MOSFET Gate Drive Issues: Unstable Switching and Overheating in Buck Converter

[gavinray]

Hi everyone,

I’m designing a synchronous buck converter (input 24V, output 5V @ 3A) using IRFZ44N MOSFETs for both high-side and low-side switching. The circuit works in simulation (LTspice) with ideal switches, but the real prototype has two critical issues:

1. Problem Description​

• High-Side MOSFET Overheating: The high-side MOSFET (Q1) gets extremely hot (>100°C) even at 50% load.
• Unstable Switching Node (Vsw):Oscilloscope shows severe ringing and delayed turn-off (see waveform sketch below).
  • Waveform Details:
    • Rising edge: Ringing amplitude ≈ 8V (frequency ~30MHz).
    • Falling edge: Turn-off delay ~200ns (vs. 50ns in simulation).

2. Current Design​

• Gate Driver: TC4420 (non-isolated, 9V drive voltage).
• PWM Frequency: 200kHz.
• Dead Time: 100ns (configured via microcontroller).
• Gate Resistors: 10Ω for both Q1 and Q2.
• Decoupling: 10μF ceramic + 100μF electrolytic at input.

3. Attempted Fixes (No Success)​

1. Increased Gate Resistor (to 22Ω): Reduced ringing but increased switching delay, leading to higher Q1 conduction losses.
2. Added Snubber (RC: 100Ω + 1nF): Damped ringing slightly, but Q1 still overheats.
3. Replaced MOSFETs with Lower Rds(on) (IRF3205): No improvement.

4. Key Questions​

• Q1: Could the gate driver’s sink/source current (3A) be insufficient for the IRFZ44N’s Q_g (110nC)?
• Q2: How to balance gate resistor selection between ringing suppression and switching speed?
• Q3: Is the layout inductance (e.g., high-side gate loop) causing the turn-off delay? If so, how to estimate its impact?
• Q4: Are there SPICE model parameters (e.g., Cgd, gate resistance) that I should adjust to better match real-world behavior

5. Supporting Details​

• Layout: Hand-soldered prototype with ~5cm traces for gate signals.
• Probing Setup: Used 10x probe with ground spring (bandwidth 200MHz).

Request:

• Has anyone faced similar issues with TC4420 and IRFZ44N?
• Recommendations for gate driver ICs (isolated/non-isolated) or layout optimizations?
• Tips for modeling PCB parasitics in LTspice.

Thank you in advance!

Waveform Sketch (Vsw):

	Rising Edge:
	24V |¯¯¯\_/¯¯¯\_/¯¯¯
	| | |
	Ringing (8V)
	Falling Edge:
	5V |‾‾‾‾\___________
	| 200ns delay

 

[KlausST]

Hi,

6 minutes before you started this thread .. you replied in a different thread (most probably AI content) this:

Share your schematic and scope/probe setup details if the issue persists. Community members can help diagnose further!

The same applies to you.

Klaus

 

[dick_freebird]

I think 100 ohms is pretty high for a gate resistor.

 

[Easy peasy]

you'll get ringing in DCM - do you disable the lower switch in DCM to avoid reverse current in the choke and lower switch ?

the IRFZ44N internal diode may not be that flash - hence the high side sw losses when this lower diode is recovering => use better parts

also make sure the lower device stays OFF when the high device is slammed on ( GD impedance - use reverse schottky across GD R ) - if all else fails reduce fsw to 65kHz say - bigger choke - and see if sw losses come down accordingly . . .

--- Updated Wednesday at 2:55 AM ---

you can see here:

the Trr is 95nS @ 25 deg C - so worse at higher temps, 260nC in 30nS say is 8 or 9 amps reverse at 25 deg C, and at the paltry 100A/uS of reducing dIf/dt
so this part is a poor choice, you can see from note (3) I sd can be 25A for falling dif/dt of 230A/uS ( slow ).

--- Updated Wednesday at 2:56 AM ---

Also - we can't see your heatsinking - if any ?

--- Updated Wednesday at 2:57 AM ---

The circuit works in simulation (LTspice) with ideal switches, but the real prototype has two critical issues:

did you use the sim to measure the losses in the high side device ? I suspect you might be surprised if you now do this

 

[D.A.(Tony)Stewart]

Commutation cross-conduction failure or deadtime can create abrupt ringing with ESL. (source trace inductance)

Ringing is an RLC normal characteristic. Excess ringing needs RC snubbing to dampen switch opening.

Can you measure this in xxx ns? out of 5us cycle time? This may reduces as temperature rises.

Normally pulling down Vgs 10 ohm gate resistors is done faster with small signal diode anode to gate to ensure faster turn-off.

It is critical that any design follows the recommended layout for signals, ground and component orientation in SMPS.

--- Updated Wednesday at 6:00 PM ---

Is this your 1st SMPS rodeo?

There are a few details in the design you must specify and then validate in your design with details including DCR, ESR, L, C, Cout, Cin , deadtime, Trr. Failure to pay attention to these variables will cause the symptoms you have making it impossible to guess for the reader here.

If you wish to learn from your attempts , we must know everything about these parameter and the layout with validation DSO photos. I assume you know how to estimate Q and SRF from a series RLC network from a ringing pulse response, Wiki or Grok may explain

If you want an improved design TI has a tool for choosing the optimum design using their parts. Others like AD and Microchip as well.

Notice how the schematic includes DCR and ESR. Too little and it rings with no load, too much and it becomes lossy. But layout is critical.

e.g.

--- Updated Wednesday at 6:02 PM ---

Notice the above example costs < $2 in parts is > 97% efficient and does not need any external FETs for 5A out.

--- Updated Wednesday at 6:12 PM ---

The common issues with SMPS are that using lower RdsOn also increases Cout and high Qout means the stored energy will cause ringing during transition with L.
Thus high levels of integration optimizes these FET properties and reduce path length when it adds ~ 0.8nH/mm can affect Vgs to Vds.

FET commutation like the arcing of brush motors becomes a design problem with fast turn-off low, RdsOn and thus resonance of the FET junction capacitance with the switching inductor and the parasitic source ESL which requires great care with very short paths.

Also only two input decoupling caps can be a problem without knowing the non-ideal parameters and the source impedance which can raise the Power Delivery Network (PDN) impedance at some interactive parallel resonance.