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Heating in LLC HB mosfets ?
Heating in LLC HB mosfets ?
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Old 13th April 2019, 07:06 PM   #1
TroelsM is offline TroelsM  Denmark
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Default Heating in LLC HB mosfets ?

Hi All.

I have been playing with a small LLC for a while.

The first prototypes had very little heating in the primary HB mosfets.

Now Ive assembled a new prototype and the mosfets heat up more than before. It works and output is stable, but the mosfets get to 80degC with an output of approx 75W.

Mosfets are IRFP450 and heatsink is approx 40x60mm alu plate. Same setup as the old prototype. It is pure ZVS-switcing according to the voltage-dip at the gate before turn-on.

The gate-waveforms look fine to me. Deadtime is OK. around 100Khz and thats a little above resonance.

A potential problem is that the highside-gate is not "lifted" more than 10V above Vcc. Although IRFP450 should be fully on with Vgs at 10V it looks odd. Vcc to L6599 is around 15V.

The HB switchnode it dos not get a all the way to Vss or Vcc and that look strange to me. Although the circulating currrents in the LLC-network is pretty high, I dont think they are high enough to justify that big a "on-resistance-loss".

The IRFP450 are NOT fakea, but the L6599 is from ebay.

Technical details:

Input: 230Vac
Output: +/-60Vdc
Res-cap 88nF (split-cap-design with 2x44nF)
Inductance: 105uH + 17uH

I know its hard for you guys to troble this remotely but any hints and ideas are welcome.

I've ordered a 100x100mm PCB for the next prototypes. If it works I'l be happy to share the files.

Kind regards TroelsM
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Last edited by TroelsM; 13th April 2019 at 07:09 PM.
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Old 13th April 2019, 07:30 PM   #2
5th element is offline 5th element  United Kingdom
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To my understanding there are two things that can cause FET heating in a situation like this.

The first is useful work being done in which case the current through the transformer and then output from it should be proportional to what you'd expect should be flowing through the FETs.

You should be able to back calculate ball park expected losses within the FETs and see if matches the temperatures you're seeing.

The other is cross conduction through the FETs either through too little deadtime compensation or some other issue with the switching causing what you don't want to happen. Ie the heating is from small pulses of short circuits through the half bridge. Not long enough to destroy anything but nonetheless unwanted.

Have you scoped the waveforms of the high and low side FETs together to check for any cross conduction?
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Old 13th April 2019, 08:26 PM   #3
rayma is online now rayma  United States
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Quote:
Originally Posted by TroelsM View Post
A potential problem is that the highside-gate is not "lifted" more than 10V
above Vcc. Although IRFP450 should be fully on with Vgs at 10V it looks odd.
First, I'd get a full 15V on the gates, to minimize the conduction loss.
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Old 14th April 2019, 04:55 PM   #4
TroelsM is offline TroelsM  Denmark
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Thanks for the input. I havnt checked for shoot-through yet, but gate-waveform indicate an ok dead time. Also, shoot-throug would only explain the heating, but not the HB not getting close to the rails?

I would like to drive with 15V, but for some for reason the driver Will not deliver that.
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Old 14th April 2019, 04:58 PM   #5
rayma is online now rayma  United States
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Quote:
Originally Posted by TroelsM View Post
I havnt checked for shoot-through yet, but gate-waveform indicate an ok dead time.
Can you post the schematic and some scope waveforms?
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Old 15th April 2019, 10:47 AM   #6
TroelsM is offline TroelsM  Denmark
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Schematic is very basic standard LLC:
[IMG]Click the image to open in full size.[/IMG]

The transformer (T1) has a little leakage-inductance and the rest is implemented in external inducor (L2). Capacitors C2 and C9 are the resonant C in LLC.

I'l post some waveforms later.
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Old 15th April 2019, 12:59 PM   #7
kees52 is offline kees52  Netherlands
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Quote:
Originally Posted by TroelsM View Post
Hi All.

I have been playing with a small LLC for a while.

The first prototypes had very little heating in the primary HB mosfets.

Now Ive assembled a new prototype and the mosfets heat up more than before. It works and output is stable, but the mosfets get to 80degC with an output of approx 75W.

Mosfets are IRFP450 and heatsink is approx 40x60mm alu plate. Same setup as the old prototype. It is pure ZVS-switcing according to the voltage-dip at the gate before turn-on.

The gate-waveforms look fine to me. Deadtime is OK. around 100Khz and thats a little above resonance.

A potential problem is that the highside-gate is not "lifted" more than 10V above Vcc. Although IRFP450 should be fully on with Vgs at 10V it looks odd. Vcc to L6599 is around 15V.

The HB switchnode it dos not get a all the way to Vss or Vcc and that look strange to me. Although the circulating currrents in the LLC-network is pretty high, I dont think they are high enough to justify that big a "on-resistance-loss".

The IRFP450 are NOT fakea, but the L6599 is from ebay.

Technical details:

Input: 230Vac
Output: +/-60Vdc
Res-cap 88nF (split-cap-design with 2x44nF)
Inductance: 105uH + 17uH

I know its hard for you guys to troble this remotely but any hints and ideas are welcome.

I've ordered a 100x100mm PCB for the next prototypes. If it works I'l be happy to share the files.

Kind regards TroelsM
IRFP is 0.40 ohm on resistance, maybe the reason, and you need really to drive the mosfets so it is in on completely.

The LLC is quite critical, you need experiment with it. I do not now it is for audio amp purposes, because LLC is maybe
to slow reacting on peaks of power.

regards
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Old 15th April 2019, 03:10 PM   #8
TroelsM is offline TroelsM  Denmark
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I borrowed a better scope at work and made a few measurments.

This is a test-setup with a DC-source instead of the mains.

The two gate-signals look fine to me: ( shot taken without anything connected to the HB-midpoint, to get a "clean" reading).
Yellow is highside Gate. Blue is low side gate. Vcc 30v
Click the image to open in full size.




Blue is low side Gate. Yellow is HB mid-point.
No load in the secondary and freq is locked around 100Khz.
With 22Vdc i get this:
Click the image to open in full size.
Gate swings nicely to approx 15V. HB mid comes close to GND. Looks good

But raising the voltage above that, reveals something odd: HB-mid does fall quickly any more.
This is with Vcc at approx 30V. Gate still looks good.
To my understanding this is a sign of the lower mosfet not turning on fast enough.
IF it was cross-conductiong for that long is would heat up very fast even with no load.
Power draw from psu is around 40mA at 30Vdc

Click the image to open in full size.

Without load fets are still cool- but with load they heat up. I makes sense that the I^2/R rises if the mosfet does not turn fully on fast enough, but WHY is that slow, when the gate is fast?
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Last edited by TroelsM; 15th April 2019 at 03:24 PM.
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Old 15th April 2019, 03:20 PM   #9
rayma is online now rayma  United States
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Quote:
Originally Posted by TroelsM View Post
Schematic is very basic standard LLC
Can you also post the gate drive circuit? What value are the gate drive resistors?
There seems to be a problem in the device switching. If it switches slowly,
the conduction loss can be high with a load. Are you using floating differential
voltage probes? The upper gate-source voltage waveform would also be useful.

Last edited by rayma; 15th April 2019 at 03:30 PM.
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Old 15th April 2019, 03:29 PM   #10
TroelsM is offline TroelsM  Denmark
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Drive is text-book ( I think, beginning to suspect some rookie-mistake in my prototype)
Click the image to open in full size.

The signals change very little with different values for the drive-resistors.
If the gate-drive was totally off it would show up on the gate-drive-waveforms above?

No floating probes for this as the voltage is only 30Vdc for this test
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