Energy Conservation

One obvious way to reduce standby power is to minimize the power required by the system during standby. Unfortunately, this is typically out of the hands of the power-supply designer, and the power-supply designer is saddled with having to deliver around 300 mW from a limited input power budget. While this may seem easy enough to accomplish, the PFC and 250-W main power supply typically draw more than enough power at no-load operation to push the losses well above the acceptable limits. As a result, it is usually necessary to disable all unused supplies, including the PFC, during standby. Typically, this is accomplished by gating the bias power to the supply controllers.

Fortunately, IC manufacturers have taken notice of the need for efficient light-load controllers and now offer controllers targeted for these applications. An example of a PFC and green-mode flyback converter standby supply is shown in Fig. 6. This circuit uses an energy-efficient UCC28600 to minimize power loss while in standby mode. The UCC28600 enters a burst-mode operation at light loads and provides a signal to disable the bias power to the PFC controller.

The circuit shown in Fig. 6 is adequate for reducing the standby power consumption to below 3 W, but may not be enough to achieve less than 1 W of input power. PFC controllers require resistor dividers to sense the ac line voltage and the PFC output voltage. These resistors can easily dissipate more than 200 mW. In addition, the leakage current of the PFC output capacitor can lead to another 200 mW of unwanted loss. Combined, these losses can push the standby losses above the acceptable limit. In these situations, it may be necessary to use a relay to disconnect the ac power to the PFC and all downstream converters. This relay may be used in conjunction with a dedicated standby supply. In addition, this relay may be a solid-state type as long as it does not require significant bias power while the system is in standby mode.

Lower Power with LEDs

An example of an LED driver circuit for a portable DLP projector is shown in Fig. 7. In this circuit, a TPS40071 controller is used to control a synchronous buck power stage that is operated as a current source while the LED is on, and as a voltage source while the LED is off. The LED on/off signal, sent from the microprocessor, turns the LED on by turning on FET Q1 and moving switch S1 to the down position, which provides the current feedback signal. When the LED is turned off, S1 is returned to the up position, which allows the TPS40071 to regulate the driver output voltage. The LED current is controlled by varying the pulse width of a 10-kHz digital pulse train sent from the microprocessor. This PWM signal is filtered and summed into the feedback pin of the TPS40071.

In Fig. 7, the resistor divider of R1 and R2 is designed so that the regulated voltage in the LED off state closely matches the forward drop of the LED during the on state. This keeps the output of the TPS40071 internal error amplifier at nearly the same level during the two states and minimizes the rise time of the LED current when it is switched on. This is significant because a fast current rise time provides more flexibility in the digital control of the projected light.

The waveforms of Fig. 8 show the output voltage and LED current during this transition. For these waveforms, the LED driver is powering two 1-A green LEDs connected in series. The wide bandwidth of the synchronous buck-current source, about 100 kHz, helps to minimize the current rise time.

Benefits Beyond DLP

Controllers like the ones described previously have been developed to power HID lamps. Because the controller required green-mode operation, it was necessary to develop variable frequency control that shuts portions of the power system off.

Meanwhile, LEDs have brought improved reliability to DLP projectors, while advancing the development of rapid slew rate power supplies. Finally, the need for transition-mode control in the PFC stage has led to cost and size reductions in this circuitry.

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© 2007 Penton Media, Inc.