1. Voltage load
In applications with DC voltage stabilized high-power circuits, the drain-source voltage VDS is usually considered first. The reason here is that in the actual working environment, the maximum peak drain-source voltage of the MOSFET is less than 90% of the nominal drain-source interrupt voltage of the device specification. Is:
VDS_peak≤90%* V(BR)DSS
Note: Generally, the temperature coefficient of V(BR)DSS is positive. Therefore, it should be used as a reference for the V(BR)DSS value at the lowest operating temperature of the device.
2. Loss and heat dissipation
A small Ron value is good for reducing the loss during conduction, and a small Rth value is good for heat dissipation, because it can reduce the temperature difference (under the same power consumption condition).
3. Drain current
Next, consider the choice of drain current. The basic principle is that in the actual working environment, the maximum periodic drain current of the MOSFET is 90% or less of the specified maximum drain-source current, and the maximum drain pulse current is the specified peak value of the nominal drain pulse current. 90% or less. which is:
ID_max≤90%* ID
ID_pulse≤90%* IDP
Note: The temperature coefficients ID_max and ID_pulse are usually negative. Therefore, please use the ID_max and ID_pulse values of the device at the highest junction temperature as a reference. The selection of this parameter of the high-performance DC regulated power supply is very uncertain, which is mainly due to the mutual restriction of the working environment, heat dissipation technology and other equipment parameters (resistance, thermal resistance, etc.). The final decision will be based on the junction temperature (ie, "Power Dissipation Limitation" in Article 6 below). Experience has shown that in practical applications, the limit value of power loss and temperature rise causes the specification ID to be a multiple of the actual maximum operating current. During the pre-calculation, this parameter must be adjusted continuously according to the power loss limit in Section 6. It is recommended that the first choice is 3-5 times ID = (3-5) * ID_max.
4. First calculation of power loss
There are eight main parts to calculating MOSFET losses:
PD = Pon + Poff + Poff_on + Pon_off + Pds + Pgs + Pd_f + Pd_recover
The detailed formula should be determined according to the specific circuit and working conditions. For example, synchronous rectification applications should also consider the loss during the forward conduction of the body diode and the reverse recovery loss when turning off the steering.
5. Drive requirements
The drive requirement of MOSFEF is determined by the parameter of total network cost (Qg). If you meet other parameter requirements, please try to choose a smaller Qg to simplify the drive circuit design. Choose the drive voltage to keep Ron as low as possible, as long as it is not close to the maximum gate-source voltage (VGSS) (usually using device specification recommendations).
6. Dissipated power constraint
The maximum steady-state power loss PD of a DC-stabilized high-performance power supply device should be based on the maximum temperature limit of the device's operating transition temperature. If the working environment temperature of the device is known in advance, the maximum power consumption can be estimated as follows.
PD，max≤(Tj，max-Tamb)/Rθj-a
Among them, Rθj-a is the total thermal resistance from the device junction to its working environment, including Rθ junction case, Rθcase sinking, Rθsink environment, etc. If there is an insulating material between the two, the thermal resistance should be considered.