Implementing Simple Brown-Out Solution for NCP101X Series

AND8272/D
Implementing a Simple
Brown−out Solution on the
NCP101X Series
Prepared by: David Sabatie
ON Semiconductor
http://onsemi.com
APPLICATION NOTE
Introduction
Some AC−DC applications require a clean start−up
sequence when the mains reaches a certain level to avoid
stressing the power MOSFET at low−line, especially when
the bulk capacitor starts to charge up. Also, at power−off, the
power supply shall not go into hiccup as the bulk capacitor
depletes and can no longer deliver the required energy. To
provide such a behavior, the present application note
describes a low−cost brown−out circuit successfully tested
on the NCP101X switcher series.
The NCP101X series integrates a fixed−frequency
current−mode controller and a 700 V MOSFET. Housed in
a PDIP−7, PDIP−7 Gull Wing, or SOT−223 package, the
NCP101X offers everything needed to build a rugged and
low−cost power supply, including soft−start, frequency
jittering, short−circuit protection, skip−cycle and a Dynamic
Self−Supply (no need for an auxiliary winding). Unlike
other monolithic solutions, the NCP101X is quiet by nature:
during nominal load operation, the part switches at one of
the available frequencies (65 − 100 − 130 kHz). When the
current setpoint falls below a given value, e.g. the output
power demand diminishes; the IC automatically enters the
so−called skip−cycle mode and provides excellent
efficiency at light loads. Because this occurs at typically 1/4
of the maximum peak value, no acoustic noise takes place.
As a result, standby power is reduced to the minimum
without acoustic noise generation. Skip cycle is
implemented by monitoring the feedback voltage. As it
drops below a certain level, a dedicated comparator blanks
the switching events. If a circuit maintains the pin below the
skip level, we have a mean to stop the circuit, e.g. for a
brown−out implementation…
Figure 1 shows a possible implementation where no
hysteresis is necessary.
Vbulk
2
3
R1 R4
To opto−coupler
Q2
Q1
BC547
R2
0
0
0
: Components to add
Figure 1. In This Application, There is No Need for
Hysteresis
We set the startup Vbulk voltage by pulling the feedback
pin low until enough voltage builds−up on the bulk
capacitor. When Vbulk is too low, Q1 is blocked and Q2 base
is pulled−up by R4 to VCC (≈ 8 V). Q2 being saturated, the
feedback pin goes directly to ground. As we explained, the
product does not start thanks to the presence of the internal
skip−cycle comparator. When Vbulk reaches the selected
value (determined by R1 and R2), Q1 saturates and Q2 base
goes to ground, naturally blocking the transistor. The
feedback is now solely driven by the opto−coupler and the
system can freely operate.
To implement this brown−out function, highlighted
components are used, two standard NPN transistors and
three resistors. The remaining components in the design are
not modified. The design equations are as follows:
1. select the bridge current flowing in R1 and R2:
50ĂmA to avoid wasting power at high line.
As explained in the introduction, some particular
applications impose a minimum input voltage below which
the power supply shall not work. To start the SMPS, a circuit
constantly monitors the rectified mains voltage and pulls the
feedback pin down to ground until the mains voltage reaches
the desired value. When this happens, we need to introduce
a certain amount of hysteresis to avoid going into a hiccup
mode as the ripple on the bulk increases.
August, 2006 − Rev. 0
8
VCC GND
GND GND
7
GND
DRV
4 FB
5
NCP1014
BC547
Brown−out
© Semiconductor Components Industries, LLC, 2006
U1
1
1
Publication Order Number:
AND8272/D
AND8272/D
2. calculate R2 assuming a Vbe around 650 mV:
R2 + 0.65 + 13 kW
50 m
4. R4 is fixed to 10 kW, a choice limiting the
Dynamic Self Supply (DSS) loading current to a
reasonable value.
(eq. 1)
3. If we want a startup around 100 Vdc, then the
upper resistor is found to be:
R1 + 100 * 0.65 + 1.98 MW
50 m
Adding Hysteresis to the Brown−out Detection,
Auxiliary Winding Solution
Now, let us see how we can add some hysteresis via the
usage of the auxiliary winding. Figure 2 portrays the idea:
(eq. 2)
Vbulk
R3
U1
D1
1
R1
8
VCC GND
GND
2
GND
7
3
GND
DRV
5
4 FB
NCP1014
R4
R5
From auxillary winding
1N4148
+
+
C1
C2
0
0
To opto−coupler
Q2
BC547
Q1
: Components to add
BC547
R2
0
0
0
Figure 2. A Small Hysteresis is Added via R3.
The main circuit actually does not differ compared to that
of Figure 1. However, the addition of a small voltage offset
changes the behavior at turn−off. During the startup phase,
there is no auxiliary voltage. Hence, R3 right terminal is
pulled down to ground. Thanks to D1 which isolates the
device from the NCP1014 DSS level, there is no voltage.
When the system starts to work, the auxiliary branch goes up
and brings its level around 23 V (in our particular example).
Via R3, it slightly raises Q1 Vbe voltage, now forcing Vbulk
to reduce to a further down level, in comparison with the
startup level. To implement this brown−out function,
Figure 2 shows the additional components. the highlighted
components are used. The other components in the design
are not modified.
The design equations now include the presence of R3,
coming in parallel with R2 during the start−up sequence.
However, we can neglect its contribution as its value is much
higher than R2 as we will see:
1. select the bridge current flowing in R1 and R2:
50ĂmA to avoid wasting power at high line.
2. calculate R2 assuming a Vbe around 650 mV:
R2 + 0.65 + 13 kW
50 m
4. Once the auxiliary voltage comes into play, the
voltage over R2 including R3 is given by (applying
superposition):
Vbe + Vbulk
solving for R3 gives:
R3 +
V
R1R2(Vaux * Vbe)
be(R1 ) R2) * VbulkR2
(eq. 5)
Keeping similar values as in the above example, if we
want a cutoff voltage of 70 Vdc with a 23 V auxiliary level
(in this particular example), then R3 = 1.5 MW.
5. R4 is fixed to 10 kW, a choice limiting the
Dynamic Self Supply (DSS) loading current to a
reasonable value
Adding Hysteresis to the Brown−out Detection, DSS
Solution
When using the NCP101X series in a non−auxiliary supply
solution, the controller receives its operating supply from the
DSS, bringing VCC between 7.5 and 8.5 V. Rather than
connecting the hysteresis resistor to the auxiliary voltage,
why not wiring it to the DSS supply. The arrangement is then
slightly different as the DSS is already present at startup. The
trick consists in shunting a resistor in series with the lower
side element once the circuit has started: it artificially raises
the level on the bottom transistor Q1 base.
(eq. 3)
3. If we want a startup around 100 Vdc, then the
upper resistor is found to be:
R1 + 100 * 0.65 + 1.98 MW
50 m
R2 ø R3
R1 ø R2
) Vaux
,
R2 ø R3 ) R1
R1 ø R2 ) R3
(eq. 4)
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2
AND8272/D
Vbulk
U1
8
VCC GND
2
GND GND
7
3
GND
DRV
FB
4
5
NCP1014
1
R1
R4
Q2
BC547
Q1
R2
To opto−coupler
BC547
Q3
: Components to add
BC547
0
R3
0
0
0
Figure 3. Altering the Bottom Resistor once the Circuit has
Started Helps Creating the Necessary Hysteresis
Designs equations are not complicated, compared to the
previous calculations. At power−up, Q3 is biased by R4 and
R3 goes off the picture (we neglect Q3 Vce,sat):
R3 +
Again, if we stick to our 70 Vdc cutoff level, we have R3 =
5.6 kW.
R2
Vbe +
V
R2 ) R1 bulk
Conclusion
The method to evaluate R2 and R1 is similar to bullets 1 to
3 above.
When the circuits starts to operate, in other words Q1
collector being low, Q3 is blocked and R3 appears in series
with R2. The above equation becomes:
R2 ) R3
Vbe +
V
R2 ) R3 ) R1 bulk
Vbe(R1 ) R2) * VbulkR2
Vbulk * Vbe
As we have shown here, various possibilities exist to
implement brownout protection on NCP101X boards.
Despite the natural Vbe variations with temperature, the
obtained results are good for low−cost adapters. For more
precise trip points, designer should consider a more efficient
solution built around the NCP1027 which features a real
programmable brown−out protection.
(eq. 6)
Solving this equation gives the series resistor value where
Vbulk represents the turn−off value:
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