In the power electronics field, the ongoing trend in power supply manufacturing is shrinking the size and weight of magnetic components via increasing the working frequency. The traditional VD-MOSFET (Planar MOSFET) faces problems like high temperature-rising in high frequency application environment. However, SJ-MOSFET(Super Junction MOSFET) has a lower Cj(Junction Capacitance), which is more suitable for high switching frequency. Based on the power consumption analysis and practical use case, below is a detailed analysis of the application advantages of SJ-MOSFET.
MOSFET switching power loss analysis
When Power MOSFET works in the high frequency, the charging and discharging of high input capacitance will lead to large switching loss. The on&off waveform as below:
During Tr, Pt can be ignored comparing with Pton, Ptf;
(Full line) CCM：
(Imaginary line) DCM:
( formula 3)
( formula 4)
Referring to formula 4, Tf is positive proportion to RG, CGS, CGD;
With the same Rdson, the CGS, CGD of SJ-MOSFET will be smaller 2-3 times, and the corresponding turn-off loss is lower 2-3 times than VD-MOSFET.
From formula 3, the higher switching frequency f is, the higher Ptoff is, if decreasing Ib and tf, the loss increasing aroused by the frequency increasing can be offset.
Referring to the above analysis, SJ-MOSFET is more suitable for higher switching frequency, such as increasing the switching frequency from 65KHZ to 100KHZ-200KHZ.
The advantages analysis of high frequency application
( formula 5)
Power supply with the same power PIN, the higher switching frequency fSW is, the smaller IPK MOSFET .is; in this way, the low current MOSFET can be used which will save more cost for the user. To further verify the above view, we test the VDS, IDS stress of two MOSFETs based on two different switching frequency.
b. The high frequency transformer can be of smaller size and lower cost.
The primary side inductance Lp of the transformer with IC controller UC3842 CCM is calculated as below
( formula 6)
From formula 6, fSW is negative proportion to Lp; the lower Lp is, the less transformer winding turns NP is, and the magnetic core size can be smaller and thinner. The ultra smaller size Power Supply become possible to be designed. The more important point is that the cost of the more compact transformer is lower.
Comparing with VD-MOSFET, SJ-MOSFET has smaller Ciss or Qg, smaller turn-off loss, which is more suitable for high switching frequency. If MOSFET at high frequency power supply primary side, there will be a smaller current stress, lower current specification components can become optional. In the meantime, the transformer in high frequency power supply can be of smaller size and lower cost.