[Technology Report]
New Techniques Enhance Efficiency Across All Loads
Governments around the world are moving from voluntary to mandatory power-supply efficiency efforts. Challenges may lie ahead, but designers now have methods to cope with them.
Under normal load conditions, when the control current exceeds a predetermined value, the duty cycle is controlled, providing an approximate constant-voltage characteristic. For light-load/ no-load conditions, when the duty cycle drops below 3%, the switching frequency is reduced to cut energy consumption.
With that CV/CC output characteristic, the flyback transformer is always in discontinuous current mode (DCM). DCM is another way of saying that current, and thus the magnetic field in the coil, may reach or cross zero. With DCM, all of the energy is delivered to the load during the MOSFET off time for each switching cycle. (In continuous-current mode, current and the magnetic field never reach zero.)
Over the years since it introduced the LNK501, Power Integrations has made the improvement of power efficiency across all ranges of power output and load conditions into a crusade. Last year, the company upgraded its higher-power (48 W sans heatsink, 150 W with heatsink) TOPSwitch ac-dc line with the HX series. The TOPSwitch HX chips are monolithic ICs that integrate a 700-V power MOSFET with a controller and supervisory functions.
No-load power consumption for HX parts is less than 200 mW. At higher loads, high efficiency is achieved via a multimode control scheme. At high loads, the chips use a fixed-frequency pulsewidth- modulation (PWM) control technique. As load decreases, the controller transitions to a variable-frequency mode and then to a lower fixed-frequency PWM mode that avoids audible frequencies. At very low loads, the controller transitions into a cycleskipping mode that delivers maximum power to the output for 1-W input and consumes very little power in standby.
TEXAS INSTRUMENTS Texas Instruments’ portfolio of switch-mode regulators is too broad to cover completely. Some of TI’s buck converters, such as the TPS51117, approach low-load efficiency by varying switching frequency. “If auto-skip mode is selected, the TPS51117 automatically reduces the switching frequency during a light load condition to maintain high efficiency,” says the TPS51117’s data sheet.
“As the output current decreases from a heavy load condition, the inductor current is also reduced and eventually comes to the point that its valley touches zero current, which is the boundary between continuous conduction and discontinuous conduction modes. The rectifying MOSFET is turned off when this zero inductor current is detected,” it continues.
“Since the output voltage is still higher than the reference at this moment, both high-side and low-side MOSFETs are turned off and wait for the next cycle. As the load current decreases further, the converter runs in discontinuous conduction mode, taking longer time to discharge the output capacitor below the reference voltage. Note the ON time is kept the same as during the heavy load condition,” the datasheet says.
“In reverse, when the output current increases from a light load to a heavy load, the switching frequency increases to the preset value as the inductor current reaches... continuous conduction,” it concludes.
Another chip, TI’s eight-pin UCC28600 quasi-resonant, flyback green-mode controller, incorporates three control modes: quasi-resonant/DCM mode, frequency-foldback mode, and “green mode.” Quasi-resonant (QR) and DCM operation occur for high loads, since the rising edge of the gate drive to the switching MOSFET always occurs at the valley of the resonant ring after demagnetization (resonant valley switching).
Resonant valley switching is also imposed at maximum switching frequency. In frequency-foldback mode, the voltage-controlled oscillator is restricted to 40 to 130 kHz. At the lightest loads, green mode maintains the oscillator at 40 kHz, and switching enters a hysteretic “burst” or “hiccup” state.
NATIONAL SEMICONDUCTOR National Semicondcutor’s LM26480 embodies its own forms of synchronous switching frequency control in a multi-output controller. It contains two high-current, step-down dc-dc converters and a pair of linear regulators. The chips are for portable systems running off lithium-ion batteries.
One of the buck regulators can provide any voltage between 0.8 and 2.0 V at up to 1.5 A. The other supports 1.0- to 3.3-V output levels, also up to 1.5 A. The low-dropout regulators (LDOs) can be set to output between 1.0 and 3.5 V, with ±3% accuracy at up to 300 mA with 25-mV (typical) dropout.
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