Single-Output Off-Line DC-DC Converter ICs
Off-line dc-dc converters operate from the rectified ac powerline voltage, so they are optimized for a high voltage input. Isolated topologies must be used because these systems require galvanic isolation from the powerline. These self-contained converter ICs are usually oriented toward use as ac adapters in battery-based systems and in computer peripherals, such as printers and scanners. They’re usually rated at 100 W and below.
The VIPer17 off-line switch-mode power IC from STMicroelectronics can be configured as a flyback converter (Fig. 7). It is intended for off-line power supplies up to 6 W with a wide-range input voltage and up to 10 W with the European input voltage range. The IC includes both the power section and controller in one package.
The power section is an avalanche rugged MOSFET with 800-V breakdown, which ensures robustness and reliability for power supplies. The power section includes a SenseFET, temperature sensor, and high-voltage startup circuit.
The controller operates with current-mode control. It features circuits to minimize current consumption in the normal mode and reduces consumption even further in standby. Also, it can produce a power supply with a standby consumption under no-load conditions of less than 50 mW, which complies with energy-saving standards.
Furthermore, this IC offers functions that guarantee high performance with a lower component count: the soft startup function, simple feedback management, and frequency modulation jittering, which reduces EMI and helps meet standards regarding electromagnetic disturbance.
The VIPer17 has two possible fixed operating frequencies: 60 kHz (VIPer17L) or 115 kHz (VIPer17H). Both versions are available in DIP-7 (VIPer17LN and VIPer17HN) and SO-16N packages (VIPer17LD and VIPer17HD). Additionally, the IC offers:
• PWM operation with frequency jittering for low EMI
• Standby power less than 50 mW at 265 V ac
• Limiting current with adjustable set point
• Adjustable and accurate overvoltage protection
• On-board soft start
• Safe auto restart after a fault condition
• Overload protection (OLP)
• Brownout protection
• Hysteretic thermal shutdown protection
Finishing The Design
Capacitors employed with the controller are another design consideration that affects the switch-mode converter. The model for a capacitor is actually a resistor (ESR) in series with an ideal capacitor. ESR should be low to eliminate high-frequency switching noise from the output of the converter. Also, a low-ESR capacitor at the converter’s input prevents switching noise from being conducted through the input supply.
The designs of dc-dc controllers usually employ a tantalum or electrolytic capacitor in parallel with a low-ESR ceramic capacitor at the input and output. The tantalum capacitor filters lower-frequency noise, and the ceramic capacitor is better for filtering high-frequency switching noise. However, a very low-ESR capacitor may cause the converter to oscillate. Therefore, it is best to use the components suggested by the IC manufacturer.
Another output capacitor characteristic affects the controller’s output ripple—filtered output voltage variations. Typically, this ripple is a sawtooth at the switching frequency and may contain voltage spikes. Ripple can heat the output tantalum or electrolytic capacitor, which can shorten its life.
Board Layout
Component layout on a printed-circuit board (PCB) is an important system consideration. Use wide copper power traces and a ground plane. IC manufacturers usually supply a copper etch pattern on their datasheet. Large copper areas also provide good heat transfer to the ambient. Other thermal management factors include board size, shape, thickness, position, location, and board temperature.
Board layout and circuit components can also affect EMI conduction or radiation. Filter the converter input with low-ESR capacitors to prevent noise from entering the IC. Layout affects performance because fast-switching currents and associated wiring inductance can generate EMI. Also, keep the digital ground separate from the ground of the dc-dc converter. Connect the two grounds together at the output of the primary dc voltage source.
When designing high-frequency switch-mode converters:
• Avoid capacitive and inductive coupling of the switching waveform into high-impedance circuitry such as the error amplifier, oscillator, and current-sense amplifier.
• Keep printed-circuit traces associated with the converter IC as short as possible. Avoid long pc- board traces and component leads.
• External components should be as close to the converter IC. Also, locate oscillator and compensation circuitry near the IC.
• Use high-frequency decoupling capacitors on the reference voltage (VREF), and, if necessary, on VDD.
• Return high-di/dt currents directly to their source and use large-area ground planes.
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