Micro Linear announced a patented chip for the design of green switching power supplies in systems such as PCs, workstations and servers.

The ML4802 is a low-power BiCMOS device that contains an intelligent "Green Mode" circuit that automatically increases power supply efficiency at light loads. Prior to this chip, power supply efficiency could fall by up to 50 percent when Green systems reduced power loads by turning off unused circuits.

The chip works with a wide range of power line input voltages (85 to 265V) which makes it possible to design universal power supplies that automatically adapt to power outlets in any country. This eliminates the need for power line adapters, and costly and confusing switches, which if set improperly, can destroy a system.

The ML4802, pin compatible with the industry-standard ML4841, also includes a sophisticated power factor correction circuit that reduces supply harmonic radiation, protects systems from line noise and brownouts, with excellent response to load transients. This circuit uses a patented LETE (Leading Edge/Trailing Edge) modulation method that can cut the cost of the PFC high-voltage capacitor in half. The power factor circuit meets the European IEC1000-3-2 regulations for equipment sold on that continent.

According to Thomas Goddard, Product Marketing Manager, "Our device comes at the perfect time because of consumer demand for Green systems, growing interest in Energy Star programs, and the approaching deadline requiring that electronic systems sold in Europe meet IEC1000-3-2 regulations. Our integrated chip makes life easier for power supply engineers by offering an easy path to meeting the conflicting demands of low power and high efficiency."

Green Mode
When the ML4802 output current sense circuit detects that the power demand has dropped, it automatically activates the green mode of operation. In this mode, the PFC goes into a "Charge and Coast" mode saving power during coasting periods, while the PWM generates fewer, but widely spaced longer pulses, reducing total number of power pulses and the amount of energy that is wasted turning the FET on and off. Although this wasted "on and off" current is not significant when the power supply current is high, it is a significant proportion of total supply energy use when the power supply current is low, causing efficiency to drop over 50 percent in conventional power supplies.

Unless you already know the ML4841, I don't believe that you will understand what this product is, or does, just by reading the release. It is actually a PWM controller for power factor-corrected switched-mode power supplies in designs for line input voltages where there is generally no input transformer. The ability to change modulation modes -- in some manner -- at lower loads is an increasingly popular technique in lower voltage controllers so the idea here is not new, but this is the first implementation I have seen in higher-voltage controllers. I personally don't think that the "Green" (why capitalized, like the political party?) mode is that important in this product. Efficiency saving techniques are obviously critical in battery-powered devices but here the power savings are trivial from a line supply; every little bit helps, of course, in meeting IEC-1000 standards, but I think this product will sell to the power supply design houses because of the savings in the power factor correction capacitor -- only a small savings in price in some cases but a fairly large savings in board area, probably the most critical specification of a power supply after cost.

The correction circuit's ability to switch the two parts of the circuit -- the PFC and the PWM -- synchronously is a beautifully simple and elegant technique. The heart of the circuit is a gain modulator which controls the response of the current loop with three different senses: a current representing the instantaneous input voltage (amplitude and form factor), a voltage proportional to the long-term line voltage rms amplitude, and the output of the voltage error amplifier. Even if you are not going to use this part the data sheet is a "must-read" on how to design this sort of solution.

The other things I like about this part, in the group of ML4800-05 (with different modulations, with or without PFC, with or without "green"), is the extremely low start-up current and the fact that the gain modulator and LETE switching produces a much lower ripple current in the capacitor between sections and low noise in the feedback loop because of the current drive.