RDS on in MOSFETs -In today’s post, we explore one of the most critical parameters in power MOSFETs — the Drain-to-Source Resistance, commonly known as RDS on. We’ll compare four popular MOSFETs under identical test conditions to understand how RDS(on) impacts power loss, heat dissipation, and overall efficiency in electronic circuits.

RDS on in MOSFETs

🔍 What is RDS on in MOSFETs ?

RDS on is the resistance between the drain and source terminals when the MOSFET is fully turned ON. A lower RDS on means less power dissipation and greater efficiency, making it a crucial factor in selecting MOSFETs for power electronics applications. It is important when used in inverter applications powered by DC sources. Higher efficiency means longer backup time as losses are minimized. Thus, while using a number of them in parallel to share the load, very little loss appears, and less heating too, requiring small heatsinks. Enables compact and lightweight designs, especially important in mobile or space-constrained systems

🧪 Test Setup and Methodology for RDS on in MOSFETs

We tested four popular different N-channel MOSFETs:

IRFZ44N
IRF540N
IRF644
IRF250

Each MOSFET was:

Connected to the same power supply.
Driven by a positive gate drive through a resistor.
Tested using identical circuit configurations.
Measured for voltage drop across the load and current flow using voltmeters and ammeters.

🔧 MOSFET Analysis and Power Loss Calculations for RDS on in MOSFETs

1. IRFZ44N

Application: Low-voltage circuits
RDS(on): 0.022 Ω (22 mΩ)
Measured Current: 9.83 A
Voltage Drop: 0.21 V
Power Loss:
- Theoretical: I² × R = 9.83² × 0.022 ≈ 1.9 W
- Measured: V × I = 0.21 × 9.83 ≈ 2 W

➡️ Conclusion: Low RDS(on) results in minimal power loss.

2. IRF540N

Application: General-purpose MOSFET
RDS(on): 0.040 Ω
Measured Current: 9.68 A
Voltage Drop: 0.39 V
Power Loss:
- Theoretical: 9.68² × 0.040 ≈ 3.74 W
- Measured: 0.39 × 9.68 ≈ 3.7 W

➡️ Conclusion: Slightly higher RDS(on) leads to significantly more heat.

3. IRF644

Application: High-voltage circuits (up to 250V)
RDS(on): 0.28 Ω
Measured Current: 8.11 A
Voltage Drop: 2.27 V
Power Loss:
- Theoretical: 8.11² × 0.28 ≈ 18 W
- Measured: 2.27 × 8.11 ≈ 18 W

➡️ Conclusion: High voltage tolerance comes with high RDS(on), increasing thermal losses significantly.

4. IRF250

Application: High-power circuits (200V, 30A rating)
RDS(on): 0.085 Ω
Measured Current: 9.33 A
Voltage Drop: 0.80 V
Power Loss:
- Theoretical: 9.33² × 0.085 ≈ 7.4 W
- Measured: 0.80 × 9.33 ≈ 7.4 W

➡️ Conclusion: Moderate RDS(on), but still requires good thermal management.

📊 Power Loss Comparison Summary for RDS on in MOSFETs

MOSFET	RDS(on)	Current (A)	Voltage Drop (V)	Power Loss (W)
IRFZ44N	0.022 Ω	9.83	0.21	2.0
IRF540N	0.040 Ω	9.68	0.39	3.7
IRF644	0.28 Ω	8.11	2.27	18.0
IRF250	0.085 Ω	9.33	0.80	7.4

💡 Key Takeaways of RDS on in MOSFETs

Lower RDS(on) = Lower power loss = Less heat = Smaller heat sink
Always select MOSFETs based on your:
- Voltage requirements
- Current handling
- Acceptable power loss
High-voltage MOSFETs often come with higher RDS(on), making thermal management critical.
Understanding RDS(on) helps in optimizing efficiency and reliability in switching applications.

In conclusion-Certainly! Here are several key advantages of using low RDS(on) MOSFETs in power electronics and switching applications:

✅ 1. Lower Power Loss (Conduction Loss) in RDS on in MOSFETs

Power loss due to conduction is P = I² × RDS(on).
A lower RDS(on) significantly reduces heat generation during operation.
This improves overall efficiency, especially in high-current applications.

✅ 2. Reduced Heat Generation in RDS on in MOSFETs

Less power loss = lower thermal dissipation.
This often eliminates the need for large or expensive heatsinks.
Enables compact and lightweight designs, especially important in mobile or space-constrained systems.

✅ 3. Improved System Efficiency in RDS on in MOSFETs

Lower RDS(on) improves energy transfer efficiency in:
- Power supplies
- DC-DC converters
- Battery-powered devices
This leads to better battery life and lower energy costs.

✅ 4. Higher Current Handling Capability

A MOSFET with low RDS(on) can handle more current without overheating.
Suitable for high-current switching in:
- Motor controllers
- Inverters
- LED drivers
- Power tools

✅ 5. Better Thermal Management and Reliability

Lower heat stress improves the long-term reliability of the component.
Reduces risk of thermal runaway and component failure.
Allows operation in higher ambient temperatures.

✅ 6. Higher Switching Frequency Potential

Although RDS(on) isn’t directly related to switching speed, lower conduction losses allow operation at higher frequencies without excessive heating.
Enables smaller passive components (inductors, capacitors), further reducing circuit size.

✅ 7. Cost Efficiency in the Long Term

While low RDS(on) MOSFETs may be slightly more expensive, the:
- Reduced cooling requirements
- Improved efficiency
- Longer lifespan
  …often result in total system cost savings.

✅ 8. Ideal for Synchronous Rectification

In applications like buck converters, synchronous rectifiers using low RDS(on) MOSFETs replace diodes to drastically reduce losses from forward voltage drop.

✅ 9. Enhances Parallel Operation

In parallel MOSFET configurations, lower RDS(on) improves current sharing and reduces imbalance between devices, ensuring uniform thermal performance.

Would you like these formatted into a blog section or infographic-style list for your post or video?

📌 What’s Next?

In the next part, we’ll explore another critical parameter — Gate Capacitance — and how it influences switching speed and efficiency.

RDS on in MOSFETs: Mastering the Crucial Power Loss in MOSFETs Using 4 Real-World Devices