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MOSFET (Si/SiC) Forum Discussions

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What is the specific MOSFET parameter involved in the dead time setting in an LLC converter?
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1 Solution
G_Anshika
Moderator
Moderator 100 replies posted 50 solutions authored 100 sign-ins
Moderator

Hello,

The rational selection of dead-time for the MOSFETs of LLC converter is essential to achieve the ZVS(Zero Voltage Switching) within a wide adjustment range for reducing the electromagnetic interference and improving the operational efficiency. Dead time is required to discharge completely the effective capacitance appeared in parallel with drain–sources of the power MOSFETs to realize the ZVS operation at the worst-case conditions applied to the converter.

For identifying the required dead time, two important limitations must be considered:
1. Depends on the power MOSFET’s minimum required dead time, which can be obtained from the
datasheet easily, by considering MOSFET parameters such as body diode reverse recovery time, turn-on and
-off delay times, rise, and fall times.
2. Another is related to the minimum necessary dead time for achieving ZVS operation even under the worst-case conditions, i.e., when maximum input voltage is applied to the converter and the output voltage is adjusted at minimum value under light or no load conditions. In the situations that ZVS operation needs longer dead time than that of the power MOSFET’s necessary value, the greater value can be used for satisfying both cases, but this approach will sacrifice the converter efficiency.

There are three kinds of Coss values in datasheets of CoolMOS™: Coss at specified VDS value, Co(er) and Co(tr).Co(tr) is an equivalent fixed capacitance of MOSFET to determine the charging/discharging time of COSS of MOSFET regardless of topology. The dead time selection should ensure ZVS of two primary side MOSFET at
maximum switching frequency, where the magnetizing current to charge and discharge MOSFET Coss is the
minimum. Equation  shows the relation of dead time and MOSFET Co(tr):
                                       Screenshot 2022-03-10 084819.png
Imag_min is the minimum magnetizing current to charge and discharge Coss.
I hope this helps. 

Regards,
Anshika

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2 Replies
Not applicable
The Co(tr). Of course this will be always in combination with the designed peak magnetizing current.
0 Likes
G_Anshika
Moderator
Moderator 100 replies posted 50 solutions authored 100 sign-ins
Moderator

Hello,

The rational selection of dead-time for the MOSFETs of LLC converter is essential to achieve the ZVS(Zero Voltage Switching) within a wide adjustment range for reducing the electromagnetic interference and improving the operational efficiency. Dead time is required to discharge completely the effective capacitance appeared in parallel with drain–sources of the power MOSFETs to realize the ZVS operation at the worst-case conditions applied to the converter.

For identifying the required dead time, two important limitations must be considered:
1. Depends on the power MOSFET’s minimum required dead time, which can be obtained from the
datasheet easily, by considering MOSFET parameters such as body diode reverse recovery time, turn-on and
-off delay times, rise, and fall times.
2. Another is related to the minimum necessary dead time for achieving ZVS operation even under the worst-case conditions, i.e., when maximum input voltage is applied to the converter and the output voltage is adjusted at minimum value under light or no load conditions. In the situations that ZVS operation needs longer dead time than that of the power MOSFET’s necessary value, the greater value can be used for satisfying both cases, but this approach will sacrifice the converter efficiency.

There are three kinds of Coss values in datasheets of CoolMOS™: Coss at specified VDS value, Co(er) and Co(tr).Co(tr) is an equivalent fixed capacitance of MOSFET to determine the charging/discharging time of COSS of MOSFET regardless of topology. The dead time selection should ensure ZVS of two primary side MOSFET at
maximum switching frequency, where the magnetizing current to charge and discharge MOSFET Coss is the
minimum. Equation  shows the relation of dead time and MOSFET Co(tr):
                                       Screenshot 2022-03-10 084819.png
Imag_min is the minimum magnetizing current to charge and discharge Coss.
I hope this helps. 

Regards,
Anshika

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