AN-D10

Supertex inc.
AN-D10
Application Note
Off-Line Compact Universal Linear Regulator
Introduction
An off-line compact universal linear regulator is shown in
Figure 1. The regulating device is the Supertex LND150N3.
The LND1 is a 500V N-channel depletion mode MOSFET
with gate-to-source ESD protection. The regulated voltage,
VOUT, is an ideal supply for CMOS ICs and a variety of other
circuits that require low current.
the 120VAC input line. D5 does not conduct during normal
operation.
Calculations for Component Values
C1 is a 0.1µF 200V capacitor, chosen to minimize ripple on
the 170VDC which would affect the regulated output voltage.
The minimum value of C1 is calculated as follows:
Circuit Description
The 120V AC input voltage is rectified by a full bridge, consisting of diodes D1, D2, D3, and D4. A small filter or smoothing capacitor, C1, is used to hold the rectified voltage to approximately +170VDC.
The unregulated 170VDC is connected to the drain of the
LND1. The LND1 and trim pot R1 are configured as a 1.0mA
constant current source. The 1.0mA constant current flows
through R2 which is a 5.1kΩ resistor to ground. A constant
voltage drop of 5.1V is developed across R2. VOUT is taken
as the voltage across R2 and is used to supply, for example,
a simple CMOS timer circuit.
Capacitor C2 is a low voltage bypass capacitor to supply
any peak current required by the CMOS timer circuit during switching transitions. D5 is a 5.6V Zener diode used to
clamp transient voltages that may occur during powering up
VIN = Asin2πft; A = 170V, f = 60Hz
I = C1
dv
; I = 1.0mA
dt
dv = ΔV = A - VOUT - (ID(ON) • RDS(ON) ); dt = Δt =
C1 ≥ I
Δt
= 1.0mA •
ΔV
•
1
2f
1
2(60Hz)
170V - 5.1V - (10mA) • (1KΩ)
C1 ≥ 0.054μF
The LND1 can maintain a virtually constant current over a
wide input voltage range. Large ripple voltages on the drain
of the LND1 will have very little effect on the output current. The device can also withstand transient voltages up
to 500V.
1500pF
+
120VAC/
240VAC D4
VIN
_
D2
D6
D1
D3
C1
0.1µF
3M
0.1mA
LND150N3
Potter &
Brumfield
KUMP
11A-18-240
3M
1500pF
VDD
D1, D2, D3, D4, D6, D7: IN4005
VDD
R1
10k
+
D5
5.6V
_
CK
R2
5.1k
T
Reset
C2
1.0µF
MC14030
14-BIT
Counter
VN0650N3
Load
100k
CMOS Timer
Figure 1. Linear Regulator
Doc.# DSAN-AN-D10
A090313
Supertex inc.
www.supertex.com
AN-D10
VDRAIN and VOUT vs Time
170V
VDRAIN
0V
5V
VOUT
0V
Figure 2. Input/ Output Waveforms
The value of the constant current source is a function of R1,
VGS(OFF), and IDSS where VGS(OFF) and IDSS are characteristics
of the device. R1 is a variable resistor adjusted for 1.0mA
and is approximated by:
R1 =
Where,
ID VGS(OFF)
ID
on VOUT over a period of time. C2 can be calculated as follows:
• ( √ ID / IDSS - 1 )
IOUT = output current
dt = required time duration of IOUT
dVOUT = acceptable change in VOUT
For example, a 10.0mA output peak current for a duration of
1.0µsec with a maximum VOUT drop of 100mV will require a
C2 value of 1.0mA(1.0µsec/100mV) = 0.1µF or greater. C2 is
chosen to be 1.0µF.
= desired constant current value,
VGS(OFF) = pinch-off voltage, and
IDSS
C2 = IOUT(dt/dVOUT) where,
= saturation current at VGS = 0 V.
Figure 2 is an oscilloscope picture showing the actual voltage waveforms on the drain of the LND1 and VOUT.
VGS(OFF) and IDSS may vary from lot to lot. The range of adjustment for R1 is calculated for operation over the range of
LND1 values for VGS(OFF) and IDSS.
Symbol
Parameter
Min
Max
VGS(OFF)
Pinch-off voltage
-1.0V
-3.0V
IDSS
Saturation current
-1.0mA
-3.0mA
Figure 3 is an output characteristic showing the regulation of
the circuit over a wide range of input voltage.
VOUT
5.0V
For the above values, R1 is calculated to be from 0 to 1.3kΩ.
A 10kΩ trim pot is chosen for R1.
Since the constant current is adjusted to 1.0mA, R2 is chosen to be 5.1kΩ to obtain a VOUT of 5.1VDC. The value of C2
is selected to supply the peak current required by the load
Doc.# DSAN-AN-D10
A090313
6.8V
VIN
Figure 3. VOUT vs VIN
2
Supertex inc.
www.supertex.com
AN-D10
For a 10V source, R2 can be replaced with a 10KΩ resistor. Applications requiring multiple voltage references can be
generated using a string of resistors as shown in Figure 4.
500V since the LND1 breakdown voltage is guaranteed to
be greater than 500V. A simple, low cost, transient protection (e.g., MOV) will protect the circuit from virtually anything,
other than a direct lightning strike.
The constant current can easily be changed by readjusting
R1 for the desired current. However, the power dissipation
on the LND1 should be taken into consideration. PD for the
LND1 in the TO-92 package should not exceed:
Regulation can also be achieved with AC or DC voltages
from 6.8 to 240V with no modifications of the circuit. This allows manufacture of one model of equipment for worldwide
usage without any voltage setting tappings.
IDS (VIN - VOUT ) = 600mW.
Conclusion
Alternative Applications
The Supertex LND1 can be configured as a simple, constant
current source to create an economical compact off-line, low
current regulated, voltage supply for powering CMOS ICs
and other low current loads. The need for transformers can
be eliminated.
Universality
The universality of the linear regulator can benefit a variety of industrial or consumer applications as it can be used
from a very wide range of input voltages, anywhere in the
world. Input voltages can be up to 450V for linear regulation. Protection is afforded for line voltage transients up to
+
VIN
_
120VAC/
240VAC
D4
D2
D1
C
D3
1.0mA
LND1
5.0VDC
1kΩ
4.0VDC
1kΩ
3.0VDC
1kΩ
2.0VDC
1kΩ
1.0VDC
1kΩ
0VDC
Figure 4. Multiple Voltage References
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.
Doc.# DSAN-AN-D10
A090313
3
1235 Bordeaux Drive, Sunnyvale, CA 94089
Tel: 408-222-8888
www.supertex.com
Similar pages