# Boost SMPS getting very hot. Why?

I tried to experiment with boost SMPS with multiple supply rails and I almost burned myself when touching parts when running.

Here is the circuit and the part getting hot:

simulate this circuit – Schematic created using CircuitLab

I am trying to kick the 8.8V from my regulator to 15V. The PWM comes from Arduino, assuming near perfect regulation from the regulator powering this experiment.

In my first experiment with 600Hz PWM and 10mH the inductor is very noisy and gets hot. I get 10V across R1.

In my 2nd attempt running 31.5kHz PWM and 100uH inductor at L1 the MOSFET start to get extremely hot. I am also getting 53V (!!!) across R1. I probably damaged the diode.

Can anyone tell me what did I do wrong in this?

## 1 Answer

Your circuit topology looks basically right.

However, my first guess is that L1 isn't really a 10 mH inductor. If it were 10 µH, that could explain things. You are switching at 31.5 kHz, which has a period of 31.8 µs. You show a square wave, so the switch on and off times are half that, or 16 µs. Even with the inductor current starting at 0, at the end of one pulse you'd get (8.8 V)(16 µs)/(10 µH) = 14 A. You didn't provide any datasheets, but that's quite likely well past the saturation of L1, and possible the current capability of M1 and the supply.

Another possibility is that M1 isn't turning fully on. Again, since you didn't provide any datasheet, I can't comment further. Check what gate voltage M1 is specified for, then compare that to whatever you are driving it at. If the PWM is only a 5 V or 3.3 V digital signal, then quite likely M1 isn't turning fully on when the signal is high.

The next possibility is that the gate isn't being driven with enough current when switching. That is easily determined just by looking at the gate signal. Is it a nice and sharp square wave, or does it take it's time getting from one state to the other. If the latter, then the FET is spending too much time in between full on and full off. One possible solution is to use a FET driver chip.

## Added:

Now that the datasheet for the FET has been provided (although only in comments), we can see this FET is clearly inappropriate for this gate drive. The low Rdson applies at 10 V gate drive. The gate threshold voltage can be as high as 4 V, and there is no guarantee what it does with only 5 V on the gate.

Since you only want 15 V out, a low voltage FET that is actually specified for 5 V gate drive would be good choice. For example, check out the IRFML8244. It can be driven directly from a digital logic output in many cases.

However, replacing the FET only addresses one of the possible issues. At such a slow switching frequency, you have to be careful about inductor saturation. You also need to do something to prevent the output from going too high. This could be as simple as a voltage divider from the output feeding a comparator input of the micro, which is set up to kill the PWM output when above some threshold.

• My MOSFET is indeed IRF540 but the inductor's seller did not provide me a datasheet. I am driving it at 5V gate voltage though, but without the boost I have only 9V at most. Also, my power supply have protection that will shut itself down if current draw exceeds 1A. Jan 19 '15 at 18:16
• Great advice from Olin. His last point is very likely part of the problem. Digital PWM outputs from microcontrollers do not supply enough current to drive larger FETs even at relatively low switching frequencies. The switching losses are too high. I would investigate using a FET driver chip as Olin suggests. Jan 19 '15 at 18:18
• The IRF540 is NOT a logic level FET and 5V is not enough gate drive. If you only have 5V choose a more appropriate FET, or use a gate driver chip and the 9V rail (You still won't get the recommended 10V gate drive at that point, but the FET should be well enhanced.) Jan 19 '15 at 18:20
• @Maxth: OK, so what about the remaining things I asked for? Again, provide a link to the FET datasheet. If you don't have a datasheet for the inductor, what makes you think it's 10 mH? That is quite a large inductor. What is it's DC resistance? Again, what gate voltage does the FET need? What does the gate waveform look like? Jan 19 '15 at 18:21
• @JohnD IRL540 cost me twice as much so I don't think I would be able to afford using them in experiments. I do have IR2110 driver chip though Jan 19 '15 at 18:22