AN4250 STMICROELECTRONICS | Alldatasheet
Document overview
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- PDF pages: 15
Technical content
Datasheet sections
- 1 Power factor
- 2 Boost converter
- 3 Power factor correction
- 4 Fishbone diagram
- 5 Fishbone diagram for Power MOSFET in parallel
- 6 Revision history
Fishbone diagram for power factor correction Rosario Costanzo, Gianluca Messina, Antonino Gaito Introduction This report aims to show through a Fishbone diagram, all possible causes of failure of the Power MOSFET mounted on a PFC. This work is divided into 5 sections:
- The first one describes the power factor
- The second describes the Boost converter
- The third describes the PFC system
- The forth paragraph shows all critical conditions causing the failure of the Power MOSFET and builds up the Fishbone diagram
- Last paragraph presents a specific Fishbone parallel configuration
1 Power factor
Figure 1. Schematics of a single phase diode bridge rectifier
While the apparent power: Equation 5 Due to equations 4 and 5, the power factor is: Equation 6 The power factor depends on the phase displacement due to the contribute of the factor ϕ and on the harmonic content due to the contribute of the factor ϑ. IV effeffS = ϕϑϕ coscoscos.. 1 == I I eff FP
2 Boost converter
The Boost converter is generally used in the SMPS as a PFC. Section 3 gives a complete description of the PFC block and the Boost converter. The Boost circuit is a DC-DC converter providing a higher output voltage than input voltage. Here below the classic Boost topology. Figure 2. Boost converter schematic state of the switch: on-state, off-state. Figure 3. Boost converter schematic during the Power MOSFET turn-on
Figure 4. Boost converter schematic during the Power MOSFET turn-of phenomenon is an extra voltage with a sign, which can oppose to the decreasing current. one, allowing the current to flow through it. conduction mode), DCM (discontinuous conduction mode).
3 Power factor correction
compares the input main voltage with the output voltage and acts on the duty cycle. Boost converter features two conduction modes; current waveforms depend on them. reaches zero, and turns off when the inductor current meets the desired value. Figure 7. Current waveforms on the inductor for the CCM/DCM Boost converter
4 Fishbone diagram
Figure 8. Pre-charge circuit causes the failure of the device. cause the failure of the device.
- If the “pre-charge” circuit is not present, the switch works in start-up phase with a huge
- the “pre-charge” circuit is present till it works, then current flows onto the switch like the
the spike, the overall absorbed current could cause the failure. Figure 9. Recovery current inside a diode turn-on can happen during the turn-off commutation. High values of Crss can cause the undesired turn-on of the switch.
Figure 10. Fishbone diagram
Fishbone diagram for Power MOSFET in parallel AN4250
5 Fishbone diagram for Power MOSFET in parallel
When the power levels are very high, some PFC makers put in parallel two or more Power MOSFET devices. In this case, other aspects must be taken in account. Causes linked to the application Parasitic inductance It is generated by interconnection wiring and discrete components. They cause delays and power losses which affect the balance of the current. It strictly depends on the PCB layout. Temperature unbalance A temperature rise causes a decrease of Vth and an increase of RDS(on). Decrease of Vth -> switching loss rise -> thermal runaway. Increase of RDS(on) -> conductive loss rise-> current limitation -> unbalance. Several factors let two Power MOSFETs work according to different temperatures: – Another device is mounted on the same heatsink – Different air flow according to the fan Boost diode During the turn-on, it impacts on the current spike. If devices with different Vth are used (500 mV), a fast diode or a very fast diode involves a different peak current. Gate circuitry Decoupling resistor mismatch causes the current unbalance. The device with lower R gate leads more current than other one and its temperature, as well as its RDS(on) increase. Causes linked to the material Different Vth During the switching phase (turn-on, turn-off), the difference of Vth leads the device, with its lower value, to conduct earlier, causing an unbalance of the currents. Different RDS(on) The difference of RDS(on) causes, during the conduction phase, an unbalance between two currents. In particular, if one of the two Power MOSFETs has a lower RDS(on), an increase of its ID current is observed, causing an increase of the Power MOSFET temperature. Gfs influence Different gfs values, during the commutation, cause substantial differences in relation to the two currents. This parameter is guaranteed by design and process.
Figure 11. Additional Fishbone diagram for parallel configuration
6 Revision history
Table 1. Document revision history 11-Mar-2014 1 Initial release.