High efficiency PFC circuit diode selection solution

New options for diodes in PFC

    In power factor correction (PFC) circuits, 600V boost diodes are key components, especially for PFCs operating in continuous mode and under harsh switching conditions. During each switching cycle, the recovery current of the diode flows through the MOS transistor, which results in high "switch-on" power losses in the switch. For this application, the fastest 600V diodes are required.

    In order to improve the efficiency of PFC, the usual method is to connect three 200V epitaxial recovery diodes in series. This necessitates the addition of a balancing network (a capacitor and a resistor in parallel with each diode) to ensure that each diode operates within its rated voltage.

    ST Microelectronics offers a novel solution: two 300V diodes are connected in series and packaged in an insulated TO-220 package to form a 600V Tandem (series diode). This silicon device is an ultra-fast diode that suppresses the balancing network in most cases.

Comparison with ordinary diodes

    PFCs operating in continuous mode and under harsh switching conditions (Figure 1) see the current in the diode decrease very quickly (hundreds of amps/microseconds) when the transistor turns on.

    There are two types of power consumption here:

    ·Conduction and switching power losses in diodes;

    ·Power dissipation in the transistor due to the reverse recovery current of the diode.

    Figure 2 shows the power consumption comparison of the same PFC using different diodes (ordinary 600V diode STTA806D or 600V Tandem STTH806 TTI). These results are obtained under the following operating conditions: Pout=400W, Fs=150kHz, dI/dt= 200A/μs, Tj=125℃, Vmains=110V. It can be clearly seen from Figure 2 that the main part of the switching power consumption is in the MOS transistor; the total power consumption of using a 600V Tandem (STTH806TTI) is lower than using an ordinary 600V diode (STTA806D). This is due to the small recovery current of the diode Caused by.

Tandem diode selection index

    The choice between 600V Tandem and ordinary 600V diodes mainly depends on the following parameters:

    3. Influence of junction temperature

    The maximum reverse recovery current increases with junction temperature, so the power consumption difference between STTA806D and STTH806TTI increases with junction temperature. Figure 4 shows the graph of the power consumption difference between STTA806D and STTH806TTI and the diode operating junction temperature. The working conditions to obtain this result are: Pout=330W, F=110kHz, Vmains=85V, dI/dt=165A/μs.

in conclusion

    600V Tandem diodes are the fastest silicon 600V diodes on the market today. The analysis in this article shows that the performance relationship between Tandem diodes and ordinary diodes depends on the application parameters of the PFC circuit.

    Tandem diodes are best suited for low input voltage (110V), high junction temperature and high switching frequency.

    Reliability testing shows that no balancing network is required when using Tandem diodes in conventional PFC designs.