Electronic Design

CMOS DC-DC Controller Tackles High Voltages

One Chip Melds A 200-V MOSFET, A PWM Controller, Fault Detection, And Protection Circuitry

Pressure to speed designs to market without compromising reliability and performance isn't the only compelling reason to adopt more integrated voltage converters or controllers in power supplies. The ability to build smaller, lighter, and more energy-efficient power supplies also pushes designers toward integrated solutions.

Although some integrated power converters have helped to reduce parts counts and cut the overall bill-of-materials cost, they have failed to integrate high-voltage power MOSFETs on the same die. Historically, these huge discrete high-voltage transistor circuits have remained outside of the power-supply controller's package, which means more room on the board, higher cost, and lower system reliability.

Designers at Power Integrations now seem to have licked this problem for high-voltage dc-dc converters with a CMOS process technology. Besides integrating a 200-V power MOSFET with a pulse-width modulated (PWM) controller on one die, Power Integrations' newest dc-dc controller achieves a high degree of functional density on the same silicon chip. Consequently, the new DPA-Switch family eliminates nearly 40 external components from a traditional high-voltage dc-dc converter design. It truly takes miniaturization of high-voltage converters to the next level.

Aimed at distributed power architectures, the DPA-Switch combines a 200-V power MOSFET and a high-frequency PWM controller with fault protection and control functions on a monolithic chip (Fig. 1). The ability to pack both high-voltage MOSFET and low-voltage control circuitry on the same chip is attributed to a patented high-voltage CMOS process. By a modified, reduced-surface field (RESURF) principle, it integrates a lateral 200-V MOSFET with a specific on-resistance that's 33% lower than traditional double RESURF technology.

Plus, the IC employs junction isolation to maintain harmony between the high- and low-voltage circuits. Depending on the output power, the maximum on-resistance of this 220-V MOSFET varies from 0.27 to 1.59 Ω.

In fact, the dc-dc controller is built on a proven CMOS technology previously employed to realize its popular ac-dc switcher family (TOPSwitch) in the mid-1990s. Now, the company is extending the process to high-voltage dc-dc converters, states Rich Fassler, director of product marketing at Power Integrations. Therefore, the DPA-Switch guarantees the same reliability as the TOPSwitch line. An estimate of the failure rate for the TOP-GX series is 108 million hours mean time between failure.

In essence, the DPA-Switch integrates all primary side functions to dramatically simplify single-ended forward or flyback dc-dc converter topologies. It will cut the size of a high-voltage dc-dc converter by almost 50%. According to Fassler, a 30-W dc-dc converter evaluation board implemented with the DPA-Switch IC contains about 40 fewer components than a traditional converter with discrete components (Fig. 2).

Beyond standard features like high-voltage start-up, cycle-by-cycle current limiting without a sense resistor or current sense transformer, loop compensation circuitry, auto-restart, and thermal shutdown, the DPA-Switch comes equipped with many advanced features, improving performance and design flexibility and reducing cost. Some highlights are a fully integrated 5-ms soft-start for minimum stress and overshoot, a high switching frequency, 73% maximum duty cycle without requiring slope compensation, cycle skipping, line undervoltage (UV) detection, synchronous rectification, and the ability to synchronize with an external clock.

Although the 5-ms soft-start limits peak currents and voltages during start-up to suppress output overshoot, cycle-skipping operation at light loads minimizes standby power consumption. Cycle-skipping maintains high conversion efficiency even at 5% to 10% of full-load. Depending on the load condition, the controller automatically switches between normal and cycle-skipping modes as necessary.

In reality, it's initiated at duty cycles below 4%. In compliance with the ETSI specifications, the line UV detection ensures glitch-free operation at both power-up and power-down. Moreover, pin-selectable high-frequency (400/300-kHz) switching permits the use of a miniature transformer while offering high bandwidth for loop control.

Simplifying Converter Design: It takes just three user-configurable pins (L, X, and F in Figure 1) to configure the device for line UV/overvoltage sensing, variable maximum duty cycle (DCMAX), accurate current limiting, remote on/off, and frequency selection. A single external resistor provides line sensing, remote on/off, and line feed forward with DCMAX reduction.

By appropriately setting the value of this external resistor, DCMAX, set at a default value of 73%, can be decreased to 43% when input line voltage increases. Limiting DCMAX at higher line voltages prevents transformer saturation due to large load transients in forward converter designs. This pin also lets the user synchronize the DPA-Switch with an external clock. But here, the external clock frequency must be lower than the internal switching frequency. Under this condition, the effective DCMAX is 0.73 × Fsync × Fosc.

Normally, the default current limit of the DPA-Switch is preset internally. But by connecting a resistor between the external current limit and the source pins, the current limit can be programmed externally between 35% and 100% of the default current limit, a nearly 3:1 range. Moreover, to implement a true power-limiting operation against line variation, the controller permits the designer to connect a second (optional) resistor between the external current-limit pin and the rectified dc high-voltage bus. As a result, the current limit is reduced with increasing line voltage.

Switching frequency also is pin-selectable. Although the DPA-Switch's nominal fixed switching frequency is 400 kHz, it can be lowered to 300 kHz by simply connecting the frequency and control pins. Aside from minimizing the transformer size, the high switching frequency improves the dc-dc converter's loop response.

Other features include hysteretic thermal shutdown, bandgap reference, synchronous rectification, and auto-restart. To lower EMI, the package tab is internally connected to the source. For higher efficiency, the DPA-Switch supports any self-driven synchronous rectification scheme, as used in discrete solutions (Fig. 3). Typical efficiency at full load is over 86% using Shottky barrier diodes for rectification (Fig. 4). This can be further improved by implementing synchronous rectification instead of diodes.

The DPA-Switch accommodates both single-ended forward or flyback topologies. It also handles a wide input range of 16 to 75 V dc. This new four-member family provides maximum power outputs of between 20 and 111 W (see the table). Note that this maximum is limited by the device's internal current limit and assumes the provision of adequate heatsinking to keep junction temperature below 125°C.

Price & Availability
Designed for distributed power architectures with industry-standard bus voltages of 24 or 48 V, the DPA-Switch family initially offers four members, DPA423/24/25/26. Maximum power range for the family is 20 to 111 W, letting designers scale up easily as power requirements increase.

Sampling now, the high-voltage dc-dc controllers are slated for production in September. Implemented in a 3-µm CMOS process, all four versions come in six-lead TO-263 packages and are specified over the industrial temperature range. They will be shipped in tape and reel format. In 1000-piece quantities, the 20-W DPA423R-TL is priced at $3.02, while the 111-W DPA426R-TL runs $5.47.

Power Integrations Inc., 5245 Hellyer Ave., San Jose, 95138; (408) 414-9200; www.powerint.com.

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