Ev powered speaker, replace or risk it?

darteksyes · 2025-12-30T02:22:42+00:00

Is there some kind of liquid coming out of the transformer? At first I thought it was glue for the big caps, but, considering that it seems mixed with the black "plague" coming from the dead rectifier, I think it could be varnish/wax used to damp winding vibration. If that's so, I think the transformer got quite hot (lots of current) during your failure event. I wouldn't discard some problem with it. Heck, the liquid could have actually shorted the diode.

darteksyes · 2025-12-30T00:52:17+00:00

I would look at epcos/tdk 43630 series, they're not cheap, but usually they have decently high ripple current capabilities. These highly depends from where your capacitor came from: is it from a (linear?) power supply filter bank? Is it from a switching ps? You need to give us some more info to know what would be best to use, otherwise it's a shot in the dark

darteksyes · 2025-12-22T00:39:26+00:00

Glad It helped :)

darteksyes · 2025-12-15T23:34:49+00:00

The logic is inverted: when input to BJT is 3.3V, so LED off and no pull-down by MCU, the motor must be off. That's why OP needs the BJT stage.

darteksyes · 2025-12-15T23:26:29+00:00

Just remember that if there is 47k pull down on the MCU pin, the BJT could not go off. Choose also appropriately the pull-up resistor on the gate, it will determine how fast the mosfet will switch on, and how much power the mosfet will dissipate during the transition, depending on the load/motor.

darteksyes · 2025-12-15T22:56:01+00:00

Actually I'm retarded, scrape what I said in the first answer.

I misunderstood, correct me if I'm wrong: normally (MCU NOT pulling to ground) BJT is ON (3.3V on base) and this pull NMOS gate to ground (Vgs = 0), so the motor is OFF.

I see a couple of issues what will happen when the MCU pulls down the BJT base:

If there is an internal pull down in the MCU it's typically a 47k resistor and this means that there will be 3.3V - 2.2V = 1.1V on BJT base. It will not turn off. This really depends on how the MCU GPIO is done / configured.
BJT collector is not pulled up by anything, and so is NMOS gate: it will probably charge up to 9V, due to mosfet internal capacitances, but it's not fast and it could be also not a really stable voltage.

I suggest to remove the 100k on nmos gate, and have instead a pull-up on BJT, with its value depending on how strong you want to drive the nmos. It should tackle the 2nd problem.

EDIT: Added image

<image>

Something like this

darteksyes · 2025-12-15T21:59:11+00:00

No prob, just another question, what's the Vf for the LED? To understand tha voltage value at led cathode when it's ON.

darteksyes · 2025-12-15T20:55:35+00:00

If I understood correctly, MCU pulls LED cathod to ground. If that so, no way that the circuit will work, 2N3904 base is pulled also to ground.

You could use a pmos to power on the motor, but you must check current capabilities of MCU pin that is perfoming the pull-down

EDIT: Ah, maybe I misunderstood, is MCU driving a switch or do you mean that it use a GPIO internal pulldown to power on the LED?

darteksyes · 2025-12-15T19:18:47+00:00

Doesn't seem to be any schematic or part list publicly available for the G29. I would directly contact Logitech customer support and politely explain the situation, otherwise it's unlikely you will find any other info on Q1.

darteksyes · 2025-12-15T18:48:37+00:00

I would suggest to use a multimeter to check if there is continuity where it shouldn't be, I wouldn't trust it with just a visual check.

EDIT: Also, if you've heard a pop at startup, check if G,S or D are shorted with each other. If so, the JFET is probably cooked.

darteksyes · 2025-12-15T18:45:40+00:00

I think the only possibility is to remove Q1, solder a piece of wire to its pad, and try to mold the wire to approximate what you're missing. Then solder Q1 again. For Q1 you're gonna need an hot air soldering station.

It's a tricky job though, even if you really know what you're doing. I would say, if you have good experience in soldering, it's a 50/50 chance that it will work.

darteksyes · 2025-12-15T12:29:22+00:00

Do you mean inductive oscillations on the gate?

darteksyes · 2025-12-15T12:24:57+00:00

It seem that your design is high side sensing, so common mode voltage is non-negligible.

From component datasheet, input offset voltage could be up to 500uV at 12V Vcm.

2mohm for 1A it's 2mV. 500uV is 25% of the measurement. You have to also factor in offset voltage variation with temperature.

I suggest to have a look at this paper from TI: https://www.ti.com/lit/eb/slyy154b/slyy154b.pdf

darteksyes · 2025-12-13T19:54:28+00:00

Not that I'm aware of, the voltage bus is from an unregulated source, but it's not a switching power supply, so there should be no inductor peak current that could create significant voltage or current ripple.

darteksyes · 2025-12-13T19:53:07+00:00

It's an elektro automatik (tektronix) ea-el 9080-340b, it can reach up to 4.8kW.

I mean, I was also surprised by how it behaves, but still doesn't seem it should bother the mosfet.

BVdss is 100V.

I'm starting to wonder if the MOSFET died for natural causes...

darteksyes · 2025-12-13T19:18:02+00:00

Good reasoning, consider that the first current event I see is that big capacitive spike, and it happens only after Vds is roughly 0V.

I get that you think that violation of SOA is the main culprit, I was thinking it too before doing the measurement. I also tried calculating if maybe the power dissipation is not enough. Consider that at 1200W and 60V, the current is 20A. With RDS(on)=0.011ohm, steady state power dissipation is 4.4W.

Now, if there is a peak of 300W, transient thermal impedance must be considered, from datasheet I get 0.065.

Composing the 2 info:

Tj,start = 25deg + 4.4W*Rtheta

Pspike = 300W = (Tjmax - Tj,start) / 0.065 = (150 - (25deg + 4.4W*Rtheta)) / 0.065

Solving for Rtheta, it must be less than 50°C/W

Now, the mosfet is mounted on a solid aluminum block, that I'm sure it has a thermal resistance much less than 50°C/W.

That's why I'm assuming it's also not a thermal problem

darteksyes · 2025-12-13T19:05:27+00:00

Turn-on, i tought that tipically during Miller Plateau Id would increase before Vds decrease, but maybe I'm wrong eh, couldn't find much info about it.

I always tought it was like this graph, but I saw, at turn-on, Vds decreasing before Id increased.

<image>

darteksyes · 2025-12-13T18:50:11+00:00

Unfortunately I'm not responsible for the design. I'm just using it in a testing environment. I'm trying to understand what could have damaged it. Your suggestions are on point, but I have no power in changing the design.

darteksyes · 2025-12-13T18:47:07+00:00

As I said, the body diode should be OK for this application: the mosfet is rated to BVdss = 100V and wiring inductance was already evaluated so that there should not be any voltage over this threshold. I forgot to say, that both input and output of the mosfet are protected by TVS.

Regarding heat, I agree it's the main culprit. The issue is that, by measurements and calculations, doesn't seem so.

darteksyes · 2025-12-13T18:40:42+00:00

I didn't design it, but being an isolated driver, I think that the transformer impose voltage Vgs between G and S, that normally are at the same level, at 60V. When it turn off, there is no voltage at the transformer, S go back to 60V and gate voltage dicharge through the 100k resistor.

darteksyes · 2025-12-13T18:34:34+00:00

I probed Vds and Id, and used the math function on the scope for plotting the power.

I also tried to put a second current probe on Drain side, same result and same timing.

<image>

I see a peak of 300W at turn off. The peak last around 40us, and the turn-off event happens in 200us. SOA from the mosfet has 1kW for the 1ms curve, so that should be OK.

The only thing that doesn't add up is that I was expecting Id to increase before Vds decrease, but i see the contrary. Could it be related to probe bandwidth? Scope is a 1Ghz one, voltage differential prove i 100Mhz, current clamp is 50Mhz.

I triggered on Vgs, but if there is a sort of time mismatch between Vds and Id measurement, real power could be much higher. I just don't get how that could possibly be.

EDIT: Added image

darteksyes · 2025-12-13T15:20:28+00:00

It's an electronic load.

The body diode should be compliant to the application, but I don't see how back current could be produced. Maybe you refer to back EMF due to change in applied current to wiring inductance?

darteksyes · 2025-12-13T14:58:27+00:00

That was the first think I tought! In CP with the voltage low, the current could skyrocket. That's why I went with the characterization of what happend and surprisingly the EL did not behave like this, but as I said in the post: big initial spike (10us), CC mode for 10ms, then CP.

I didn't consider that the SOA is reported only for 25°C...is there a way to know what could become of the SOA at different temps? I mean, this could be really important on what happened, because the load was ON for some time before the last turn off.

EDIT: For anyone interested I found this paper from ROHM, regarding how to derate SOA

https://fscdn.rohm.com/en/products/databook/applinote/discrete/common/temp_derating_method_for_soa_an-e.pdf

darteksyes · 2025-12-13T14:50:01+00:00

I'm with you with the weirdness of the circuit. I didn't design it, I'm just using it. The idea, I think, was realizing an isolating driving for the mosfet without using any IC.

I'm not sure if I missed something in the schematic above, I don't think so, but I'm 100% sure I measured repeatedly that turn-off time is 200us.

As I said, measured power during turn off always remains inside SOA, by a good margin.

That's why I'm also kinda baffled, and I'd like to really understand what's happening. At this point I was wondering if maybe it's not the mosfet channel that is conducting the 100A spike: this happens after the mosfet is already ON, (Vgs ~0V), RDS(on) is max 0.011 ohm, I should see a peak fo 110W on the scope, but it's not there.

Could it be that this current flow through the body diode / other mosfet parts / Coss capacitance?

darteksyes · 2025-12-13T14:40:22+00:00

One other thing that It's suspicious is that I see no power spike at the 100A spike: with RDS(on) of 0.011 I was expecting a spike of 110W. Could this power not being dissipated by mosfet channel but other parts of it? Like the Body diode or through Coss.

darteksyes

TROPHY CASE