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Power tip: Boost cell phone charger design

Posted: 22 Oct 2012     Print Version  Bookmark and Share

Keywords:low-power chargers  FET  valley switching 

With worldwide sales of cell phones reaching almost two billion per year, the cell phone charger�s size, cost and efficiency are under strict examination. For example, Amazon and Apple have established new benchmarks of small size and aesthetics, while reducing circuit losses and cutting the cost of low-power chargers. This has been accomplished with the use of an advanced topology and clever control methods.

The most popular topology for a charger at the 5-10 watt power level is the discontinuous flyback. However, it has evolved into the quasi-resonant flyback, which reduces some switching losses. In the traditional discontinuous flyback, the switching frequency is fixed and the control IC simply sets the peak current in the power transformer. This represents a fixed amount of energy that is delivered to the load during a switching cycle. The power switch's drain waveform is shown in figure 1. During the charge interval (portion of switching cycle when waveform is zero volts), energy is stored in the primary inductance.

When the power switch is turned off, energy flows into the secondary where it is stored in the output capacitor and delivered to the load. Once the power transformer is demagnetized, the FET drain voltage collapses and rings around the input voltage. In the traditional approach, the FET is turned on at the next switching interval, regardless of the FET drain voltage. It can be at a minimum, maximum or somewhere in between. The losses associated with switching this voltage can be appreciable, often resulting in a two to three per cent loss of efficiency. Quasi-resonant flybacks minimise the switching loss by only turning the FET on when the drain voltage is at a valley.

Figure 1: The quasi-resonant flyback turns the FET on at its minimum drain voltage to reduce CV2 losses.

Recent control methods do more than just valley switching. Figure 2 shows how two parameters (switching frequency and peak primary current) are varied to control the output voltage. At full load, the power supply is operated at maximum peak current and maximum frequency. As the load is reduced, the switching frequency is reduced. Since both output power and switching losses are directly related to the power supply's frequency, this results in an almost flat efficiency in this mode of operation. Note that valley switching is still occurring, so the power supply switching frequency is not fixed.

The turn on of the power FET hops from one valley to another, with an average switching frequency as shown in figure 1. Audible noise considerations limit how low the switching frequency can be as switching the power supply may induce audible noise in magnetics and ceramic capacitors. Many times, you may not want to allow the power supply switching frequency to drop below 10-20kHz and an alternate control scheme is employed. In this case, once the minimum allowable operating frequency is reached, the peak current in the primary FET is controlled to regulate the output voltage at low-power levels.

Figure 2: Primary peak current control and frequency modulation enhance efficiency over load.

Figure 3 presents a typical power supply schematic of a universal input, 5 watt output charger. The schematic is very simple; it does not require a reference or optocoupler to regulate the output voltage. It uses the reflected output voltage on the primary bias winding for feedback. Referring to figure 1, which shows the FET drain voltage, this waveform is an analogue of both the bias and output voltage. When the drain voltage flies up, the drain voltage is related to the output voltage plus diode and resistive drops in the secondary. The drain voltage decreases linearly as the reflected inductance is discharged through the output diode. At the end of the diode conduction, this voltage and the voltage on the bias winding is a reflection of the output voltage plus a diode junction voltage. A feedback loop is closed around this reflected voltage and gives a reasonable regulation tolerance (three to five per cent).

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