVER-VOLTAGE PROTECTION ON V
CC
PIN
The NCP1239 hosts a dedicated comparator on the V
CC
pin. When the voltage on this pin exceeds 25.5 V typically
(32.0 V for F and J versions) for more than 20 ms, a signal
is sent to the internal latch and the controller immediately
stops the driving pulses while remaining in a lockout state.
Depending controller options, this OVP on V
CC
pin can be
auto-recovery or latched. For latching-off versions, the part
can be reset by cycling down its V
CC
, for instance by pulling
off the power plug but also if a brown-out recovery is sensed
by the controller. This technique offers a simple and cheap
means to protect the converter against optocoupler.
PROTECTING FROM A FAILURE OF THE CURRENT SENSING
A 1-mA (typically) pull-up current source, I
CS
, pulls up the
CS pin to disable the controller if the pin is left open.
In addition the maximum duty ratio limit (80% typically)
avoids that the MOSFET stays permanently on if the switch
current cannot reach the setpoint when for instance, the input
voltage is low or if the CS pin is grounded. In this case, the
OCP timer is activated. If the timer elapses, the controller
enters in auto-recovery or endless hiccup mode depending
on the controller option. This unexpected operation can lead
to deep CCM with destructive consequences.
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NCP1239KD65R2G

Mfr. #:
Buy NCP1239KD65R2G
Manufacturer:
ON Semiconductor
Description:
Switching Controllers NCP1239K, 65KHZ
Lifecycle:
New from this manufacturer.
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