TIDUBE5A Design guide

TIDUBE5A January 2022 – October 2022

2.4.1.7 MOSFET Ratings

MOSFET Voltage Rating
When MOSFET is in OFF state, it will have to block DC bus voltage which can rise up to 400V maximum. S0, with leaving 50% room as safety factor, 600V MOSFET is selected in this design.
MOSFET Current Rating
When MOSFET is ON, it will conduct peak value of inductor current when inductance is at its lowest value
Equation 23. $I_{F E T_{(P E A K)}} = I_{L_{(P E A K)}}$
Power Dissipation in MOSFET
Total power dissipation in MOSFET consists of conduction loss, turn off loss, turn on loss, C_OSS loss and gate drive loss. It can be estimated using following formula
Equation 24. $P_{F E T} = P_{R_{D S (O N)}} + P_{S W I T C H (t_{O N})} + P_{S W T I C H (t_{O F F})} + P_{C_{O S S}} + P_{G A T E}$

MOSFET Conduction Loss
Conduction loss in MOSFET can be calculated by following formula
Equation 25. $P_{R_{D S (O N)}} = I_{F E T (R M S)}^{2} \times R_{D S (O N)}$
where FET RMS current can be calculated using following formula:
Equation 26. $I_{F E T (R M S)} = \sqrt{f_{L I N E} \times \sum_{n = 1}^{I t e r a t i o n} \{\int_{0}^{[\frac{D (n \times S t e p)}{f_{S W}}]} {[\frac{I_{I N} (n \times S t e p)}{2}]}^{2} d t\}}$

MOSFET Switch ON Loss
- MOSFET switching loss due to turn on operation
  Equation 27. ${P_{S W I T C H}}_{(t_{O N})} = f_{L I N E} \times \sum_{n = 1}^{I t e r a t i o n} [I_{I N_{t_{O N}}} (n \times S t e p) \times V_{O U T} \times t_{O N (d e l a y)}]$
- Where current at ON state is given by
  Equation 28. $I_{I N_{t_{O N}}} (t) = (I_{L} - \frac{∆ I_{L_M A X}}{2}) \sin (ω \times t)$
- MOSFET turn ON delay time can be given by
  Equation 29. $t_{O N (d e l a y)} = \frac{Q_{G S (m i l l e r)}}{I_{G A T E (O N)}}$
- I_GATE(ON) is average FET gate drive current during turn ON operation
  Equation 30. $I_{G A T E (O N)} = \frac{V_{G S (M A X)}}{2 \times R_{G A T E (O N)}}$
  Where, R_GATE(ON) = R70 = R71. That is resistance in gate current path during turn ON operation.
MOSFET Switch OFF
- MOSFET switching loss due to turn off operation
  Equation 31. ${P_{S W I T C H}}_{(t_{O F F})} = f_{L I N E} \times \sum_{n = 1}^{I t e r a t i o n} [I_{I N_{t_{O F F}}} (n \times S t e p) \times V_{O U T} \times t_{O F F (d e l a y)}]$
- Where current at OFF state is given by
  Equation 32. $I_{I N_{t_{O F F}}} (t) = (I_{L} + \frac{∆ I_{L_M A X}}{2}) \sin (ω \times t)$
- MOSFET turn OFF delay time can be given by
  Equation 33. $t_{O F F (d e l a y)} = \frac{Q_{G S (m i l l e r)}}{I_{G A T E (O F F)}}$
- I_GATE(OFF) is average FET gate drive current during turn OFF operation. Current in this mode flows through diode and resistor in series with diode. So, considering forward voltage drop, I_GATE(OFF) can be given by:
  Equation 34. $I_{G A T E (O F F)} = \frac{V_{G S (M A X)} - V_{f_D i o d e}}{2 \times R_{G A T E (O F F)}}$
Part of the total FET losses are contributor C_OSS (C_OSS(AVG)) during charging and discharging during a PWM switching cycle and can be given by
Equation 35. $P_{C_{O S S}} = \frac{1}{2} \times C_{O S S (A V G)} \times V_{O U T}^{2} \times f_{S W_P F C}$
Where C_OSS varies with line voltage and is not a linear function. The following equation and information from the FETs data sheet can be used to calculate C_OSS(AVG). C_OSS(SPEC)is the typical C_OSS measured at a specified voltage V_DS(SPEC).
Equation 36. $C_{O S S (A V G)} = 2 \times C_{O S S (S P E C)} \times \sqrt{\frac{V_{D S (S P E C)}}{V_{O U T}}}$
MOSFET Gate Loss can be determined by
Equation 37. $P_{G A T E} = Q_{G S} \times V_{G S (M A X)} \times f_{S W_P F C}$