Overview
During hardware development for a router product, the customer required a non-standard POE power output with selectable voltages of 12/24/30/48V and a maximum output power of 24W. The design used a flyback power supply reference design based on the MP3910. During debugging, the NMOS MOSFET (SUD50N06) overheated severely. Without load the outputs were normal. When loading began (about 50% load), the MOSFET heated quickly and the output voltage was pulled down to about 9V regardless of the selected output. The TLV431 reference measured around 1V instead of the expected 2.5V reference. Initially the TLV431 was suspected, and a different board appeared to regulate correctly, but the MOSFET still became very hot and smoked within ten seconds under load.
Root Cause
After consulting the chip vendor FAE, the cause was identified: the gate series resistor that limits current between the MP3910 driver gate pin and the MOSFET gate was the wrong value. The schematic specified 4.99 ohm, but the BOM used 4.99 kohm. After replacing the resistor with the correct value, the output voltage returned to normal and the MOSFET no longer overheated.
Oscilloscope Observations
Using an oscilloscope to observe the MOSFET gate waveform, the measured rise time was about 1.32 μs and the fall time was under 160 ns (measured ~50 ns).
The MOSFET datasheet specifies driver rise/fall requirements of rise time < 35 ns and fall time < 80 ns.
Conclusion: the excessively long rise time forced the MOSFET to operate in its linear region rather than as a switch, causing significant heating during turn-on.
Solution and Result
The gate series resistor (Rg) was replaced. A 4.99 ohm resistor was the intended value, but a 22 ohm resistor was used at the time due to parts availability. After this change, the gate waveform improved and Ton/Toff approached the datasheet requirements. At 24V output with a 27 ohm load, the output power was 21.3W, the output voltage was normal, and the MOSFET remained barely warm.
Summary 1: Common Causes of MOSFET Heating
1. Circuit design that forces the MOSFET to operate in the linear region rather than as a switch. For N-MOSFET switches, the gate voltage must be several volts above the source to fully turn on; for P-MOSFET the opposite applies. Insufficient gate drive leads to large voltage drops and increased power dissipation.
2. Excessively high switching frequency, which can increase switching losses if not managed properly.
3. Inadequate thermal design. Even if the drain current is below the MOSFET's nominal rating, insufficient heat sinking can cause excessive temperature rise.
4. Incorrect MOSFET selection, including underestimating on-resistance and other parameters that affect losses.
Summary 2: MOSFET Operating States
MOSFET operating states include: turn-on transition, conduction state, turn-off transition, and cutoff. Main loss mechanisms are switching losses, conduction losses, leakage losses, and avalanche energy. Switching losses often dominate.
Main failure mechanisms: overcurrent (sustained or transient), overvoltage (D-S or G-S breakdown), and electrostatic discharge.
Summary 3: Turn-On/Turn-Off Tradeoffs
The MOSFET switching process involves many variables. Slower switching reduces the risk of Miller-induced oscillation but increases switching losses and heating. Faster switching reduces switching losses but can provoke Miller oscillation and increase losses if not properly controlled. Good layout for both the driver and the main current loop is important. A typical guideline is to keep the turn-on process under 1 μs.
Summary 4: Key MOSFET Parameters for Selection
Qgs: charge required to charge the gate from 0V to the Miller plateau, associated with Cgs (input capacitance).
Qgd: charge required across the Miller plateau.
Qg: total gate charge, including Qgs and Qgd.
These charge values are typically in nanocoulombs.
Rds(on): on-resistance. Lower Rds(on) reduces conduction losses for a given voltage rating.
Selection guideline: choose devices with low Qgs, Qgd, Qg and low Rds(on) to minimize both gate drive demands and conduction losses.
