PS (ON)
■SELECTION OF COMPONENTS
• Pch MOS FET
The P-ch MOSFET for switching use should be rated for at least 20% more than the maximum input voltage. To minimize continuity loss, use a FET with low RDS(ON) between the drain and source. For high input voltage and high frequency operation, on/off-cycle switching loss will be higher so that power dissipation must be considered. In this application, the Toshiba TPC8102 is used. Continuity loss, on/off switching loss, and total loss are deter- mined by the following formulas. The selection must ensure that peak drain current does not exceed rated values, and also must be in accordance with overcurrent detection levels.
Continuity loss : PC
PC = ID 2 ⋅ RDS (ON) ⋅ Duty
On-cycle switching loss : PS (ON)
= VD (Max) ⋅ ID ⋅ tr ⋅ fOSC
6
Off-cycle switching loss : PS (OFF)
| PS (OFF) = | VD (Max) ⋅ ID (Max) ⋅ tf ⋅ fOSC |
| 6 |
| |
Total loss : PT
PT = PC + PS (ON) + PS (OFF)
Example: Using the Toshiba TPC8102
CH1
Input voltage VIN (Max) = 19 V, output voltage VO = 5 V, drain current ID = 3 A, Oscillation frequency fOSC = 500 kHz, L = 15 ∝H, drain-source on resistance RDS (ON) =: 50 mΩ, tr = tf =: 100 ns.
Drain current (Max) : ID (Max)
ID (Max) | = IO + | VIN − VO | | ton | | | |
2L | | | | |
| | | | | | |
| = 3 + | 19 − 5 | ⋅ | 1 | ⋅ 0.263 |
| 2 ⋅ 15 ⋅ 10−6 | 500 ⋅ 103 |
| =: 3.25 (A) | | | | | |
Drain current (Min) : ID (Min) | | | |
ID (Min) | = IO − | VIN − VO | ton | | | |
2L | | | |
| | | | | | |
| = 3 − | 19 − 5 | ⋅ | 1 | ⋅ 0.263 |
| 2 ⋅ 15 ⋅ 10−6 | 500 ⋅ 103 |
| =: 2.75 (A) | | | | | |
