APPLICATION NOTES BPC SERIES POWER RESISTOR APPLICATIONS BI Technologies offers the BPC Series of noninductive, planar, thick film, power resistors for current limiting, power supply circuits and snubber circuits. Manufactured using 96% alumina substrates with ruthenium oxide resistors, the BPC Series power resistors are capable of withstanding a 20X overload for 8 ms and offer excellent performance and high reliability. BPC Series power resistors are available in 3, 5, 7.5 and 10 watt power ratings as shown below. Model Power Rating, Watts Dimensions, mm Length Width Thickness Range Standard Tolerance Operating Temperature Range Operating Voltage, Maximum Temperature Coefficient of Resistance BPC3 3 BPC5 5 BPC7 7 BPC10 10 27.7 10.2 2.54 27.7 12.7 2.54 27.7 19.1 2.54 27.7 25.4 2.54 3 to 200 KΩ 200 mΩ to 200 KΩ ±10% (Standard), ±1%, ±2%, ±5% (Optional) -55°C to +125°C 300 Vac, 500 Vdc ±100 ppm/°C, Max. ±250 ppm/°C for values below 1Ω WHY USE BI TECHNOLOGIES BPC SERIES POWER RESISTORS • Vertical mounting means power is dissipated above board level – enhancing board cooling due to natural convection flow • Planar thick film cermet resistor offers very low inductance for high frequency operation • 20X rated power surge capability • Small footprint requires only 1/3 of the board area used by a typical wirewound resistor • Low operating temperature at board surface means high temperature solder is not required • Large surface area to thickness ratio – maximizes power dissipation, minimizes hot spots 5-16 INRUSH CURRENT SUPPRESSION The BPC may be used to limit inrush current when a power supply is switched on. In Figure 1 is a typical PFC circuit in which two BPC 10 resistors in series function as inrush current suppression circuit which operates principally at switch-on BPC 10 BPC 10 5Ω 5Ω C T HM31 Vo 400 VDC EMI Filter L1 HM00 CT HM31 + Load Co AC Line 5 Gate Driver PFC PWM FIGURE 1. Typical PFC Circuit SNUBBER CIRCUIT STARTUP CIRCUIT EMI Filter The BPC may be used in series with a capacitor in a snubber circuit to filter spikes in the rising and falling edges of square waves. In Figure 2 a 500Ω BPC 3 is used. Figure 2 also includes a startup circuit that provides 18 volts to the PWM while the power supply is turning on. In this case, a 50KΩ BPC 5 is utilized. BPC 3 500Ω BPC 5 50KΩ To Output Circuitry AC Line 18V + PWM 47µF FIGURE 2. Typical startup and snubber circuits 5-17 APPLICATION NOTES INRUSH CURRENT LIMITING This resistor limits the amount of current allowed to surge into the system at turn-on. Power resistors in this circuit must be able to withstand a 20 X nominal surge for 8 ms. This is typically a 15Ω, 7 to 10 watt, 10% resistor. A typical location of this resistor is shown in Figure 3. This resistor carries the main current load. * + + AC Input DC Circuitry * EMI/RFI Filter * Other Power Resistors Inrush Power Resistor BPC 7, BPC 10 FIGURE 3. Circuit showing a BPC resistor in an inrush application OTHER SNUBBER CIRCUITS In these applications the power resistor bleeds the current of the magnetics to prevent inductive spikes. This resistor will experience voltage spikes at 2 X the switchmode switching frequency. This is typically a 500 Ω to 1K Ω, 3 watt to 5 watt, 2% to 5% resistor. Circuit configurations are found in Figure 4. 1,000 pF 1,000 Vdc Ceramic L101 FBL-00-103 FBL-00-103 BPC 5 500 Ω 5%, 5W Snubber Circuit 1 BPC 3 1K Ω 2%, 3W FBL-00-256 240 pF 20%, 1kV Ceramic Snubber Circuit 2 FIGURE 4. Snubber circuits found in power supplies. 5-18 APPLICATION NOTES BIASING (CURRENT CONDITIONING) These resistors do not experience surges or spikes. This is a steady state application. In these applications the resistor values vary considerably from 200 Ω to 20K Ω, 3 watt to 5 watt, 2% to 5%. An example is shown in Figure 5. BPC 5 Biasing Resistor PRELOADING 1 To DC Section This resistor experiences a steady state condition. The typical resistor is 3 Ω to 1K Ω and needs 10% tolerance. The preloading is usually a function of the circuit. An example is shown in Figure 6. + FIGURE 5. Cicuit from a high current, power supply which uses a BPC resistor for biasing. 5 10K Ω Sensing Resistor 0.1 Ω 180 Ω 470 pF 1,000 V Ceramic 1,500 µF 35 V + BPC 3 650 Ω 3W + 4.7 µF Output Preload Resistor From Switching Circuitry BPC Power Resistor FIGURE 6. Part of proprietary power supply showing two power resistors. 5-19 APPLICATION NOTES CURRENT LIMITING BPC resistors have uses as current limiters in highvoltage power supplies. An example of a highvoltage power supply, which is used to drive a laser, is shown in Figure 7 and the BPC resistor is highlighted. 0.01µF (5 KV) 1,800 pF 47K 1,800 pF (10 KV) BPC5 5W High Voltage Diodes Laser + Vsw Switching Regulator Vin + Vfb Vin + FIGURE 7. Laser power supply showing a BPC5 for circuit limiting. 5-20 APPLICATION NOTES PRELOADING 2 Another example of preloading is in a Battery Charger circuit as shown in Figure 8 where a resistor is used as a dummy load to prevent overcharging the battery. BPC 10 3.65 M 6.8 V 7.5 Ω 10W BPC 10 1M 8M TIP121 1M + 1A Solar Array 300 K Si9405 12 V 5 Ahr 1M Load + VN2222 1.2 V FIGURE 8. Shunt battery charger using a BPC10. 5 Vertical Mounting: Enhances board cooling due to natural convection airflow. Planar, Thick Film Cermet Resistor: Virtually eliminates inductance for high frequency designs compared with wirewound alternatives. Automated Laser Trimming: Assures accurate and stable resistance values. Large surface Area to Thickness Ratio: Maximizes power dissipation and minimizes hot spots. Robust Leads: Are firmly attached to both sides for superior mechanical integrity. Built-in Standoffs Low Operating Temperature: At board surface means high temperature solder is not required. Vertical Mounting: Minimizes required board area. Uses only 1/3 the board area versus a typical wirewound resistor. Model BPC5 (5 watt) 5-21 APPLICATION OVERLOAD CHARACTERISTICS - INRUSH APPLICATIONS 20X Nominal Power Rating Power Nominal Power Rating 0 8 Time (milliseconds) SIZE COMPARISON 1" 1" W 10 bound e Wir 0.10" 0.312 " DIA . 5-22 NOTES