# Buck converter MOSFET (IRF4905) heating up

I tried to build a buck converter without any ICs, just transistors, a diode and passive components. Its purpose is to convert 12V to 5V at 2 Amps. It works, but the switching MOSFET (IRF4905) heats up very quickly and I can't figure out exactly what the issue is.

The idea was to build a sort of hysteretic buck converter. R3 and the trimmer R4 form a voltage divider, when the voltage across R4 is high enough (about 0.7V), Q5 opens and closes Q4, which closes the MOSFET. If the voltage on the output is too low, the MOSFET is open.

It regulates the voltage, but it heats up very quickly at 2A. I measured the switching frequency to be about 42kHz. I suspect that the MOSFET is not opening and closing fast enough, or not opening and closing completely. I can't verify that because I don't have an oscilloscope.

List of things I tried:

• Decreasing/increasing frequency by changing C2.
• Increasing inductance L1 to ~280uH.
• Adding a 1k resistor from the MOSFET gate to +12V.
• Adding 100nF and 4.7uF capacitors between Vcc and GND near the totem pole.

Does anyone know what could cause the MOSFET to heat up? Any suggestions would be appreciated.

• Welcome to EE.SE! Please show Vgs and Vds in the same oscillogram with good probing. You can’t develop an SMPS without an oscilloscope. – winny Dec 16 '19 at 14:43
• When EEs refer to opening and closing circuits we do it oppositely to that which a plumber might say thus, if a transistor "opens" it stops conducting and vice versa. Hence we have an open circuit that means there is no connection. Did it become too hot to touch? Heating up is expected. What flyback diode did you use? What inductor did you use? – Andy aka Dec 16 '19 at 14:54
• OK, I will try to use the terms properly next time. The transistor reaches 100°C in about 2 minutes. I used a schottky diode meant for switching power supplies (SR***). I'm not quite sure which one it is because I currently don't have the circuit near me. The inductor is a 100uH ferrite core inductor pulled from an old PC power supply. I later tried switching it for a 280uH toroidal core inductor. – Wonka Dec 16 '19 at 15:16
• The heating up of the P-MOSFET should not happen provided that it is switched on/off properly and quickly enough. Use an oscilloscope to monitor the Vgs of Q1. Also make sure that inductor L1 doesn't saturate. These are standard checks on any DCDC converter. I must state that it will be challenge to make a "good" 2A DCDC converter using only discrete components. – Bimpelrekkie Dec 16 '19 at 15:21
• Note that if you live in a 1st-world country you're going to spend more money on the extra circuit board space than you would on some 20 year old jellybean switcher chip, like an MC33063 -- and the chip will perform better. Note, too, that if you're willing to do surface-mount, you'll spend less money on a brand new switcher chip that operates at 200kHz to 1MHz than you will on the inductor you'll need for a 40kHz, 10W switcher -- it's why the new chips sell. – TimWescott Dec 16 '19 at 15:39

The transistor reaches 100°C in about 2 minutes.

The transistor unmounted on a heatsink has a thermal resistance of 62 degC per watt. That is how much it will heat up if dissipating a watt of power.

Its purpose is to convert 12V to 5V at 2 Amps

5 volts at 2 amps is 10 watts and losses in the MOSFET of about 1 watt (or a little more) are a likely scenario. Modern buck regulators are quite often quoted as having an efficiency of around 95% and a home spun one will be somewhat worse than this at around 90% so, I don't think your MOSFET is doing anything out of the ordinary.

It might continue to warm of course so you should consider mounting it on a small heatsink. Note that it does have a maximum operating temperature of 175 degC.

Sorry for not updating sooner. But I finally got the circuit to work.

I followed the advice that Russell McMahon gave me and I connected a 1 Megaohm resistor in parrallel with a 220pF capacitor from the base of Q5 to the collector of Q4. This should add positive feedback to the regulator.

The MOSFET barely heats up now and I measured the efficiency of the circuit to be around 81% with an input voltage of 12V and output of 5V at 2A. I will try to optimize it further but I think that most of the losses are from the freewheeling diode. A synchronous buck converter would be too complicated to make with discrete components, so I'll leave the diode.

It works better than I originally expected. And it should be possible to improve it further.

In the absence of a scope, I'd simulate that and see what's happening.

Probably what's going on is that you're correct and the FET isn't getting switched fast enough. Probably the least that you need to do is to add some positive feedback to the amplification stage at Q5. If I were determined to keep the circuit to only transistors, I'd add a capacitively-coupled PNP stage there that's set up with positive feedback so that Q5 switches, well, positively.

Here is an LTspice simulation of the schematic. Slightly different components were used due to library limitations, but even with these changes, the simulation works:

The traces are plotted logarithmically to try and show both small and large-scale detail.

So the simulation looks promising. But the only way to know for certain what the circuit is doing is to 'scope the gate lead of Q1. The simulation does hint at some non-ideal rounding and delay on the gate lead, but it's not too bad considering the spartan component count. Another possibility is high-frequency ringing at the switched node; if you have any ferrite beads, try adding one to Q1. Short of that, you'll have to get an oscilloscope.

Here is the .asc file:

Version 4
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SYMATTR Value 12v
SYMATTR SpiceLine Rser=0.1
TEXT 306 304 Left 2 !.tran 20m startup uic
TEXT 216 352 Left 2 ;Simulation shows 0.37W losses in Q1.\nWaveform is not ideal squarewave\nbut close enough. Try adding ferrite\nbeads to Q1 S and D pins.


I also agree with Hearth that your FET (R DS(on) = 0.02Ω) was operating in partial linear mode except you were able to measure a frequency of 42kHz at 5V 2A.

I suggest this design is inferior because the oscillator is not stable and depends on parasitics with phase margin reduced by the 5 transistor stages and Series Resonant Frequency of a couple hundred Hz with selected LC values.

The FET is overkill for a 2 A load rated for > 50A with 260A peaks but that adds to the Coss and very low RdsOn to cause spurious resonance where the phase margin must be negative in order to sustain oscillation.

There may be ways to increase the gate drive to a sharp pulse but it is not worth the overkill on FET switch and unstable design.

The PFET also guarantees high inrush power on start driving the ESR of the load capacitor then the inductor saturates.

All around , I'd say it's not surprising that it can regulate or oscillate but the load regulation is terrible without a voltage reference and step load overshoot for a wide range of loads would be terrible.

Conclusion.
The major missing ingredients to a good SMPS regulator design are " Good Specs, and good tolerance margins."

Poor topology.

• The load regulation and overshoot is not that important for what I need. I just want to reduce the heating and improve efficiency. – Wonka Dec 16 '19 at 19:11
• Like I said, where are your design specs? You have a nice sine wave oscillator with 5Vdc. You need a pulse oscillator for the Gate regulated with a Vref – Tony Stewart EE75 Dec 16 '19 at 19:22

I've run a simulation, and it doesn't look like your FET is switching at all. I think you may have built what amounts to a linear regulator, instead of a switching one; there's some oscillatory behavior when the potentiometer is changed, but it decays away into a stable linear mode rather quickly.

Under the assumption that you have accidentally made a linear regulator, the switching element heating up is to be expected. With the 2.5 Ω load, the power dissipation in the transistor is over 13 watts, which is more than enough to heat it substantially even with a decently-sized heatsink.

• I measured the frequency on the FET's gate using a multimeter and it shows about 42kHz. I also measured the ac voltage on it and it shows about 5V RMS. – Wonka Dec 16 '19 at 17:52
• @Wonka Seems my simulation is missing something then, perhaps parasitics affect circuit operation or maybe the simulator is flawed somehow. – Hearth Dec 16 '19 at 18:00
• What simulator did you use? – Wonka Dec 16 '19 at 19:14
• @Wonka CircuitJS. – Hearth Dec 16 '19 at 19:14
• falstad.com/circuit/circuitjs.html – Wonka Dec 16 '19 at 19:15