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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING New LinkSwich chips for building AC/DC converters. Reference data
Encyclopedia of radio electronics and electrical engineering / Application of microcircuits LinkSwitch is the name of a new series of microcircuits from Power Integration for building low power AC/DC network flyback converters. These converters are used in low-power isolated mains power supplies, wired and cordless phones, CD players, home appliances as a standby power source, chargers, and anywhere a low power source is required - from 2 to 5 watts. The series consists of LNK500 and LNK5O1 chips. In fact, this is the same crystal, the difference is only in the percentage of output voltage spread. At LNK500, when switched on without feedback, the output voltage spread is greater, respectively, and the price is lower. For LNK5O1, the opposite is true. The microcircuits include (Fig. 1): a high-voltage power switch - a POWER MOSFET transistor, a current limiting comparator, a thermal protection unit, a soft start circuit, an error amplifier, a generator, a PWM comparator. Microcircuits are manufactured in packages of the DIP-8B type (option P, Fig. 2) and SMD-8B (option G), which do not have one output.
The microcircuits include (Fig. 1): a high-voltage power switch - a POWER MOSFET transistor, a current limiting comparator, a thermal protection unit, a soft start circuit, an error amplifier, a generator, a PWM comparator. Microcircuits are manufactured in packages of the DIP-8B type (option P, Fig. 2) and SMD-8B (option G), which do not have one output.
Converters based on these microcircuits are quite compact, since they use a small number of components. Moreover, the converter board turns out to be much smaller in size and weight than a transformer of the corresponding power at 50 Hz. On-chip nodes reduce the number of add-on components, simplifying installation and increasing system reliability. The operating frequency of the microcircuit is 42 kHz. At this frequency, the filtering of the output voltages of the converter is simplified. Both microcircuits are used in converters not only for a fixed input voltage, but also for an extended range (85 ... 265 V). As a rule, in cheap equipment that does not require high output voltage stability, an open-loop switching circuit is used (Fig. 3). In this case, the output voltage instability increases up to ±10% for LNK501 and up to ±20% for LNK500. For devices with high requirements for the stability of the supply voltage, a feedback switching circuit is used (Fig. 4).
Chips comply with EcoSmart, Energy Star, Blue Angel and EU guidelines. In the absence of load and voltage in the 265 V network, they consume less than 300 mW, and microcircuits do not need an external current sensor to control the current. Pin assignment: D (drain) - connected to the drain of a powerful MOSFET, it supplies power to the entire control circuit. The pin is connected to the internal current limiting circuit. C (control) - input of the error amplifier, current feedback circuit (duty cycle adjustment) and control of the current limiting circuit. The built-in parallel regulator is connected to an internal current source in the normal state. The input is also used to connect a smoothing capacitor and a compensation/auto-restart capacitor. S (source) - is the output of a powerful key for connecting the load, the output of the primary winding control circuit. Description of the operation of a typical AC / DC converter circuit Power on During the process of applying voltage, the capacitor C3 (Fig. 3, 4), connected between the pins C and S of the microcircuit, is charged by a through current from the input D through an internal current source. When the voltage at pin C reaches 5,6 V with respect to pin S, the current stops, the internal control circuit is activated, and the MOSFET begins to switch the primary winding. At this point, the charge on the capacitor C3 is used to power the control circuits of the microcircuit. Maintenance of the set current The shape of the output voltage follows the slope of the voltage curve applied to the primary winding of the transformer. Current IС (Fig. 5) at terminal C increases. When the value of IС equals IDCT, the internal circuit limits the rise of IС upon reaching threshold ILIM. Internal layout provides V-shape IС to maintain normal power during power outages.
Maintaining a given voltage When the current IС exceeds the value IDCS (Fig. 5), the duty cycle of the pulses decreases. Since the value of IС depends on the supply voltage, the duty cycle is limited depending on the peak current set by the internal control circuit of the key (hence the name LinkSwitch). Depending on the position of the operating point in the graphs of Fig. 5, the microcircuit operates either in the mode of maintaining voltage or current. At a minimum input voltage (in the case of using a microcircuit in a power supply with a universal input), this transition occurs at approximately 30% duty cycle. When the duty cycle is less than 4%, the switching frequency is reduced to reduce power consumption. The value of the resistor R1 (Fig. 3) is therefore chosen so as to ensure the equality of the currents IC и IDCTwhen VOUT takes the desired values at a minimum input voltage. Auto restart mode If any deviations occur in operation, for example, during a short circuit or load break, the current stops at pin C of the microcircuit. Capacitor C3 is discharged to 4,7 V. This activates the auto-restart circuit, which turns off the MOSFET and puts the control circuit in low power mode. In the auto-restart mode, the microcircuit starts up periodically, but goes into normal mode only after the fault has been eliminated. The output voltage regulation is affected by the voltage across the capacitor C4, which in turn depends on the EMF of the self-induction of the primary winding of the transformer. Resistor R3 and capacitor C4 form a filter on which the error voltage is generated. On fig. 4 shows a typical circuit for switching on microcircuits with a feedback optocoupler. For primary purposes, elements R4, C5 and an optocoupler transistor DA2 are added. The optocoupler LED is included in the secondary circuit along with the elements R5, R6, VD7. Resistor R6 sets the operating current VD7. Resistor R5 limits the through current through the optocoupler LED and VD7. As soon as the voltage on the secondary winding of the transformer T1 exceeds the opening threshold of the LED and the zener diode, the phototransistor opens and shunts the resistor R4, increasing the voltage across the capacitor C4. A change in the voltage on this capacitor causes a decrease in the duty cycle of the pulses supplied to the powerful key, and as a result, a decrease in the voltage on the side of the secondary winding. The output characteristic of microcircuits is shown in fig. 6.
Publication: cxem.net
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