Achieve higher efficiency and reliability in your power supply
03 February 2016
The LinkSwitch family of ICs is a range of offline switchers designed for applications where output power is typically below 15W.
To address the ongoing requirement for improvements in efficiency and reliability and reduction in manufacturing cost for external power supplies and battery chargers used in mobile, consumer and medical applications, Power Integrations has developed the LinkSwitch-4 IC family that integrates a number of new features to boost switching performance and increase efficiency.
The advanced switching techniques are especially beneficial in high-voltage applications where they reduce the slow turn-off switching losses seen in conventional Bipolar Junction Transistor (BJT) base-only switched designs and eliminate the Bipolar Junction Transistor failures due to secondary breakdown. With its highly integrated design and ability to use a low cost external Bipolar Junction Transistor switching element, the LinkSwitch-4 IC simplifies the design and cost of Constant Voltage (CV) and Constant Current (CC) based power supplies.
LinkSwitch-4 is the latest generation of LinkSwitch products from Power Integrations and is based around a dual mode quasi-resonant PWM/PFM switching controller driving an external low cost Bipolar Junction Transistor. Targeted at applications required to meet the latest exacting efficiency standards from the USA Department of Energy (DoE) and European Code of Conduct (CoC), the device is aimed at improving efficiency whilst driving down manufacturing costs. Using adaptive base and emitter switching technology, LinkSwitch-4 ICs reduce AC switching losses improving efficiency and extend the Reverse Bias Safe Operating Area (RBSOA) of the bipolar switching element, increasing overall reliability. Dynamic base current drive also reduces the sensitivity to the external BJT characteristics allowing the use of low cost, high voltage BJTs in the design. In addition LinkSwitch-4 ICs eliminate the requirement for an optocoupler and additional secondary CV/CC control circuitry whilst retaining tight output regulation (3% CV and 2% CC) and fast transient response. With typical efficiency around 80% and no load power dissipation less than 30mW with a 230 VAC input, the LinkSwitch-4 family is suitable for standby power supplies in many applications.
The input voltage is rectified via the full bridge rectifier Dbridge and filtered via Lfilt, Cin1 and Cin2. The resistor RHT biases the base of Q1 which turns on and starts supplying current to the LinkSwitch and charging Cvcc. Once Cvcc reaches approximately 13V the LinkSwitch IC enters its initialisation mode and issues drive pulses to the base of Q1. It then internally checks if the rectified mains voltage is within tolerance to enable the next stage of the start-up routine. If it is too low, the part will not issue any more drive pulses, Cvcc will be discharged and the part will enter a sleep mode until Cvcc charges up again and the initialisation routine can begin again. If the supply voltage is sufficiently high, drive pulses will be output on the Base Drive (BD) pin and the part will enter run mode and power itself via the transformer voltage supply winding. If the input voltage falls below Vmainslow for any reason the LinkSwitch IC will go back into sleep mode and re-initialise when the input voltage is restored.
In Constant Voltage mode, regulation is achieved by sensing the voltage at the feedback (FB) input which is connected to the voltage supply (Vcc) winding, or a dedicated feedback winding, via a potential divider network, Rfb1 and Rfb2, which is used to set the output voltage. Figure 2 shows the typical waveforms for the voltage Feed Back (FB), Current Sense (CS) pin, Base Drive (BD) and Emitter Drive (ED) pins. The feedback waveform is continuously sampled and analysed at time Tsamp to measure the reflected voltage and provide input to the under voltage detection circuit and CV control loop. Tsamp is identified by the slope of the feedback voltage and is coincident with zero flux in the transformer.
In Constant Current operation, the output current is regulated by applying a voltage to the CS pin which is an estimated version of the regulated output current scaled by the value of Rcs. This voltage is negative and when it exceeds the internal threshold set by the CC control loop the Base Drive pin is driven low, turning Q1 off. The switching frequency varies from Fmin at No Load to Fmax at peak current.
The LinkSwitch-4 device can also compensate for voltage drop in the output cable. The compensation value Gcab is given by the following formula where Rcab is the output cable resistance.
The LinkSwitch-4 IC determines both the primary switch peak current and switching frequency to control the output power, ensuring discontinuous operation at all times. Analysing the switching waveforms in Figure 2 shows that during the ON time the emitter is switched to GND via the ED pin and the base current IBD is modulated to achieve fast turn on time, low on voltage drop and fast turn off time. At the start of the base drive (BD) pulse a fixed peak current value is applied ton to provide fast turn on. Its amplitude and duration are then modulated to provide low Von whilst allowing the transistor to de-saturate towards the end of its ON period so it can turn off quickly reducing switching losses. Quasi-Resonant switching is used, in which Q1 is switched when the voltage across it rings down to a minimum value to further reduce losses and help control EMI emissions. Emitter switching also ensures fast turn off of the switching element and results in less time spent in the turn off zone, reducing power dissipation and improving the Reverse Bias Safe Operating Area (RBSOA). Duty cycle is a function of Primary to Secondary turns ration on the transformer and is usually chosen as 50% at the minimum value of the rectified mains voltage.
When the external switching element Q1 turns off, the current drops to zero and the voltage across it rises to the input supply voltage. Power dissipated is a product of the voltage across the device and the current flowing through it. By using a combination of Base and Emitter switching the LinkSwitch-4 controller can extend the RBSOA of the BJT switching element to be close to that of a high voltage MOSFET (see Figure 4a and 4b). This increases the reliability of the design and also permits the use of a low cost high voltage BJT as the switching element.
Power Integrations has published a Reference Design for a CV/CC 5V, 2A, USB charger. It describes how to design a low cost, Constant Voltage, Constant Current flyback switcher and includes the schematic, bill of materials, two layer PCB plots, transformer calculations and construction information, performance and EMI measurements.
Additional components have been added to improve input EMI performance (R14 and L2). An RCD clamp (R3, R12, D1 and C3) limits voltage spikes on the primary side of the transformer. A snubber circuit (R10 and C6) across the output rectifier diode reduces noise, ringing and radiated EMI on the Vout line.
The performance of this design has been rigorously tested to ensure operation under the full load across the full input voltage range to ensure efficiency remains above 80% as illustrated in Figure 6. This wide input operating voltage range is ideal for power supplies which have to operate in any region of the world. Measurements of the main component temperatures were also taken to understand the temperature rise on critical active components. With 230 VAC input, 2A load and 25.7°C ambient temperature the component temperatures were as follows:
LinkSwitch-4: 70.8°C;
BJT: 74.8°C;
Input bridge rectifier: 44.6°C.
All these major components are well within their rated operating temperature range and an appropriately-designed two layer 1oz copper PCB would be sufficient to dissipate the heat.
The ripple increases as the output current increases but remains well within the 2% range even at full power.
The plot indicates the conductive EMI performance for a typical floating output 2A resistive load. Full plots for conductive EMI and radiated EMI for different applications can be found in the reference design published on Power Integrations website.
In addition to the performance plots, Power Integrations has a full range of application notes, tools to assist in the design process and component selection and a range of power supply design examples which can be used as the basis for a custom design.
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