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Si8283 データシート(PDF) 7 Page - Silicon Laboratories |
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Si8283 データシート(HTML) 7 Page - Silicon Laboratories |
7 / 43 page 2.12 DC-DC Converter Application Information The Si828x isolated dc-dc converter is based on a modified fly-back topology and uses an external transformer and rectifying diodes for low cost and high operating efficiency. The PWM controller operates in closed-loop, current mode control and generates isolated output voltages with up to 2 W average output power at VDDP = 5.0 V. Voltage feedback is referenced between VDDB-VSSB. Although there is only one voltage feedback path, two output voltages are realized by the tight coupling of the two secondary transformer windings. Options are available for 24 Vdc input operation and externally configured switching frequency. The dc-dc controller modulates a pair of internal, primary-side power switches (see Figure 2.5 Si8281/83 Block Diagram: 3 V–5 V Input to Split Voltage Output on page 8) to generate an isolated voltage at external diode D1 and D2. Divider resistors, R1 and R2, gener- ate proper 1.05 V for the VSNS pin. Closed-loop feedback is provided by an internal compensated error amplifier, which compares the voltage at the VSNS pin to an internal voltage reference. The resulting error voltage is fed back across the isolation barrier via an inter- nal feedback path to the controller, thus completing the control loop. For input supply voltages higher than 5 V, an external FET Q2 is modulated by a driver pin ESW as shown in Figure 2.6 Si8282/84 Block Diagram: >5.5 V Input to Split Voltage Output on page 9. A shunt resistor based voltage sense pin, RSN, provides current sensing capability to the controller. The Vin must be able to support the Si828x VDDB-VSSD static load current (approximately 9 mA), the output drive load requirement, and the dc-dc power dissipation (loss). The driver power requirement is dependent on the IGBT gate charge and the driver switching frequency. Below are the equations to calculate the Vin power requirement. Pvin = (9 × 10−3 × (VDDB + VSSB) + Qg × Fsw) η where: Qg = IGBT total gate charge Fsw = driver switching frequency ƞ = dc-dc efficiency (approximately 78%) Additional part number features include an externally-triggered shutdown of the converter functionality using the SH pin and a program- mable soft start configured by a capacitor connected to the SS pin. The resistor value on pin SH/FC and the capacitor value on pin SS are used during power-up to set the dc-dc switching frequency. Note that pin SH/FC and SS pins are available on the Si8283 and Si8284 only. The Si828x can be used with a low-voltage power rail or a high-voltage power rail. These features and configurations are explained in more detail in other sections. 2.12.1 External Transformer Driver The dc-dc controller has internal switches (VSW) for driving the transformer with up to a 5.5 V voltage supply. For higher voltages on the primary side, a driver output (ESW) is provided on the Si8282 and Si8284 that can switch an external NMOS power transistor for driving the transformer. When this configuration is used, a shunt resistor based voltage sense pin (RSN) provides current sensing to the controller. 2.12.2 Output Voltage Control The isolated output voltage, VOUT (VDDB–VSSB), is sensed by a resistor divider that provides feedback to the controller through the VSNS pin. The voltage error is encoded and transmitted back to the primary side controller across the isolation barrier, which in turn changes the duty cycle of the transformer driver. The equation for VOUT is as follows: VOUT = VSNS × (1+ R1 R2 ) The VDDB-VSSB voltage split is depended on the ratio of the two secondary windings and can be calculated as follows: VDDB − VMID = VOUT × ( S1 S1 × S2 ) VSSB − VMID = VOUT × ( S2 S1 + S2 ) 2.12.3 Compensation The dc-dc converter operates in current mode control. The loop is compensated by connecting an external resistor in series with a ca- pacitor from the COMP pin to VSSB. The compensation network, RCOMP, and CCOMP are set to 200 kΩ and 1 nF for most Si828x applications. Si8281/82/83/84 Data Sheet System Overview silabs.com | Smart. Connected. Energy-friendly. Preliminary Rev. 0.5 | 6 |
同様の部品番号 - Si8283 |
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同様の説明 - Si8283 |
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