Powering the DLP System

Fig. 2 presents a block diagram of a typical DLP HDTV power system. Total power supplied is on the order of 200 W. Because these products are supplied to the European market, a PFC circuit is often provided to meet their harmonic requirements. The PFC circuit provides a regulated 400 V, which feeds the lamp power supply and low-voltage logic and analog circuits. In addition, there is a standby power supply that powers a small sustaining load during the off condition. Typically, this standby supply will need to be energy efficient, or green. To be Energy Star compliant, this supply must consume less than 0.5 W of input power at no load.

Using LEDs as a light source is another trend that directly impacts the power-supply design of DLP products. In addition to eliminating the need for a ballast, LEDs also provide much longer lamp life, are more light efficient and eliminate the color filter. LEDs offer a whole new range of possibilities for generating superb images. Color segments are no longer tied to the color filter design and speed of rotation, allowing more mixing options, faster on/off switching speeds and intensity control through current-level management. The smaller sizes of LED light engines offer a distinct advantage in portable products.

Fig. 3 shows a block diagram of a power supply for a LED projector. It is very similar to a DLP LED HDTV in that it provides a standby supply, a PFC circuit, a main power supply and a supply for the LEDs. In this block diagram, the LEDs are driven from one of the main power supply outputs. Alternate configurations supply the LED drivers from the 400-V output of the PFC. Although these supplies seem very simple from the block diagrams, each has its own design challenges.

Transition or Continuous-Conduction Mode?

The differences between CCM and transition-mode control are shown in Fig. 4. A PFC that employs CCM uses a fixed-frequency PWM to regulate the average current in the inductor. As a result, the PFC MOSFET must turn on while current is still flowing through the inductor and diode. This can lead to high switching and reverse-recovery losses. Ultrafast diodes, which add cost, are typically used in CCM PFCs to lower the reverse-recovery losses.

By contrast, a transition-mode PFC regulates the peak inductor current and waits until the inductor current returns to 0 A before beginning the next pulse. This substantially reduces the reverse-recovery and turn-on losses, but also leads to much higher peak currents. High peak currents can result in proximity losses in the PFC inductor and a substantially larger EMI filter. In addition, the transition-mode switching frequency is variable, which further complicates the EMI filter design.

Transition-mode controllers tend to be simpler and less expensive than CCM controllers. As illustrated in the table, the typical rule of thumb is to use transition mode for output powers less than 200 W, and to use CCM for output powers greater than 200 W.

When the light in the TV comes from a HID lamp, an electronic ballast is needed to control the lamp. The HID lamp is comprised of two opposing electrodes in a high-pressure gas-filled bulb. The high-pressure gas must be broken down for current to flow in the lamp, and consequently, a high-voltage circuit is used to generate a 30-kV impulse to create an arc within the bulb's gas.

After the gap has been broken down, it has a nearly constant-voltage characteristic of around 40 V. The voltage changes in the short term as the gas in the bulb heats up, thereby increasing the pressure. It also has a long-term variation as the tips of the electrodes erode and the gap length increases. The electronic ballast must regulate the bulb's power to keep the lamp output constant over time.

Several protection features must be built into the HID ballast supply as illustrated in Fig. 5. Once the igniter has been fired, a decision is made as to whether the lamp sustained an arc. If it has not, a digital counter is incremented and a decision is made as to whether to retry ignition. If there is a sustained arc, the power from the ballast supply is limited and the output voltage is then monitored. If the voltage monitor senses an overvoltage condition caused by an aging lamp or open circuit, the supply is disabled. Finally, there is quite a bit of housekeeping required; the warming up and cooling down of the bulb is controlled, and if the supply enters a standby mode, the PFC must be disabled. With all this overhead required, a microcontroller makes the best choice for overall power-supply control and fault monitoring, and it has become feasible for the PWM portion of the supply.

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