Step Up Converter 12v to 220V

[cometa_gsm]

Hello, My application is a 12V to 220V step-up converter, and I'm using the UCC28086 PWM controller with an IRF3205 MOSFET. The load is 30W, but the MOSFET is getting hot. What could be the reason? The oscilloscope showed an undershoot on the gate of the MOSFET. Is this undershoot normal, or is something wrong? Thank you.

[coromonadalix]

do you have a schematic to post, it will help others ... to help you   

[cometa_gsm]

The third picture in the attachment shows my circuit diagram.

[inse]

Where is the ground connection of the scope?
Ideally probe directly at G/S of the MOSFET with nothing else connected at the scope.
A differential probe would be the best for this task.

[cometa_gsm]

Ground scope connected to Mosfet source.

[Doctorandus_P]

The most common cause for hot FET's is insufficient gate drive for the FET's.  This causes them to switch slowly, and during the switching the FET generates heat, because there is both a voltage over it and current though it at the same time. This seems to be confirmed by the scope pictures (Added later?) Around the center line of the scope, you see the "miller plateau".

There are several ways to fix this:

• Use a gate driver that can deliver more current.
• Use fets with lower internal capacitances.
• Reduce the switching frequency.

[Ian.M]

Also, there are a couple of things I don't like about that schematic:

• The circuit doesn't show any snubbers on the MOSFET drains - that means every time a MOSFET switches off the residual energy stored in the transformer's primary's leakage inductance has nowhere to go except to avalanche the MOSET.   An IRF3205 is rated to 55V abs. max. Vds, and although it *is* avalanche rated, if you want them to have a reasonable chance of running cool, unless the transformer has been wound to minimise primary leakage inductance, you need to clamp or snub the turnoff spike below that.   Clamping at about 48V would be good as that's three times nom. Vin.  With a center-tapped feed push-pull driven transformer, the drain will swing to double the input voltage when the other MOSFET turns on, so the clamp *MUST* be above *DOUBLE* the max. input voltage.
• No current limiting, which means if anything goes wrong you let the magic smoke out!
  I'd strongly recommend doing the design calculations as per UCC28086 datasheet and patching in a suitable current sense resistor and associated components Rs and Cf to the CS pin, and Rset to ground on the ISET pin.   The current sense resistor also gives you a good place to scope the MOSFETs total source current so you can check you aren't transiently exceeding the MOSFET SOA.

@Doctorandus_P,
Running the numbers from the scope traces, the falling miller plateau is about 0.6% of the cycle time, and if we assume the rising miller plateau is similar, that's about 1.2% total - not ideal but not unreasonable.    The frequency is only 42.5 43.5 kHz which isn't  that high.

[youngda9]

Your schematic is a copy of the vendor schematic on sheet 1 of the datasheet.  Except you removed the feedback and current sensing.

https://www.ti.com/lit/ds/symlink/ucc28086.pdf?ts=1733930446213&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FUCC28086

[cometa_gsm]

Yes, I edited the vendor schematic because I don’t need the current sense circuit. My main question is: Is the small undershoot normal, or is it a problem for this operating mode?

[trobbins]

I'd anticipate the gate ringing after it is turned off is the Cdg coupling from the drain waveform.

Are you using just a simple resistive loading to cause 30W secondary side load?

Perhaps show a photo of your primary side layout, as there may be parasitic loops and other quirks arising, especially as your schematic implies you only used 1uF of 12V input decoupling for a 30W converter.

Also what transformer did you use - commercial or diy or ?  - even at 44kHz, leakage inductance could harbour significant power that is going to be dissipated as a loss.

[bostonman]

The most common cause for hot FET's is insufficient gate drive for the FET's.

I have a question regarding gate drive.

I thought the gate takes very little current to turn on (one of the benefits of FETs) but takes higher voltage such as >3.5v to turn on (the downside of FETs).

Is it the charging rate that's affecting the turn on time?

[cometa_gsm]

Thanks everyone, I replaced the mosfet with a 100V one and there is no undershoot anymore, I think it was the cause of the avalanche.