NCL30001: Input Overvoltage Protection Circuit

DN05060/D
Design Note – DN05060/D
Input Over Voltage Protection Circuit
Device
NCL300001:
NCP431 and
NCS2220 or
LM2903
Application
Input Voltage
Output Power
Topology
Offline Power
Conversion, LED
lighting
90 – 315 Vac
Various
Various
Typical Shutdown Voltage
Typical Restart Voltage
Bias Current (LM2903)
Bias Current (NCS2220A)
Circuit Description
Overvoltage events on the power grid can be thought of
in broad terms as impulse-like or “long term”. “Impulse”
events are well understood and there are a range of
surge suppression techniques (ex: TVS, MOV etc.)
utilized by power supply designers.
Longer term excess voltage events are typically due to
poor regulation or faults in the power grid and can
exceed 300 Vac and last for hours or be less than 50
msec in duration. Some form of mitigation is required in
order to provide a reliable system with a long service life.
This is especially true in applications like outdoor LED
lighting.
Essentially these long term events have limitless power
capability and the first line of defense is to select
components which are rated to withstand the anticipated
stress. Depending on the magnitude of excess voltage, it
may not be practical to use parts with suitable rating.
This design note describes an input voltage monitoring
solution and is suitable for systems which can be
disabled for the duration of the excess voltage event
circumventing unnecessary stress on some components.
Hysteresis provides more stable operation by lowering
the input voltage threshold necessary to restore normal
operation.
Typically, power converter input voltage is specified in
terms of RMS voltage. The detection method used in this
example circuit is based on peak voltage providing a
simpler implementation. Throughout the remainder of this
design note, conversion between RMS and peak
voltages will be done assuming sinusoidal waveforms.
February 2014, Rev. 0
266 Vac
253 Vac
1.25 mA
0.17 mA
Even though the power converter is not operational
during the overvoltage event, the monitoring circuit
remains active to detect when the input voltage returns to
normal levels so bias power must be supplied directly
from the rectified ac source. The simplest method is via a
linear regulator, and in this case a shunt configuration is
appropriate and cost effective.
Any linear regulator will generate heat and the amount is
proportional to the required current and the difference
between input and output voltage. To minimize
dissipation the circuit must draw minimal current. Since
the comparator draws the majority of the current, careful
consideration should be given to the tradeoff of cost and
power consumption. High resistances are selected for
dividers and pull-up devices to meet this end. Note that
an appropriate number of resistors are needed in this
shunt regulator to dissipate the power without exceeding
component ratings.
The bias in this circuit performs two functions. Not only
does this voltage provide the current to operate the
circuit, it is also used as the main reference to establish
the input over voltage threshold. A reference with tight
tolerance is desirable to control accuracy of the
shutdown threshold; however, once again there is a
tradeoff between a low cost solution such as a zener
diode and the more accurate NCP431 shunt regulator.
Details on utilizing the more accurate NCP431 are shown
in a later section. Moreover compared to an industry
standard TL431, the NCP431 requires only 8% of the
minimum operating current offering a more power
efficient solution.
Circuit Operation
Figure 1 shows a solution based on the industry standard
LM2903 comparator and a low current zener diode for
the bias voltage. Two resistors, R1 and R2, are used to
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DN05060/D
form the upper portion of the input divider due to
maximum voltage rating of 200 volts for a 1206 surface
mount resistor. Two resistors also provide finer resolution
on setting the shutdown threshold. C1 is a noise filter for
this initial divider. The peak detecting comparator U1A
accepts the input from the first divider and compares it to
a reference derived from the bias voltage. R6 provides
hysteresis and noise immunity for the peak level sensing.
R8 is a pull up for the LM2903 open collector comparator
output. D2 couples the peak detecting comparator to a
simple resistor-capacitor timer formed by R9, R10 and
C2. This R-C timer integrates over each half cycle of the
ac line providing a more constant level indicative of the
average ac input voltage. R10 provides quick response
to high peak voltages on the input. R9 provides a slow
response filling the gaps between half cycles of the
applied voltage.
When U1A detects a peak voltage above the set
threshold or trip level, C2 is pulled low. When C2 voltage
falls below the threshold of the second comparator U1B,
the output of this comparator switches to a high state
turning on Q2. FET Q2 is connected to the appropriate
control signal of the target power converter stopping all
switching and reducing voltage stress in response to the
excessive input voltage.
The second comparator also turns on Q1 forming a
conducting path for R7. This resistor increases the
hysteresis for the first comparator lowering the detection
threshold. The value of R7 is adjusted to provide the
desired input line voltage level below which the power
converter will be allowed to restart.
As long as the peak detecting comparator U1A senses
input voltage above the reset threshold, D2 will keep C2
discharged, and the second comparator will maintain Q2
in a conducting state forcing the power converter off.
When the first comparator no longer detects excessive
peak voltages corresponding to normal RMS voltage
levels, C2 will begin to charge through R9. After a delay,
the second comparator will switch to a low state
providing hysteresis through R13 for stable detection.
Subsequently, Q2 will switch off allowing the power
converter to restart. Q1 will also switch off raising the first
comparator threshold back to the higher trip level
corresponding to the RMS input voltage threshold to shut
down the power converter.
Bias Setup
Energy to power the detection circuit must be derived
from the rectified ac input. By its nature, bias power is
supplied when the circuit is operating at high input
voltage. A series connection of resistors is used to
deliver the required current dividing the voltage and
power stress amongst multiple devices. Maintaining each
device within ratings enhances reliability. Collectively,
this series connection of resistors is referred to as R15.
February 2014, Rev. 0
The design example of Figure 1 is based on the
MMSZ4689 5.1 volt zener with 5% tolerance. Note this
low current zener diode is specified at 50 µA bias current
which avoids significant dissipation compared to a
MMSZ5231 which is guaranteed 5% at 20 mA bias
current.
The required bias current is largely dependent on the
selected comparator. The LM2903 dual comparator
draws about 1 mA bias current. The remaining circuitry
draws about 0.25 mA depending on resistor values plus
50 µA for the zener diode, totaling 1.25 mA for this
circuit. A lower power solution based on the NCS2220
comparator is discussed in a later section.
The current supplied by R15 is dependent on the input
voltage. As the ac input voltage is reduced, the available
bias current will reduce. By the nature of the application,
full performance of this circuit is required only at elevated
input voltage. However, the circuit must not interfere with
power supply operation down to the minimum operating
input voltage of the system. Empirical testing shows full
bias current is required at approximately 250 volts to
avoid false operation.
In this case, R15 = (250 – 5.1) / 1.25 mA = 196 k ohms is
the maximum resistance value. During an excessive
input event of say 310 Vac less 2 volts in the bridge
rectifier, the peak voltage will be 308 * 1.414 = 436 Vdc.
Subtracting the 5.1 volt bias leaves 434.9 Vdc applied
across R15.
Dissipation in R15 follows 434.9 squared divided by 196
k = 0.96 watts. Using the typical 125 mW allowable
stress per 1206 resistor, this means 8 resistors are
required to handle the dissipation. 196 k divided by 8
equals 24.5 k ohm per resistor. Therefore, R15 is
comprised of 8 resistors of 24.5 k ohm each.
Optionally, a two watt through-hole resistor could be
used for R15. Specific types are available which are
rated for higher operating voltage.
Improved Accuracy
The bias voltage is used as the reference to establish
shutdown or trip voltage. Any deviation in the bias
voltage will directly reflect in accuracy of the trip voltage.
The MMSZ4689 low current zener provides a simple bias
regulator, but carries with it a 5% tolerance as well as
variation due to current through the device.
Changing to the NCP431 shunt regulator reduces the
error to 1% or even 0.5% depending on the version
selected.
The NCP431 requires 100 µA bias current. This is a
relatively small increase in current and dissipation.
Capacitor C3 must be less than 200 pF to ensure
stability.
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DN05060/D
Figure 2 shows the implementation of the NCP431 in
place of the zener diode. Each particular application
should evaluate these tradeoffs between accuracy,
dissipation, and cost.
In addition, the NCS2220 will sink and source current
which eliminates two resistors, R8 and R14. The solution
requires less board space, fewer components and
reduced bias current.
Design Example
A design example is presented based on the NCL30001
High Efficiency, Single Stage, High Power Factor LED
driver. A similar approach could be used for any off-line
power converter.
If the NCS2220 comparator is used with the NCP431
higher accuracy reference, the typical bias current is
reduced to about 14% of the circuit shown in Figure 1.
Dissipation is reduced from 0.94 to 0.13 watts. A typical
implementation requires only 2 bias resistors compared
to 8 of the 1206 size surface mount resistors.
The circuit of Figure 1 is used in this example, The goal
is to disable the converter when the input voltage
exceeds 266 V ac. Normal operation shall be restored
when the input voltage reaches 253 V ac.
A design example based on a 60 watt output power
supply will help put these numbers in perspective. If this
power supply had an efficiency of 85%, then the input
power would be 60 / 0.85 = 70.59 watts.
The NCL30001 can be disabled through the ‘Vff’ function
of pin 5. Pulling this pin below 0.45 volts activates the
brown out function. The drain of Q2 in Figure 1 will be
connected to NCL30001 pin 5. When an over voltage
event is detected, Q2 will pull pin 5 below the threshold
causing the controller to shut off all switching. Figure 3
shows a schematic detailing connection.
Adding the Input Over Voltage Protection circuit of Figure
1 increases the input power by 0.94 watts. As a
consequence, the efficiency of this power supply will drop
from 85% to 83.9%.
Figure 4 shows the applied input voltage in yellow
transitioning from 260 V ac to 270 V ac. This step was
selected for clarity, noting that the circuit actually
responds at 268 V ac. The blue trace shows the gate
voltage of Q2 rising which initiates a shutdown of the
NCL30001 controller.
The input voltage must be reduced to below 253 V ac to
restore operation. A delay of approximately 40 ms is
used to ensure the input voltage has returned to the
proper level and the Over Voltage monitor is not
responding to normal zero crossing events.
Figure 5 shows the response of the circuit to a reduction
of input voltage from 268 V ac to 253 V ac. The blue
trace shows the gate of Q2 dropping after a delay.
The circuit provides clean transitions from excessive
input voltage to shut down state and back to normal
operation when required. The hysteresis and delay of the
circuit provide robust protection.
Reduced Current/Dissipation
Dissipation and effect on system efficiency may be a
concern in some applications. While the LM2903 dual
comparator is a cost effective solution, the bias current is
about 1 mA. This introduces significant power loss given
the circuit must be powered by a linear regulator from a
high voltage source.
If the Input Over Voltage Protection circuit of Figure 2
was incorporated, the input power increase is only 0.13
watts. The power supply efficiency would be 84.8%. The
circuit of Figure 2 represents an improvement of about
1% for this example power supply compared to the circuit
of Figure 1.
A Bill of Materials is presented in Figure 6 below showing
all parts including options for lower power comparator
and higher accuracy operation.
Design Tool
A design tool is available at the ON Semiconductor
website. This Excel® spreadsheet helps establish shut
off and start up voltages. In addition, the designer can
select which type of comparator and reference to use
and guidance is given on selecting the bias resistors.
Conclusion
Many applications be it lighting, communications, or
computing which are exposed to poor power quality may
benefit from a protection solution which does not involve
more expensive or lossy semiconductor components.
Since this solution does disable the power converter during
the over voltage event it may not be suitable for critical
applications which cannot be interrupted.
The monitor circuit will automatically restore normal
operation when the input voltage returns acceptable levels.
This design does not use any electrolytic capacitors which
enhances reliability and reduces PCB space.
ON Semiconductor offers another dual comparator which
draws less than 3.5% of the LM2903 bias current. The
NCS2220 comparator draws only 34 µA. Details for this
circuit are shown in Figure 2.
February 2014, Rev. 0
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DN05060/D
*R15 is a collection of resistors based on
dissipation and voltage derating
R15
Rectified AC
196k
R1
499k
R4
100k
R8
R9
R11
R14
100k
470k
220k
47k
Shutdown Low
R2
U1B
U1A
LM2903
8
499k
8
LM2903
3
+
R10
5
1k
1
6
+
Q2
7
2N7002
-
-
4
2
D1
MMSD4148
4
R6
2meg
R7
C1
Vcc bias
5.1 Volt
R13
1.5meg
220k
10nF
R3
R5
10k
430k
C2
R12
C3
100nF
220k
100nF
D2
MMSZ4689
Q1
2N7002
Primary Return
Figure 1. LM2903 Schematic
R15
*R15 is a collection of resistors based on
dissipation and voltage derating
Rectified AC
1.36 meg
R1
511k
R4
100k
R9
R11
470k
220k
Shutdown Low
R2
U1A
NCS2220A
511k
3
D1
8
+
-
2
U1B
NCS2220A
R10
1k
Q2
5
+
-
1
7
2N7002
6
1
R6
MMSD4148
2meg
Vcc bias
R13
5.0 Volt
C1
R7
1.5meg
220k
R16
10nF
R3
R5
10k
430k
C2
R12
C3
100nF
220k
100pF
U2
NCP431
220k
R17
Q1
220k
2N7002
Primary Return
Figure 2. NCS2220A Schematic
February 2014, Rev. 0
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DN05060/D
Figure 3. NCL30001 schematic showing connection to Input OVP circuit
February 2014, Rev. 0
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DN05060/D
Figure 4. Circuit responding to excessive input voltage
Figure 5. Restoring normal operation for normal input voltage
February 2014, Rev. 0
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DN05060/D
1
Value
Description
Tol
(+/-)
Footprint
Manufacturer
Cap
10nF
50V Ceramic X7R
10%
0603
Cap
100nF
50V Ceramic X7R
10%
0603
1
Cap
100nF
50V Ceramic X7R
10%
D1
1
Diode
MMSD4148
100V, 200mA
D2
1
Diode
MMSZ4689
Low current, 5.1V
Q1 Q2
2
2N7002
NFET, 60V, 7.5Ω
R1 R2
2
Res
499k
1/4W
R3
1
Res
10k
1/10W
R4 R8
2
Res
100k
R5
1
Res
430k
R6
1
Res
R7
1
R9
Ref
Qty
Type
C1
1
C2
1
C3
Manufacturer Part
Number
Sub
Allowed
TDK
C1608X7R1H103K080AA
Yes
TDK
C1608X7R1H104K080AA
Yes
0603
TDK
C1608X7R1H104K080AA
Yes
-
SOD-123
ON Semiconductor
MMSD4148T1G
No
5%
SOD-123
ON Semiconductor
MMSZ4689T1G
No
-
SOT-23
ON Semiconductor
2N7002LT1G
1%
1206
Yes
1%
0603
Yes
1/10W
1%
0603
Yes
1/10W
1%
0603
Yes
2 meg
1/10W
1%
0603
Yes
Res
1.5 meg
1/10W
1%
0603
Yes
1
Res
470k
1/10W
1%
0603
Yes
R10
1
Res
1k
1/10W
1%
0603
Yes
R11
R12
R13
3
Res
220k
1/10W
1%
0603
Yes
R14
1
Res
47k
1/10W
1%
0603
Yes
R15
8
Res
24k
1/4W
5%
1206
U1
1
Comp
LM2903
-
SOIC8
Tran
Dual Comparator
No
Yes
ON Semiconductor
LM2903DG
No
Optional Components when implementing low power comparator and higher accuracy reference of Figure 2
C3*
R1
R2*
1
Cap
100pF
50V Ceramic X7R
10%
0603
TDK
C1608C0G2A101K080AA
Yes
2
Res
511k
1/4W
1%
1206
Yes
R15*
R16
R17*
2
Res
680k
1/4W
5%
1206
Yes
2
Res
220k
1/10W
1%
0603
U1*
1
Comp
NCS2220A
Dual Comparator
-
UDFN8
ON Semiconductor
NCS2220AMUT1G
No
U2*
1
Reg
NCP431
Low Current Ref
-
SOT-23
ON Semiconductor
NCP431AVSNT1G
No
Yes
Figure 6 Bill of Materials
1
© 2014 ON Semiconductor.
Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor
does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to
assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated
with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its
products at any time, without notice.
Design note created by Jim Young, e-mail:[email protected]
February 2014, Rev. 0
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