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Posted on 24 March 2019

PSU ICs use Innovative Technology to Reduce 25W Charger Cost and BOM Count

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Using new Fluxlink safety-isolated communication technology, InnoSwitch ICs combine primary- and secondary-switcher circuitry to reduce component count, eliminate slow and unreliable optocouplers, outperform primary-side controllers and slash manufacturing costs of power supply designs.

By Mike Matthews, VP Product Development, Power Integrations Inc

Manufacturers of smart mobile devices, set-top boxes, networking equipment and computer peripherals are constantly challenged to develop low-cost, efficient adapters and chargers that meet increasingly demanding energy consumption regulations. Flyback designs using primary-side regulation (PSR) techniques are often used because of their simplicity and cost. However, more accurate control is delivered by secondary-side regulation (SSR); such designs are also less sensitive to production tolerances in the transformer and other external components.

The major issue with all PSR solutions is that it is only possible to see what is happening on the output after switching the primary-side transistor: effectively, every time the transistor switches you get a glimpse of the power supply output load conditions. However, high energy efficiency requirements demand that the switching frequency is reduced at light loads, therefore these ‘glimpses’ become less frequent, compromising the ability of the power supply to respond to fast transient loads. The system is always playing catch up, inevitably leading to system compromises.

A further disadvantage with PSR controllers is that they infer what is happening on the power supply output from waveforms on the primary bias winding of the transformer – rather than directly measuring output voltage and current. This means that transformer manufacturing tolerances, along with primary clamp circuit design, become factors that must be ‘allowed for’ during development and mass production.

Transformers are infamous for manufacturing variances, complicating the management of high-volume production with PSR solutions, ultimately impacting cost effectiveness if yields suffer.

Let’s now look at secondary-side regulation, which requires an isolated feedback mechanism - commonly an optocoupler - which increases circuit complexity and cost. Reliability can also be compromised if low-cost optocouplers are used as these devices suffer from aging, temperature drift and varying current gain. Another approach is to use capacitive coupling techniques. Capacitors themselves are inexpensive, but they are also difficult to integrate. High-voltage capacitive coupling on a single die is expensive to build in, especially when it is necessary to meet the typical 6kV high-potential isolation required during testing for AC/DC power supplies. But perhaps the most serious challenge that is raised by the use of capacitive feedback is coping with system ESD pulses. In many modern consumer electronic specifications, such pulses can exceed +/-15kV and are applied directly at the output of the power supply, giving rise to capacitive currents through the isolation barrier that can damage control circuitry. Also, common-mode effects due to voltage fluctuations can cause problems that require extra circuitry – and associated design and BOM costs. The third approach used to implement SSR of a power supply is to use a pulse transformer. Magnetic coupling is extensively used in high-end communications products, but has – until now – been prohibitively expensive for low-cost charger/adapters.

Leading power IC company Power Integrations took a long, hard look at the problem and with its new InnoSwitch family of highly integrated switcher ICs has come up with a digital magnetic communications function – termed FluxLink – within the IC package at virtually no extra cost. Effectively, a magnetic coupling between the primary and secondary side is created without the need for high permeability magnetic cores, using only the standard bill of materials for the manufacture of the IC package (figure 1).

Magnetic coupling between the primary and secondary side is created without the need for high permeability magnetic cores

Full internal galvanic isolation – exceeding that used in most optocouplers – is achieved, meeting UL, TÜV and all other global safety standards, while external pin-to-pin creepage of over 9.5mm is achieved with a custom surface-mount package, designed for this application. Furthermore, by occupying the space on the PCB normally reserved for the primary to secondary isolation region, the InnoSwitch IC essentially takes no PCB area. In fact, the package and pin-out are designed so that the most convenient location in most layouts is directly underneath the power transformer, making compact layout very simple for space saving and PCB cost reduction. The design allows for simple resistor divider direct sensing of the power supply output voltage while the power supply output current measurement is fully integrated inside the package, eliminating external current sense circuitry altogether.

Secondary sensing brings several other benefits. As well as eliminating the often unreliable optocoupler, it enables a simple transformer to be specified since the circuit will not be sensitive to the bias winding location or transformer inductance tolerances. Switching frequency jitter effectively spreads the EMI spectrum, enabling designs using only standard magnetic-wire primary and Triple Insulated Wire (TIW) secondary windings without the need for copper shields. But perhaps the most significant benefit of InnoSwitch ICs is the provision of simple and rugged synchronous rectification – resulting in high efficiency – without the usually expected cost penalty. Synchronous rectification (SR) improves efficiency by replacing lossy diodes with power MOSFETs on the output of the power supply. The voltage drop of a standard diode is typically between 0.7V and 1.7V, but even high-efficiency Schottky diodes will typically exhibit a voltage drop of 0.4 – 0.5V, which in a 5V system, such as a USB charger, represents a 10% loss in the output stage.

By contrast, MOSFETs can be specified with an on-resistance as low as 10 mOhm. Therefore in a typical charger design, the voltage drop might be 50mV, representing a loss of only 1% – ten times less than with Schottkys. The latest SR power MOSFETs are even 20 – 40% cheaper than Schottkys, so SR seems to be the obvious approach for flyback topology power supplies. However, anyone who has designed a flyback with SR will be aware that timing is key. Simultaneous conduction of primary transistor and SR FET creates an effective short-circuit condition across the primary transformer winding which usually leads to primary transistor damage. On the other hand, a delay in turning on the SR FET once the primary transistor has turned off compromises efficiency. In traditional SR solutions, the need for a separate secondary-side controller to drive the SR FET also adds cost and complexity to the circuit, which is why SR has sometimes had the reputation of being an expensive luxury.

This is all set to change. With InnoSwitch ICs, the FluxLink element introduces precise cycle-by-cycle, digital communication controlling both the primary transistor and secondary SR FET switch timing. For the first time, users therefore have a truly fool-proof SR solution where the complete operation is integrated in a single IC rather than having to wrestle with the independent operation of separate primary and secondary controllers normally required in SR solutions with optocoupled SSR or PSR power supplies. In addition, the instantaneous communication afforded by FluxLink technology allows the secondary controller to determine the optimum turn-on and turn-off times of the SR FET across the entire load range, whether the power supply is operating in discontinuous mode, continuous mode, and even under fault conditions. This optimized SR function allows Innoswitch ICs to easily comply with even the most stringent future efficiency standards such as the California Energy Commission, European Union Code of Conduct Tier 2 and DoE6.

A further benefit of the instantaneous FluxLink communication is extremely fast-transient response.

Switching frequency jitter effectively spreads the EMI spectrum

As can be seen in figure 2, if an event happens on the output, the primary side will receive a signal to turn on within a single switching cycle period (<10μsecs),virtually eliminating output voltage undershoot, even for 0 - 100% load transients. This allows output capacitor values to be reduced compared to PSR solutions where the slow response to transients typically requires large capacitors to meet the transient energy requirements.

A typical 2.5A, 5V mobile device charger can be achieved using just 30 components

InnoSwitch power-supply ICs include the high-voltage power MOSFET, primary- and secondary-side controllers, FluxLink feedback link and an integrated synchronous rectifier (SR) controller within a single, safety-rated, 16-pin eSOP surface-mount package. Devices feature highly accurate CV and CC control (+/-3% and +/-5% respectively) and low ripple. Operating efficiency is typically better than 84% in a 5V output 10 watt power supply at full load (as high as 88% in higher output voltage designs) – even higher under medium-load conditions – and no-load consumption is below 10 mW. InnoSwitch ICs start up using bias current drawn from a high-voltage current source connected to the Drain pin, eliminating the need for external start-up components; an external bias winding reduces no-load and increases system efficiency during normal operation. The ICs also include comprehensive system-level safety features such as output over-voltage protection, overload power limiting, hysteretic thermal protection and frequency jitter to reduce EMI.

A typical 2.5A, 5V mobile device charger can be achieved using just 30 components, roughly 33% fewer than equivalent performance solutions. And as smart mobile devices become larger, they will require higher currents for fast charging. Where previously the idea of 5V/4A chargers would have raised eyebrows, now such devices are starting to appear. InnoSwitch ICs facilitate highly efficient, cost-effective charger designs up to 25W and are designed to be compatible with emerging rapid charge technologies, easily justifying the claim to be the most effective and efficient means of implementing flyback power supply designs.

 

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