184.108.40.206.3 Boost Diode Selection
The Boost diode carries the Boost inductor current while the switch is OFF (TDCH), and carries zero current while the switch is ON (TON). Equation 34 calculates The RMS diode current over a single switching cycle, at angle θ in the Line half-cycle .
Equation 35 describes the duty cycle of Boost diode conduction for ideal transition mode operation .
Equation 36 describes the RMS Boost diode current across a complete Line half-cycle .
The maximum RMS current in the Boost diode occurs at maximum load and minimum Line.
Conduction power loss in the Boost diode is primarily a function of the average output current.
Use the previous calculations and these guidelines to select the Boost diode:
- Ensure that the Boost diode voltage rating exceeds the maximum output voltage. Under transient or Line surge testing the output voltage may rise far above its normal regulation level.
- The Boost diode must have average and RMS current ratings that are higher than the numbers calculated by Equation 37 and Equation 38.
- Diodes are available with a range of different speed/recovery charge. Fast diodes, with low reverse recovery charge, typically have higher forward voltage drop. Fast diodes have higher conduction loss but lower switching loss. Slow diodes, with high reverse recovery charge, typically have lower forward voltage drop. Slow diodes have lower conduction loss but higher switching loss. Ensure maximum efficiency by matching the diode speed rating to the application.
- When Line voltage is first applied, to the Boost converter input, an uncontrolled current flows through the Boost diode while the output capacitor charges to the Line voltage peak level. The charging current is limited only by the impedance of the Line and EMI filter stage, and may reach a very high magnitude during the output capacitor charging period. Any diode carrying this current must be rated to carry this non-repetative surge current. It is normal practice to add a bypass diode to divert most of this charging current away from the Boost diode. The bypass diode can be a slow type with lower forward voltage drop. It is therefore cheaper and more robust than the faster Boost diode.
- For this example design the STTH5L06 diode from STMicroelectronics® was selected. This diode has a voltage rating of 600 V and an average current rating of 5 A. It has a forward voltage drop of approximately 0.85 V giving a conduction loss in the Boost diode, of less than 0.5 W.