AN-D25

Supertex inc.
AN-D25
Application Note
Efficient Switchmode Power Supply Start-Up Circuit
by Jimes Lei, Applications Engineering Manager
Introduction
The purpose of this application note is to demonstrate the
many advantages of using the Supertex LND150N3 in the
start-up circuit for switchmode power supplies.
Commonly used low voltage bipolar, CMOS and BiCMOS
switchmode power supply PWM ICs usually operate from
supply voltages of up to 18V. When the input power for the
switchmode converter is available at voltages higher than
the maximum voltage rating of the IC, the voltage has to
be reduced with a start-up circuit. A frequent requirement is
for operation directly from a rectified 120V or 240V AC line
without the use of tap changing switches for the selection of
different voltages.
The circuit in Figure 1 shows the Supertex LND150N3 being
used to provide the low voltage power supply to be connected to the VCC pin of the Unitrode UC3845N PWM IC. This
circuit is capable of providing start-up over an input voltage
range of 40VDC to 500VDC.
A simple and often-used approach utilizes a power resistor
and a Zener diode as shown in Figure 2. The major difference between the two approaches is that the LND150N3
consumes negligible power after the SMPS has started
whereas the resistor will draw power continuously from the
input line.
For technical information on various aspects of designing
such power supplies, please refer to the Supertex application notes AN-H13 and AN-H21 through AN-H24.
A
VIN
C2
–
+12V
LND150
+
GND
B
VOUT
200k
+
9.1V
–
C
VCC
C1
PWM
IC
A
VN2460N8
R
B
1.0Ω
+
–VZ
C
Zener Diode +
Power Resistor
Approach
Figure 1: SMPS with LND1 Start-Up
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A082213
Figure 2: Power Resistor
Approach
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Power Resistor Approach
The Unitrode part #UC3845N specifications relevant to the
start-up are as follows:
Depending on whether the PWM IC utilizes bipolar or CMOS
technology, the power rating of the supply and the input voltage available, the continuous power dissipated in the resistor may be up to 5.0 watts or even higher. For example, if
the current required is 10mA, from a 240VAC line the power
dissipated will be 3.4 watts. The start-up current can be quite
high when the power supply uses a large MOSFET switch,
since a considerable current is required by the buffers or
MOSFET drivers in addition to the IDD or ICC of the PWM IC.
This causes 4 major problems:
a)
b)
c)
d)
Start-up current
Operating current
Start-threshold voltage
Min. operating voltage after turn-on
The start-up current is the biasing current for the IC when the
output is not switching. Once the output starts switching, the
IC is considered to be in the operating mode and will draw
no more than 17.0mA plus the load current being sourced
into the gate of the MOSFET. This load current is calculated
as fCV, where f is the operating frequency, C is the effective
input capacitance, and V is the VCC voltage. The value for the
resistor, R, must be small enough such that under the worst
case condition of minimum input line voltage, it can source
1.0mA to the IC and some biasing current for the Zener diode, IBIAS. The value of R is calculated as follows:
Excessive heat on the PC board;
Narrow input voltage operating range;
Loss of efficiency;
Large size of power resistor.
These problems can become very difficult to solve, especially in compact, off-line switchmode power supplies used
in portable equipment, e.g., laptop and notebook computers, battery chargers, etc.. In such equipment, space is at
a premium and management of the generated heat is very
troublesome. Reducing the temperature rise on densely
populated PC boards may often be impossible, resulting in
potentially reduced system reliability. For such applications,
designers often face the task of achieving very high efficiency for power supplies, with minimal board space, operating
directly from 120VAC, 240VAC, or other international utility
voltages.
ID
PWM
IC
VCC
VZ
IAUX = 0
VIN(MIN)
(1.0mA + IBIAS )
PDISS =
(VIN(MAX) )2
R
ID = 0mA
LND150N3
PWM
IC
IBIAS
+
9V
-
200k
VZ
VOUT < 8.0V
from AUX
winding
LND150N3
VCC
IIC
200k
IAUX
Figure 3a: Current Paths During Start-Up
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R=
The worst case power dissipation is determined as follows:
IIC
IBIAS
1.0mA max
17.0mA max
9.0V max
8.2Vmax
+
- 9V
VOUT = 12.7V
from AUX
winding
Figure 3b: Current Paths After Start-Up
2
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The continuous power dissipation at higher voltages from
R increases when the required input operating voltage for
the converter is widened. Consider for example a converter
required to operate at 120VAC and 240VAC. R is calculated
as:
(120) • (1.414) • V
R=
(1.0mA + 100µA)
LND1E is always in the “ON” state when there is 0V gateto-source bias. When power is available at the input of the
power supply, the LND150N3 supplies current to charge up
capacitor C and biases the 9V Zener diode through the external 200KΩ source resistor as shown in Figure 4a. The
VGS(OFF) of the LND150N3 divided by this resistor approximates the biasing current for the Zener diode. The VCC generated is approximately equal to:
R = 154KΩ
Operating the power supply from rectified 240VAC, R will
dissipate:
P=
and is almost completely constant within the input voltage
range of (VZ +3.0V) to 500V. The same circuit can therefore
be used without any modifications for both 120VAC and
240VAC. A sourcing current equal to the IDSS of the device will
cause VCC to be equal to VZ. The amount of source current
will range from 1.0mA to 3.0mA per guaranteed minimum
and maximum values of IDSS in the LND150N3 data sheet.
The guaranteed 1.0mA minimum specification satisfies the
maximum current required for the UC3835N to start up.
(240 • 1.414)2
154KΩ
P = 0.75 watts
LND150 Circuit Description
The start-up circuit portion of Figure 1 is analyzed and shown
with additional notes in Figures 3a and 3b. Figure 3a shows
the current paths during start-up and Figure 3b is after the
start-up has occurred. The following main specifications of
the LND150N3 are considered:
Parameter
Min
Max
Drain-to-source breakdown voltage
- BVDSS
500V
-
Gate-to-Source off voltage - VGS(OFF)
-1.0V
-3.0V
Saturated drain-to-source current - IDSS
1.0mA
3.0mA
VCC = (VZ - VGS(OFF)) (1 - √ IDD / IDSS )
Once VCC is generated, it allows the PWM IC to start up and
generate a DC voltage VOUT from one of the auxiliary winding. The turns ratio on the transformer should be designed
so that:
VOUT ≥ 3.0V + VZ + 0.7V
This allows the LND150N3 to be turned “OFF” as shown
in Figure 3b. By setting VOUT = 12.7V, the source of the
LND150N3 will be at 12V and the gate will be at 9.0V. The
LND150N3 gate-to-source voltage will therefore be 9.0V 12V = -3.0V, which turns it “OFF”. After the start-up described
above, the only current drawn by this circuit from the rectified AC line is the small amount of drain-to-source leakage
current, ID(OFF), typically less than 100nA.
LND150N3, the TO-92 version of LND1E, is configured as
a source follower. Being a depletion-mode MOSFET, the
VCC
(10V/DIV)
ID
(1mA/DIV)
VCC
(10V/DIV)
ID
(1mA/DIV)
IAUX
(20mA/DIV)
IAUX
(20mA/DIV)
Figure 4: Start-Up Waveforms for
VCC, ID, IAUX at VIN = 40VDC
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A082213
Figure 5: Start-Up Waveforms for
VCC, ID, IAUX at VIN = 400VDC
3
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The maximum power dissipation on the LND150N3 is determined by:
is not running. Once VCC reaches the UC3835N’s start-up
threshold voltage of 9.0V, the output will start switching the
power MOSFET, a Supertex VN2460N8 at a frequency of
40KHz. During the time the IC is driving the external switching MOSFET, it will draw 16mA from C1. The IC will continue
to operate and drive the MOSFET until VCC discharges to
the IC minimum operating voltage after turn-on specification,
which is about 8.0V, at which time the under voltage lock-out
does not allow operation. During the time the MOSFET is
switching, the auxiliary winding supplies current and charges the capacitor C1, which builds up its voltage step by step,
every time the MOSFET switches. As long as the voltage
build-up in steps is below the under voltage lock-out level,
i.e., less than 8.0V, the sequence of events above will be
repeated.
PDISS = (IDSS(MAX) ) • (VIN(MAX) )
PDISS = (3.0mA) • (240V) • (1.414)
PDISS = 1.02 watts
The 1.02 watt is only being dissipated for a short duration
during start-up. After start-up, it dissipates virtually no power.
Start-up Current Waveforms
To demonstrate the performance of the LND150N3 in the
start-up circuit, the 3.0watt power supply shown in Figure 1
was built and tested. VIN was tested with a DC input of 40VDC
and a rectified sinusoidal 285VAC, which was obtained from
the utility via a step-up transformer. The converter was powered up with its maximum load of 3.0watts connected to the
outputs. The VCC voltage, drain current of the LND150N3
(ID), and the current from the auxiliary winding (IAUX) were
simultaneously monitored during the start-up.
Once again, the LND150N3 charges C1 from 8.0 to 9.0V.
Once VCC reaches 9.0V, the IC starts switching the VN2460N8
MOSFET and the voltage on the auxiliary winding, VOUT, further increases. The cycle repeats itself several times until
the VOUT reaches 9.0V. Once VOUT reaches 9.0V, it will supply
the 16mA to the IC so C1 will no longer discharge to 8.0V.
This allows the converter to continue to operate and VOUT to
reach 12.7V. At this time, the LND150N3 is turned “OFF”, ID
goes to 0mA, VCC ramps up to 13V and IAUX charges C1 from
9.0 to 13V and settles to 16mA.
Figure 4 shows the voltage and current waveforms with VIN
= 40VDC. Figure 5 shows voltage and current waveforms of
the same circuit under the same conditions except the input
voltage is a rectified 285VAC.
The only difference between the waveforms shown in Figures 4 and 5 is that the circuit powers up faster when a
higher voltage is applied at the input of the power supply. A
rectified 285VAC line gives a 400VDC line which allows the
primary of the transformer to have a higher di/dt and consequently induce a large voltage in the secondary auxiliary
winding. This causes the capacitor C1 to charge up faster as
compared to the operation at 40V. The peak power dissipation for the LND150N3 is:
You will observe that the performances shown in Figures 4
and 5 are similar except that the latter shows a faster powerup.
The sequence of events before the start-up, as shown in Figure 4, is described as follows. Initially, all voltages are at 0V.
When 40VDC is applied to VIN, the LND150N3 starts charging the 100µF capacitor C1 at a rate equal to the LND150N3’s
IDSS minus the input current drawn by the IC, IIC. The actual
value of IDSS for this device is 2.4mA as seen by the waveform ID. VCC will therefore ramp up at the following rate:
dv
dt
=
=
for only 240 milliseconds.
(IDSS - IIC )
Higher Start-up Current
The Supertex DN2540N3 can be used for applications requiring much higher start-up currents because the Idss minimum of this device is rated at 150mA. This device has a
BVDSS rating of 400V minimum and is available in the TO92, TO-220 and SOT-89 package to suit various commercial
and industrial grade applications.
C1
(2.45 - 0.5)mA
100µF
which is 3.8V/200msec or 3.8V/division. Initially, there is no
current from the auxiliary winding because the converter
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P = (400V) • (2.4mA) = 0.96Watts
4
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This device can be used by an addition of a current limiting
resistor, RLIM as shown in Figure 6. The small current limiting
resistor, RLIM, is recommended to develop a slightly negative
VGS to set the desired output current and ensure the ID value
does not allow the DN2540 power rating to be exceeded.
DN2540
PWM
IC
Conclusion
The start-up circuit using the LND150N3 improves the overall efficiency, reduces power dissipation, widens operating
input voltage range, and reduces board space which would
be required by a large power resistor. For surface mount
requirements, the LND150N8, which is the SOT-89 (TO243AA) version of the part, allows efficient use of board
space.
RLIM
VCC
200k
+
9V
-
VZ
from AUX
winding
Figure 6: High Current Start-Up
Supertex inc. does not recommend the use of its products in life support applications, and will not knowingly sell them for use in such applications unless it receives
an adequate “product liability indemnification insurance agreement.” Supertex inc. does not assume responsibility for use of devices described, and limits its liability
to the replacement of the devices determined defective due to workmanship. No responsibility is assumed for possible omissions and inaccuracies. Circuitry and
specifications are subject to change without notice. For the latest product specifications refer to the Supertex inc. (website: http//www.supertex.com)
Supertex inc.
©2013 Supertex inc. All rights reserved. Unauthorized use or reproduction is prohibited.
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Tel: 408-222-8888
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