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.