Application Brief Michele Sclocchi Donald Ashley Highlights 1.25

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Application Brief Michele Sclocchi Donald Ashley Highlights 1.25

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High voltage, single chip DC/DC regulator optimized flyback, boost, forward power converter applications
Application Brief Michele Sclocchi Donald Ashley
Highlights 1.25 current-mode regulator integrated with power MOSFET Single chip family modules
Telecom industrial applications such local area networks (LAN), telephone interfaces (SLICs), ISDN, xDSL terminals often require low-power multioutput, non-isolated power supplies operating from non-regulated 12-36 bus. flyback converter often preferred solution because simplicity, size, cost, part count. National Semiconductor's newest high-voltage, single-chip, DC/DC regulator (LM5000) monolithic integrated circuit specially designed optimized flyback, boost, forward power converter applications. Having integrated complete current-mode controller with high voltage power switch highly efficient, low-cost regulator easily designed operate from unregulated (wide input range) voltage source.
During ON-time internal power switch, fixed voltage applied across primary winding power transformer current ramps linearly rate dI/dT (VIN)/Lp, where primary magnetizing inductance. Energy stored magnetic field core equal Ip2peak When power switch turns off, current primary inductance forces reversal polarities windings, energy from primary winding transferred secondary winding. peak secondary current equal Is(peak) Ip(peak) where turns ratio between primary secondary (Np/Ns). power delivered output (VIN* feedback loop maintains constant output voltage keeping product constant. Current mode control circuitry integrated LM5000 regulator offers users advantage being able tightly regulate output current voltage together. Pulse-width-modulation (PWM) duty cycle (ON/OFF-time) power transistor used correct input line voltage transients variations primary switch current. output will these transients because duty cycle modulation current-mode controller based error signal proportional output voltage regulation. This super-imposed across sawtooth waveform that representative output inductor current. power switch turned allowing current ramp through primary until sawtooth crosses error voltage, then shuts discharges stored energy into output. This current mode control scheme preferred over older voltage mode control scheme because provides superior bandwidth transient response well cycle cycle overload current limiting prevent thermal hazards. LM5000 operate four different selectable switching frequencies: kHz/700 (LM5000-3), kHz/1.25 (LM5000-6). high switching frequency options allow system "tuned" preferred operating frequency band. This reduces size output capacitance inductance primary secondary windings, similarly reduces
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Consider typical example output from unregulated bus, with input (see Figure
Figure LM5000 flyback converter
total unit volume transformer cost power supply. However, higher switching frequencies lead increasing transformer core losses total switching losses. Here, frequency often used keep core losses size flyback transformer minimum. Flyback converters operate discontinuous continuous modes. discontinuous mode, energy stored primary during ON-time delivered secondary load before next cycle. continuous mode operation, small amount energy remains secondary beginning next cycle. LM5000 typical application Figure continuous mode operation with maximum duty cycle used reduce output current, thereby reducing output voltage ripple. turns ratio between primary secondary windings maintains maximum stress voltage below breakdown voltage integrated FET: VSWoff (VINmax (VOUT VDIODE Npri)/Nsec reduce voltage spikes across caused transformer leakage inductance output rectifier recovery time, transient voltage suppressor (clamp) series with diode inserted across transformer primary. achieve maximum performance high switching frequency design, particular attention needs paid principal magnetic component, power transformer. Flyback transformers more like multiple inductors same core rather than typical transformers. Most power supply topologies avoid storing energy transformer. Ideally energy transferred instantaneously from primary secondary during ON-time power transistor. Uniquely, flyback transformers store energy during ON-time, deliver secondary during OFF-time. this way, currents never flow primary secondary winding same time.
Flyback transformers designed with minimal leakage inductance, minimal winding core losses with added avoid saturation. High frequency operation causes electrons flow only outermost diameter windings (known "skin effect"), increasing resistance current flow, power dissipation, heating transformer. High-frequency transformers designed using multiple strands windings Litz wire) gain cross sectional area current flow while reducing power losses. window shape transformer core used winding area should wide possible minimize number layers, winding losses, leakage inductance. E-type cores with internal air-gap best choice cost lower leakage inductance. Core losses depend core materials selected amount flux swing allowed without saturation switching frequency. reduce core losses, necessary adjust flux density limit increases core temperature rise. Limiting core loss density 100-300 mW/cm keeps temperature rise approximately 40°C. Generally, Ferrite-P material preferred materials flyback transformers. assist design flyback power supply, Mathcad file containing equations needed calculate external components found National Semiconductor site: http://power.national.com
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Application Brief Michele Sclocchi Donald Ashley Highlights 1.25

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