MOSFET Calculator
Understanding MOSFET Operation
1. Basic Characteristics
Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) are voltage-controlled devices widely used in switching and amplification applications. Their operation is based on controlling the channel conductivity through gate voltage.
- N-channel and P-channel types
- Enhancement and depletion modes
- Three operating regions: cutoff, linear, saturation
- Body effect consideration
2. Gate Drive Design
How to calculate gate resistor and drive requirements:
| Parameter | Formula | Example |
|---|---|---|
| Gate Resistor | Rg = Vdr/(Ig_peak) | 10Ω for 12V drive |
| Gate Current | Ig = Qg × fsw | 100mA at 100kHz |
3. Key Parameters
Essential parameters for MOSFET selection and operation:
VDS(max): Maximum drain-source voltage
ID(max): Maximum drain current
RDS(on): On-state resistance
VGS(th): Gate threshold voltage
Qg: Total gate charge
4. Switching Characteristics
Understanding switching behavior and timing:
- Turn-on delay time (td(on))
- Rise time (tr)
- Turn-off delay time (td(off))
- Fall time (tf)
5. Switching Loss Calculations
How to calculate and minimize switching losses:
| Temperature Rise | ΔT = Ptotal × Rth(j-a) Tj = Ta + ΔT | Consider thermal resistance path |
|---|---|---|
| Heat Sink Requirements | Rth(h-a) = (Tj(max) - Ta)/Ptotal - Rth(j-c) - Rth(c-h) Select heatsink with lower Rth | Include thermal interface material |
6. Dead Time Calculation
Calculating minimum dead time for safe operation:
| Parameter | Formula | Typical Values |
|---|---|---|
| Minimum Dead Time | td(min) = tf + tr | 100-500ns |
| Safety Margin | td(safe) = 2 × td(min) | 200-1000ns |
7. Threshold Voltage Calculation
Understanding threshold voltage variations:
| Parameter | Formula | Notes |
|---|---|---|
| VGS(th) | VGS(th) = VGS(th,25°C) + TC × ΔT | Temperature coefficient |
| Temperature Effect | TC ≈ -2 to -4 mV/°C | Negative coefficient |
8. Transconductance Calculation
Understanding MOSFET gain characteristics:
- Basic Formula:
- gm = ∂ID/∂VGS
- gm ≈ 2ID/(VGS - VGS(th))
- gm depends on operating point
- Operating Point:
- Linear region: gm varies with VDS
- Saturation region: gm more stable
- Maximum gm at ID(max)/2
9. Gate Charge Calculations
Analyzing gate charge requirements:
| Parameter | Formula | Application |
|---|---|---|
| Turn-on Time | ton = Qg/Ig | Switching speed |
| Gate Energy | Eg = Qg × VGS | Driver loss |
10. Conduction Loss Analysis
Understanding and calculating conduction losses:
| Loss Type | Formula | Considerations |
|---|---|---|
| DC Losses | P = ID² × RDS(on) | Temperature dependent |
| AC Losses | P = IRMS² × RDS(on) | Frequency dependent |
11. Design Guidelines
Best practices for MOSFET circuit design:
- Select VDS rating with 20% margin
- Consider thermal management early
- Use proper gate drive circuit
- Account for parasitic effects
- Implement protection features
- Optimize PCB layout
Quick Reference
Typical Values
VGS(th): 2-4V
RDS(on): 1-100mΩ
ID(max): 10-100A
Qg: 20-100nC
Operating Regions
Cutoff: VGS < VGS(th)
Linear: VDS < (VGS - VGS(th))
Saturation: VDS > (VGS - VGS(th))
Design Tips
- • Use proper gate drive
- • Consider switching speed
- • Monitor temperature rise
- • Check SOA limits
- • Add protection circuits
- • Control EMI/EMC