AN-D30

Supertex inc.
AN-D30
Off-Line 5.0V Output
Non-Isolated Linear Regulator
Introduction
There are many applications that call for a non-isolated, low
current DC power supply operating directly from the AC line.
A switchmode power supply would be far too complex and
expensive, whereas a simple 60Hz step down transformer
would be cost effective but physically too large. Examples of
such applications include battery chargers, proximity switches, television stand-by supplies, and internal supplies for
switchmode power supplies. When line isolation is not necessary, the circuit presented in this application note provides
a solution that is both very cost effective and compact.
The circuit presented in this note is a two-stage linear regulator capable of providing 5.0V at 50mA, but can be easily
modified for other output voltages. The first stage handles
the majority of voltage drop and power dissipation, allowing the second stage to employ standard low-voltage, lowpower linear IC regulators. The Supertex DN2535N5 high
voltage depletion-mode MOSFET is used as the first stage
pass transistor.
For an isolated output, or an output with significantly higher
output currents, please refer to Supertex application notes
AN-H13, AN-H21, AN-H22, AN-H23, and AN-H24 which
discuss the Supertex HV91XX series of switchmode PWM
controller ICs.
Application Note
Circuit Description
The circuit of Figure 1 provides a regulated 5.0V output at
50mA directly from a 120VAC input. Detailed descriptions
will be given for the three different sections: Input rectifier,
pre-regulator, and output regulator. SPICE simulation, lab
measurements, and power dissipation are also addressed.
Caution
The circuit described in this application note
does NOT provide galvanic isolation. When operated from an AC line, potentially lethal voltages can be present within the circuit. Adequate
means of protecting the end user from such voltages must be provided by the circuit developer.
Design Requirements
The circuit in Figure 1 was developed to meet the design
requirements listed in the table below. Many other output
voltages and currents can be achieved simply by changing
component values, without requiring any alterations in circuit
topology.
Input
Output
5.0VDC ±4%
120VAC
0 to 50mA
50mVP-P ripple voltage
Pre-Regulator
120VAC
C2
150pF
D1-D4
1N4001
Output-Regulator
U1
120VAC
Q1
DN2535N5
Z1
9.1V
C1
10µF
LM78L05
R1
100kΩ
Load
C3
1.0µF
Figure 1: 5V @ 50mA Non-Isolated Power Supply
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AN-D30
Circuit Description
Component
Function
Notes
Value
Rating
D1-D4
Rectifies AC line current
-
1N4004 (120VAC)
1N4005 (240VAC)
200V (120 VAC)
350V (240VAC)
100mA
Q1
Preregulator pass
transistor
Use appropriate heat
sink
DN2535N5 (120VAC)
DN2540N5 (240VAC)
350V (120VAC)
400V (240VAC)
150mA
Z1
Sets preregulator
output voltage
Other zener voltagesmay
be used for other output
voltages
1N757 (9.1V)
1/10W
R1
Provides bias for Q1 and Z1
-
100kΩ
1/16W
C1
Stores energy for use
when AC < 9.1V
-
10µF
15V
C2
Prevents high frequency
oscillations
Locate close to Q1
150pF
200V (120VAC)
350V (240VAC)
U1
Provides output
regulation
Other devices may be
used for other output
voltages
LM78L05 (5.0V)
50mA
C3
Reduces transients at output
-
1.0µF
10V
(or greater than
output voltage)
Section I: Input Rectifier
Section II: Pre-Regulator
Figure 2 shows the line rectification circuit. Diodes D1
through D4 are selected to handle the maximum input voltage and load current. Recommended rectifiers are 1N4004’s
for 120V AC line or 1N4005’s for 240V AC line.
Pre-Regulator
C2
150pF
120VAC
Q1
DN2535N5
D1-D4
1N4001
Z1
9.1V
120VAC
Figure 3 shows the preregulator, providing high voltage input
to low voltage output using the Supertex DN2535N5. The
preregulator must supply an input voltage to the output regulator within a range as determined below:
Power dissipation in the bridge rectifier is calculated to be:
Prect = 2 • (ILOAD + IBIAS) • VF
VSOURCE(MIN) = minimum specified input voltage for
the ouput regulator
ILOAD = load current (50mA)
IBIAS = bias current for Z1 and U1 (5.5mA)
VF = rectifier forward voltage drop (0.7V)
This yields 78mW dissipation in the bridge rectifier.
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A040313
C1
10µF
Figure 3: Pre-Regulator
Figure 2: Input Rectifier
where:
R1
100KΩ
VSOURCE(MAX) = VOUT +
PREG(MAX)
ILOAD(MAX)
or maximum specified input voltage, whichever is less
2
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where:
VSOURCE(MAX) = voltage at source of Q1 = input voltage to output regulator
VAC(PK) = peak line voltage and the 1.4V accounts for
rectifier drops
For the given circuit, this works out to be 241µS. The minimum value of C1 given an allowable maximum voltage droop
is:
ILOAD(MAX) = maximum anticipated load current
Preg(MAX)= power dissipation rating for the output
regulator
VZ1 = Zener voltage
VOUT = final output voltage
For the LM78L05, the minimum input voltage is specified as
7.0V. The maximum input voltage is specified as 30V. However, the [email protected] power rating of the LM78L05 limits
the maximum input voltage to 17.6V.
C ≥ tOFF •
ILOAD + IBIAS
VZ1 - VSOURCE(MIN)
where:
ILOAD = load current
Q1 acts as a source follower where the source voltage follows the gate voltage minus the gate-source voltage (VGS):
IBIAS = bias current for Z1 and U1
VZ1 = zener voltage
VSOURCE(MIN) = minimum input voltage for U1
VSOURCE = VGATE - VGS
VGS increases with increasing drain current, thus with a fixed
gate voltage, the source voltage will drop with increasing load
current. For design purposes, VGS for the transistor under
saturation and cut-off conditions (0V and VGS(OFF), respectively) can be used. These values can be readily obtained
from the transistor data sheet. For the DN2535N5, VGS(OFF)
can be a maximum of -3.5v.
This works out to 6.4µF. The next highest standard value,
10µF, was selected.
Q1 must recharge C1 immediately after the rectified AC again
rises above 9.1V. For this reason, the transistor’s saturation
current (IDSS) must be greater than the load and bias currents. The DN2535N5’s IDSS is 150mA minimum, providing
more than enough current to recharge C1.
Zener Z1 sets the gate voltage and should be selected to
provide a source voltage within the range determined above,
taking into account the variances of VGS with load. A zener
voltage of 9.1V will result in a source voltage of 9.1 to 12.6V
under all load conditions.
The power dissipation for Q1 can be calculated from the voltage drop across it times the current through it:
PQ1 = (VDRAIN - VSOURCE) • (ILOAD + IBIAS)
Bias current for the Zener is determined by VGS/R2. Using
100kΩ for R2, the bias current can vary between 0 and 35µA,
although the actual bias current will be less than 35µA since
the transistor is not operated at complete cut-off.
Vdrain is the rectified 120VACrms line minus two diode
drops (1.4V) for a drain voltage of approximately 118.6Vrms.
VSOURCE has been previously determined to be in the range
of 9.1 to 12.6 volts. Therefore, the maximum voltage across
Q1 is about 109.5V. Current through Q1 is the sum of the
load current plus the bias currents for Z1 and U1, or 50mA +
5.5mA. This yields a power dissipation in Q1 of about 6.1W.
An adequate heat sink must be provided for Q1 to dissipate
this power. Power dissipation of the other components in the
preregulator are insignificant.
Storage capacitor C1 must store enough energy to supply
the load for periods when the rectified AC voltage is below
9.1 volts. The duration of this period is:
1
(VZ1 - 1.4V)
tOFF =
arcsin
πf
(VAC(PK) )
where:
f = line frequency
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Capacitor C2 across the drain-source of Q1 is needed to
avoid possible high frequency oscillations due to parasitic
inductances. A value of 150pF is sufficient.
3
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Section III: Output Regulation
The preregulator output voltage, for both simulation and actual results, are close to the design voltage of 10 volts.
The output regulator consists of an LM78L05 linear regulator and C3. U1 provides an output of 5.0V±4% at 50mA. C3
serves to reduce output transients.
The output regulator, LM78L05, provided a solid 5.0V output.
Rejection of the 2.0V input ripple was excellent, showing no
discernible output ripple. Experimental results showed about
20mVPP of noise at the output.
2nd Stage Regulator
LM78L05
U1
Load
C3
1µF
240VAC Variation
A slight variation of this circuit is to power a 25mA load from
a 240VAC line. Adjustment of the above circuit, replacing the
100Ω load resistor with a 200W and increasing the line voltage to 240VAC. Storage capacitor C1 may be halved since
current has been halved. Overall power dissipation is about
the same since the load current was halved and the line voltage doubled. The voltage rating of Q1 could remain at 350V,
since the peak voltage of a 240VAC line is 336V. However,
this only provides a 4% safety margin which does not allow
for variations in line voltage. The DN2540N5 with a 400V rating is recommended, providing a 19% safety margin. Recommended rectifiers for the bridge rectifier are 1N4005’s.
Figure 2: Input Rectifier
Power dissipation for the output regulator can be calculated
as:
PU1 = PBIAS + PREG = (VIN x IBIAS) + (VOUT - VREG) x IOUT
= 10V x 5.5mA + (10V - 5V) x 50mA
= 0.31W
Measurements
The circuit of Figure 1 was constructed and measurements
were taken. Overall, the circuit performed close to what simulations predicted. The preregulator output voltage is very
close to simulation and calculations. Simulations predicted
that VSOURCE would drop 2.0V. Experimental results confirmed this.
VDRAIN
(DC, 50V/div)
VSOURCE
(DC, 2.0V/div)
VOUT
(AC, 50mV/div)
Figure 5: Experimental results showing VDRAIN, VSOURCE,
and VOUT (top to bottom).
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Design Verification
Based on SPICE simulations, the circuit of Figure 1 should
yield waveforms similar to those in Figure 6.
200V
0V
15V
0V
V(RECT)
V(OUT) • V(REG)
10ms
20ms
30ms
34ms
Time
Figure 6: SPICE simulation results for 5.0V, 50mA load
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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1235 Bordeaux Drive, Sunnyvale, CA 94089
Tel: 408-222-8888
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