Non-isolated Negative Output Buck AC/DC Converter

AND8190/D
Non−isolated Negative
Output Buck AC/DC
Converter
Prepared by: Jan Grulich
ON Semiconductor
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APPLICATION NOTE
The monolithic power switcher used in this application
greatly simplifies the total design and reduces time to
production. ON Semiconductor’s NCP1010 – 1014 family,
a new line of Power Switchers, is ideal for this purpose. The
NCP101x is offered in a SOT−223 package for reduced size,
and is suitable for mass production.
The design consists of the input filter, rectifier with
filtering capacitor, the power stage with switcher and
inductor, output ultrafast rectifier, output filtering capacitor,
the feedback loop with Zener diode and optocoupler, and an
indicator LED. The only component necessary for proper
powering of the IC is the VCC capacitor, since the IC is
directly powered from the HV Drain circuit via an internal
voltage regulator. To eliminate noise at the feedback input,
a small ceramic capacitor of around 1 nF should be
connected as close to the FB pin as possible.
This application note describes how to easily design a
simple, non−isolated AC/DC converter for powering the
low voltage control portion of line−powered applications
that use a triac or SCR power switch. Examples of such
applications include dishwashers, microwave ovens, coffee
machines, illumination, etc. Compared to passive solutions
using resistors or capacitors to reduce the voltage, this
design has significant advantages such as:
• Wide input voltage range 85 – 265 VAC
• Smaller size, lower weight, lower total cost
• Good line and load regulation, no need of additional
linear regulators
• Efficient design with up to 80% efficiency
• Overload, short circuit and thermal protection
• Convenient for mass production due to SMD devices
• Universal design for wide range of output currents and
voltages
E2
220 F/25 V
C1
100 nF
2
1
R1
1k5
D1
MUR160
E1
10 F/400 V
1
CON2
ARK750/2
L2
1 mH
VCC
HV
GND
FB
2
L1
1.5 mH
CON2
ARK500/2
IO2
NCP1014ST
D2
1N4007
E3
47 F/25 V
C2
1 nF
IO1
PC817
ZD1 LD1
11 V GRN
Figure 1. Complete Schematic Diagram of the 12 V/0.2 A Converter
 Semiconductor Components Industries, LLC, 2004
December, 2004 − Rev. 0
1
Publication Order Number:
AND8190/D
AND8190/D
SELECTION OF CRITICAL COMPONENTS
Inductor Selection
The current through the inductor at the beginning of the
Ton time is
The desired output power determines the minimum value
of the inductance. This value is dependent on the mode of
operation. A reduced inductor value results in
Discontinuous Conduction Mode of operation (DCM). In
practice, the switch−over point between Continuous
Conduction Mode of operation (CCM) and DCM is
commonly set to be slightly below maximum output power.
This achieves a reasonable compromise between inductor
size and ripple current, efficiency, and overall lower cost.
The only significant negative aspect of this particular
operating mode is a higher peak−to−average current ratio in
the inverter circuit.
The current ripple in the inductor during the Ton time may
be expressed by the equation
Iripple(Ton) Ton Iinit Iset Iripple
Iset = Peak switching current set by the FB loop.
The average current through the inductor over one
switching cycle can be expressed by the equation
Ic fop_min Iripple
Iinit Toff
2
Ic = Inductor operating current
fop_min = Minimum operating frequency.
The theoretical minimum inductor value is given by the
expression
Vds Vo)
(V min L min
L min Ton = ON time, internal power switch is ON
Vmin = Minimum rectified input voltage
Vds = Drain−to−Source voltage drop
Vo = Output voltage
Lmin = Minimum inductor value.
The current ripple in the inductor during the Toff time may
be expressed by the equation
Iripple
Iinit Ton
2
(2 Vo Io (V min Vds Vo))
(Iripple2 fop_min (V min Vds))
Io = Output DC current.
The theoretical maximum output power will be
Pout_max L min (Iset2 Iinit2) fop_min
V min Vds)
(V (min
VdsVo)
2
The current ripple in the inductor during the normal
operation will be
Vo
Iripple(Toff) Toff L min
Toff = OFF time, internal power switch is off.
Iripple ((V
((V min Vds Vo) Vo)
min Vds) fop_min L min)
The output current will be
Io fop_min ((Iset Iinit) Ton (Iset Iinit) Toff)
2
Table of Preselected Inductors (Vmin = 120 V, Vds = 9 V, Vo = 12 V, Iset = 0.405 A, fop_min = 59 kHz)
Inductance
(H)
Coilcraft Part Number
(see appendix for address)
Iripple
(A)
Output Current
(A)
470
RFB0810−471
0.39
0.25
680
RFB0810−681
0.27
0.32
820
RFB0810−821
0.22
0.34
1000
RFB0810−102
0.18
0.36
1500
RFB0810−152
0.12
0.40
The output current is the theoretical value and must be
multiplied by the efficiency (~ 0.7).
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2
AND8190/D
Freewheeling Diode Selection
diode with reverse recovery time trr < 35 ns must be used. For
the DCM operation a standard ultra−fast diode with trr <
75 ns is adequate.
The freewheeling diode needs to be selected according to
the mode of operation. For CCM operation an ultra−fast
Table of Preselected Freewheeling Diodes
Part number
VRRM
(V)
IF(AV)
(A)
trr
(ns)
Package
MUR160
600
1.0
75
Axial Lead
MURA160T3
600
1.0
75
SMD SMA
MURS160T3
600
1.0
75
SMD SMB
MURS260T3
600
2.0
75
SMD SMB
Electrical Specification of the Example in Figure 1:
Input: 85 – 265 VAC
Output: + 12 V / 200 mA
Note: The polarity is relative to the common line.
COMPONENT LAYOUT
Figure 2. Component Layout – Top Side
Figure 3. Component Layout – Bottom Side
PCB LAYOUT
Figure 4. PCB Layout
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3
AND8190/D
EMI Test Results:
Test Conditions:
Input: 230 VAC
Output: 11.7 VDC
Load: Resistive 68 R
Figure 5. Conducted EMI
Contact Address of the Inductor Manufacturer:
Coilcraft
1102 Silver Lake Road, Cary IL 60013
800−322−2645
847−639−6400 Fax 847−639−1469
21 Napier Place
Wardpark North, Cumbernauld
Scotland G68 0LL
Telephone (Int) : 44 (0)1236 730595
Fax (sales) : 44 (0)1236 730627
www.coilcraft.com
ON Semiconductor and
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AND8190/D