Nonisolated Positive Output Buck AC/DC Converter

AND8226/D
Nonisolated Positive Output
Buck AC/DC Converter
Prepared by: Jan Grulich
ON Semiconductor
http://onsemi.com
APPLICATION NOTE
This application note describes the way, how to easily
design the simple, non isolated AC/DC converter for
powering low voltage control part of mains applications
with triac, or SCR power switch. Some examples are:
dishwashers, microwave ovens, coffee machines, night
illumination and so on. In comparison with resistive, or
capacitive dropper is this solution more comfortable and
features some advantages such as:
• Wide Input Voltage Range 85 VAC – 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 Protected
• Simple for Low Cost Mass Production
• Universal Design for Wide Range of Output Currents
and Voltages
The monolithic power switcher, used in this application,
greatly simplifies the total design and reduces time to
production. The new line of the Power Switchers, NCP1010
through NCP1014, is ideal for this purpose. This IC in the
SOT−223 package reduces size and is suitable for mass
production. The design consists of input filter, rectifier with
filtering capacitor, power stage with switcher and inductor,
output ultrafast rectifier, output filtering capacitor, feedback
loop with zener diode and optocoupler and indicating LED.
The only component necessary for proper powering of the
IC is the VCC capacitor. The IC is directly powered from the
HV Drain circuit via internal voltage regulator. To eliminate
the noise at the feedback input, some small ceramic
capacitor with value of around 1.0 nF is necessary to be
connected as close to the FB pin, as possible.
Schematic diagram
C1
100 nF
2
E2
220 mF/25 V
D1
MUR160
E1
+ 10 mF/400 V
1
R1
1k
+
1
2
CON2
ARK750/2
L2
1 mH
+
D2
1N4007
VCC
HV
GND
FB
L1
1.5 mH
CON2
ARK500/2
LD1
GRN
IO2
NCP1014ST
IO1
PC817
E3
47 mF/25 V
ZD1
11 V
C2
1 nF
Figure 1. Complete Schematic Diagram of the 12 V/0.2 A Converter
© Semiconductor Components Industries, LLC, 2005
October, 2005 − Rev. 1
1
Publication Order Number:
AND8226/D
AND8226/D
SELECTION OF CRITICAL COMPONENTS
Inductor selection
The average current through the inductor over one
switching cycle can be expressed by Equation 4.
For the selected output power need to be selected certain
minimum value of the inductance. This value is dependent
on the mode of operation. Reduced value results in
Discontinuous Conduction Mode of operation (DCM).
Practically was found, that the borderline between
Continuous Conduction Mode of operation (CCM) and
DCM is commonly set slightly below maximum output
power. The result is low cost of the inductor, freewheeling
diode (trr > 35 ns), higher efficiency and lower cost. The
negative result is in lower output power and higher cost of
the NCP101x Power Switcher.
The current ripple in the inductor during the Ton time may
be expressed by Equation 1.
DIripple(Ton) + Ton @
Vds * VO)
ǒ(V min *L min
Ǔ
Ic + fop_min @
ǒLVO Ǔ
L min +
Ǔ @ Ton ) ǒ2DI)ripple
Ǔ @ ToffǓ
Iinit
(eq. 4)
(2 @ VO @ IO @ (V min * Vds * VO))
(eq. 5)
(DIripple 2 @ fop_min @ (V min * Vds))
IO = Output DC Current.
The theoretical maximum output power will be shown in
Equation 6.
(eq. 1)
Pout_max + L min @ (Iset 2 * Iinit 2) @ fop_min
@
(V min *Vds)
ǒ(V min
Ǔ
*Vds*VO)
(eq. 6)
2
The current ripple in the inductor during the normal
operation will be shown in Equation 7.
DIripple +
((V min * Vds * VO) @ VO)
(eq. 7)
((V min * Vds) @ fop_min @ L min)
The output current will be shown in Equation 8.
IO + fop_min @
(eq. 2)
min
DIripple
2 ) Iinit
Ic = Inductor Operating Current,
fop_min = Minimum Operating Frequency
The theoretical minimum inductor value corresponds to
Equation 5.
Where:
Ton = ON Time, Internal Power Switch in 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 Equation 2.
DIripple(Toff) + Toff @
ǒǒ
((Iset ) Iinit) @ Ton ) (Iset ) Iinit) @ Toff)
(eq. 8)
2
Toff = OFF Time, Internal Power Switch in OFF.
The current through the inductor at the beginning of the
Ton time is shown by Equation 3.
Iinit + Iset * DIripple
(eq. 3)
Iset = Peak Switching Current Set by the FB Loop.
Table of Preselected Inductors (Vmin = 120 V, Vds = 9 V, VO = 12 V, Iset = 0.405 A, fop_min = 59 kHz)
NOTE:
Inductance
(mH)
Coilcraft Part Number
(see appendix for address)
DIripple
(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 need to be multiplied by the efficiency (~0.7).
http://onsemi.com
2
AND8226/D
Freewheeling diode selection
The freewheeling diode needs to be selected accordingly to the mode of operation. For the CCM operation needs to be used
the ultra fast diode with trr < 35 ns. For the DCM operation the standard ultra fast diode with trr < 75 ns is enough.
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 at Figure 1:
Input: 85 VAC – 265 VAC
Output: + 12 V / 200 mA
NOTE: The polarity is proportional to common line.
COMPONENT LAYOUT
Figure 2. Component Layout – Top Side
Figure 3. Component Layout – Bottom Side
PCB LAYOUT
Figure 4. PCB Layout
http://onsemi.com
3
AND8226/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
Email: [email protected]−europe.com
ON Semiconductor and
are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice
to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability
arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages.
“Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All
operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights
nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications
intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should
Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates,
and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death
associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal
Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner.
PUBLICATION ORDERING INFORMATION
LITERATURE FULFILLMENT:
N. American Technical Support: 800−282−9855 Toll Free
Literature Distribution Center for ON Semiconductor
USA/Canada
P.O. Box 61312, Phoenix, Arizona 85082−1312 USA
Phone: 480−829−7710 or 800−344−3860 Toll Free USA/Canada Japan: ON Semiconductor, Japan Customer Focus Center
2−9−1 Kamimeguro, Meguro−ku, Tokyo, Japan 153−0051
Fax: 480−829−7709 or 800−344−3867 Toll Free USA/Canada
Phone: 81−3−5773−3850
Email: [email protected]
http://onsemi.com
4
ON Semiconductor Website: http://onsemi.com
Order Literature: http://www.onsemi.com/litorder
For additional information, please contact your
local Sales Representative.
AND8226/D