APPLICATION NOTE Over-Load Protection: continuously monitoring fe

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AND8127/D Implementing NCP1207 AC-DC Converter with Synchronous Rectifier
APPLICATION NOTE Over-Load Protection: continuously monitoring
feedback loop activity, NCP1207 enters hiccup operation soon power supply overloaded. soon overload condition disappears, resumes operation.
AC-DC Adaptor Board Specification
Introduction
NCP1207 controller dedicated driving current-mode free running quasi-resonant Flyback offline converter. This converter designed consumer products like notebooks, offline battery chargers, consumer electronics (DVD players, set-top boxes, TVs), etc. growing interest pollution reduction, efficiency improvement, maximum safety been taken into account while designing NCP1207. implementing NCP1207 build power supply that meet those requirements. This achieved with help following NCP1207 main features: Current-Mode Control: Cycle-by-cycle primary current observation helping prevent significant primary overcurrent which would cause transformer's core saturation consequent serious power supply failure. Critical Mode Quasi-resonant Operation: Prevents converter operation Continuous Conduction Mode input output condition. provided zero crossing detection auxiliary winding's voltage. addition reasonable delay switch turn-on instant shifted minimum (valley) drain voltage. This improves noise efficiency. Dynamic Self-Supply: Ensures proper operation applications where output voltage varies during operation like battery chargers. also supplies when overvoltage event being latched converter operation stopped. Overvoltage Protection: sampling plateau voltage auxiliary winding, NCP1207 enters into latched fault condition whenever overvoltage detected. controller stays fully latched until decreases below e.g. when user unplugs power supply from mains outlet re-plugs threshold adjusted externally.
adaptor following maximum performance ratings.
Output Power Output Voltage Output Current Minimum Input Voltage Maximum Input Voltage Maximum Switching Frequency
schematic diagram adaptor seen Figure
Transformer Design
bulk capacitor voltage than calculated:
Vbulk- VAC-
(eq.
Vbulk- VAC-
(eq.
requested output power Assuming efficiency input power equal
27.6 0.87
(eq.
average value input current minimum input voltage
IIN-AVG 27.6 bulk-
(eq.
Semiconductor Components Industries, LLC, 2004
June, 2004 Rev.
Publication Order Number: AND8127/D
AND8127/D
Taking into account absence clamping network suitable reflected primary winding voltage rated MOSFET switch
Vflbk Vbulk- Vspike
(eq.
Using time appropriate switching frequency ON-time calculated follows:
0.34 4.177 4.18 10-6
(eq.
Using calculated Flyback voltage maximum duty cycle calculated:
Vflbk Vflbk Vbulk-
(eq.
0.339 0.34
EF25 core transformer selected. cross-section area 52.5 mm2. ferrite material allows maximum operating flux density Bmax 0.25 number turns primary winding
4.18 10-6 bulk- 0.25 52.5 10-6 turns bulk- 4.18 10-6 0.635 Ippk 1.68
(eq.
following equation determines peak primary current:
IIN-AVG 10-3 Ippk 0.34
(eq.
primary inductance calculated follows:
(eq.
maximum switching frequency minimum input voltage kHz. Taking into account Quasi-Resonant (QR) valley switching operation NCP1207 time interval from instant total core demagnetization valley switch's drain voltage needs taken into account when calculating switch max. ON-time interval.
PMEC
factor transformer's core calculated follows:
(np)2
1.68 10-3 (80)2
(eq.
B250 mF/25 1N4148 1N4148 ISO1 PC817 TL431BILP STP4NB80 1N4148 BC238 BC238 IRF2807
NCP1207 Demag
BC308
Figure Schematic Diagram AC-DC Converter with NCP1207 Synchronous Rectifier
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AND8127/D
Since skin effect eddy currents play significant role Flyback topology given switching frequency Litz wire used. consists wires each with diameter 0.12 reduce leakage inductance primary winding split windings each with half number turns. secondary winding inserted between those halves primary windings. This well known sandwich arrangement. output voltage number turns secondary winding calculated (accounting synchronous rectifier) follows:
0.34) (eq. 0.34 Vbulk- turns Vs(1 max)np
crossing event. helps tune turn-on instant when drain voltage valley. Resistor also another function. Together with internal resistor divider, comparator voltage reference, forms overvoltage protection circuit. includes resistor internally connected ground. voltage that reaches roughly overvoltage latch triggered converter operation blocked until input supply plug disconnected. value resistor then calculated follows:
15.5 34.6
(eq.
secondary winding again made with Litz wire. consists wires featuring diameter 0.22 Using above number turns, auxiliary winding derived:
(VAUX Vfwd) nAUX 8.67 turns
(eq.
value delaying capacitor result tuning process real board.
Synchronous Rectifier
single wire 0.15 diameter used auxiliary winding. windings arrangement transformer following: Auxiliary Half Primary Secondary Half Primary
Primary Current Control
Primary current control path consist sensing resistor skipping resistor named maximum voltage threshold about value current sense resistor therefore given
1.57 (eq. 0.635 Ippk
synchronous rectifier consists following basic blocks: sensor secondary current, gate driver MOSFET switch. current transformer senses output rectifier current. current transformer primary winding located series with secondary switch within secondary current loop. Resistor loads secondary winding current transformer. resistor converts current into voltage. That voltage filtered limited capacitor diode then goes gate driver, which consists transistors pull-down resistor current transformer ring core selected. features cross-section area 7.83 mm2. magnetic allows maximum operating flux density Bmax appropriate number turns than easily wound that core around maximum demagnetization time converter's transformer calculated follows:
7.82 10-6 tdem cs-se Vclamp
(eq.
skipping resistor value together with internal current source gives skipping voltage level. decided skipping level maximum primary current. this case skipping voltage value skipping resistor then:
VCS-skip Iint 10-6
(eq.
This value bigger than maximum operating demagnetization time. means that current transformer enough freedom work properly even converter overloaded during start-up sequence when demagnetization time longer lower output voltage.
Feedback Loop
Demagnetization Detection
transformer demagnetization sensing based zero crossing detection auxiliary winding's voltage. this purpose zero crossing detector built-in NCP1207 connected Resistor limits current flowing through voltage clamps. Also this resistor together with capacitor delays zero voltage
feedback loop based secondary side ensure good output voltage regulation. control circuit based TL431 that internal reference voltage output voltage converter divided resistors R13. resistor divider output voltage compared with internal reference voltage TL431.
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AND8127/D
With regard TL431 input leakage current, resistor divider's current selected. resistor then calculated follows:
TL431 (eq. Idivider 10-6 Bill Materials
(eq. (eq. ISO1 (eq. Ceramic Ceramic Ceramic Ceramic Ceramic B250 1N4148 Time-lag NCP1207 TL431 PC817 2*10 Common Mode STP4NB80 BC238 BC308 IRF2807 Transformer, text Transformer, below
value upper resistor divider
VOUT VTL431 4700
17860
resistor ensures minimum current supply TL431 case converter operation near maximum output power when current flowing through diode within Optocoupler ISO1 close zero. threshold voltage being around value
VLED ITL431 10-3
resistor limits current flowing through case voltage across output terminal TL431 minimum, e.g. Considering nominal output voltage maximum current value
VLED VTL431 ILED- 10-3
Resistor together with capacitors C11.C12 creates "Pole-Zero" compensation circuit feedback loop. Their values result feedback loop response measurements adjustments board. Since NCP1207 allows direct Optocoupler connection, ISO1 connected without pull-up resistor Capacitor bypasses high frequency current pick-up.
Primary Switch Snubber Network
Since standard snubber will generate losses, different approach been used this design. cope with voltage spikes, primary switch been rated BVdss. snubber capacitor located secondary side. This capacitor functions. first purpose create together with secondary leakage inductance resonant tank. Similarly primary resonant circuit consists primary leakage inductance associated parasitic capacitances. resonant frequency secondary resonant circuit approximately times higher than resonant frequency primary resonant circuit. This frequency difference efficiently decreases voltage spike primary. second function protect secondary switch from voltage spikes.
Transformer Specifications
Ferrite Core Primary Winding Secondary Winding Epcos (Siemens) R10, Material turn (See Picture), Heat resisting plastic insulated wire, copper diameter. turns, enameled wire, copper diameter. winding beginnings application schematic.
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AND8127/D
Layout
Proper printed circuit board layout essential good operation whole converter. also influences signature both conducted radiated measurements. important ensure good grounding technique keep high frequency current loop high voltage areas
small possible avoid both magnetic electric radiation. example layout seen Figure component arrangement seen Figure board size 97.5
Figure Printed Circuit Board Layout Bottom Side
Figure Printed Circuit Board Layout Silkscreen Component Side
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AND8127/D
Practical Results
most important parameters considered during converter design overall power conversion efficiency. this reason synchronized output rectifier utilized. Table lists measured results converter working minimum specified input voltage VDC. corresponding graphical representation Table seen Figure Table lists similar results maximum specified input voltage VDC. Figure again helps results belonging Table no-load power consumption measured input voltage about about
Table Power Conversion Efficiency Input Voltage
POUT EFFICIENCY OUTPUT POWER Efficiency 91.68 91.69 91.63 91.49 91.33 90.83 90.08 89.16 87.87 85.59 81.85 77.31
Table Power Conversion Efficiency Input Voltage
POUT EFFICIENCY OUTPUT POWER Efficiency 90.70 90.56 90.42 90.28 89.76 88.97 87.85 86.39 84.75 82.16 78.20 73.62
Figure Power Conversion Efficiency Input Voltage
Figure Power Conversion Efficiency Input Voltage
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AND8127/D
following pictures basic voltage waveforms demonstrate operation converter specific conditions. Figure shows trace gate driver voltage bottom trace primary switch's drain voltage full load. Figure shows same measurement points Figure medium load condition when first valley drain voltage being skipped.
Figure Gate Driver Drain Voltage Full Load
Figure Gate Driver Drain Voltage Medium Load
Figure same previous measurements light load condition when valleys skipped.
cycle skipping operation when output load very light depicted Figure
Figure Gate Driver Drain Voltage Light Load
Figure Gate Driver Drain Voltage during Cycle Skipping Very Light Load
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AND8127/D
waveforms during overload condition depicted Figure Detailed view burst pulse during overload seen Figure This figure clearly demonstrates operation internal soft-start block.
Figure Gate Driver Drain Voltage during Over-Load
Figure Detailed View Burst Pulse
load regulation output voltage load step change from 100% vise versa seen Figure
Figure Load Regulation
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APPLICATION NOTE Over-Load Protection: continuously monitoring fe

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