IcePower module

Hi to all my fans,

just completed repairs on one of these:

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Found it fitted inside a mini-size bass instrument amplifier, which the own er had just purchased over the net and failed on him at first power up.

Smelling a rat, I checked the AC switch on the rear panel first - blow me d own, it was set to 120VAC and we live in a 240V country.

Reset the switch, then it drew zero AC current - no surprise.

Replaced the blown 5AT fuse in the IcePower module and then the PSU began h iccupping - drawing an amp or two very briefly then shutting off every few seconds.

IME feeding double AC supply voltage to a unit with a SMPS is almost always fatal, sure the AC fuse blows but semis in the HV part of the circuit fail in a fraction of a second. But these must be OK this time because the PSU was hiccupping - ie working but sensing an overload so shutting down.

Soon enough, multimeter testing showed two small power mosfets on the outpu t side of the SMPS tranny were shorted while two others seemed OK. But what the heck are mosfets doing there ?

Well, it's the fist time I have come across seen a *synchronous rectifier* in such an unit. IcePower had certainly gone all out to reduce losses and h eat in this module by using a synchronous bridge.

The particular TO-252 fets here are made by ON, number FDD86110 rated at 10

0V and 8mohms on resistance.

Removing the duds cleared all shorts and after fitting some new ones ( deli vered to me by Element14, from their Singapore warehouse ) I gingerly Varia ced the unit up from zero.

No hiccupping this time and it began to run normally - after which it passe d all my usual bench testing, no problems.

FYI:

AC current draw was 120mA at idle, 2.4A at full power (340W rms/4ohms) No PFC and inrush surges were about 25A for a few milliseconds.

There was a residual sine wave signal on the audio output of about 1V at 50

0kHz - no biggie except it makes THD testing a right PITA.

I find it odd the way the SMPS failed, just two mosfets in the secondary re ctifier, nothing else. For a brief time, the DC supply to the switching fet s must have been nearly double voltage.

Seems the two rectifier fets failed SHORT instantly and protected the rest of the circuitry - remember the power supply is NOT regulated, just a squar e wave inverter running at 100kHz.

I also feel that supplying an expensive amplifier to a buyer living in a 24

0V country with the AC inlet set to 120V and giving NO warning is criminal. BTW the amp was fitted with a regular IEC inlet which most of the world us es for 240VAC.

Any comments?

... Phil

Reply to
Phil Allison
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On Thursday, September 19, 2019 at 11:52:31 PM UTC-4, Mr. Charm & Warmth wr ote:

**There's internet in hell?

down, it was set to 120VAC and we live in a 240V country.

240V country with the AC inlet set to 120V and giving NO warning is crimina l. ** I've found that when an item fails, the customer will flip every switch in a vain attempt to "repair" the failure. Even though the amp failed on first use, we don't actually know where the switch was set when the custome r obtained the amp. There's an even (or better) chance the customer flippe d it after the amp failed.
**Good repair.
Reply to
John-Del

It may seem like this PSU is fancy, but in a really modern PSU there is no 120/240 switch and the PSU automatically works between 80 and 250V AC.

Reply to
Rob

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** Small mistake, the internet IS hell.

me down, it was set to 120VAC and we live in a 240V country.

a 240V country with the AC inlet set to 120V and giving NO warning is crimi nal.

ch in a vain attempt to "repair" the failure. Even though the amp failed o n first use, we don't actually know where the switch was set when the custo mer obtained the amp. There's an even (or better) chance the customer flip ped it after the amp failed.

** Errr - nope.

The particular "switch" is tiny - looks like a round, 20mm panel fuse holde r with sub mm white print saying 120/240 - PLUS requires screw driver to o perate.

Every knowable fact points at the supplier being in the USA and sending the buyer here a "cardboard box" with a local model inside.

The chance the amp arrived in the buyer's hand with a faulty low voltage mo sfet in the synchronous rectifier cct AND no other fault is miniscule.

** I was careful, observant and a tad lucky.

Replacing the 2 DPAK mosfets mounted flat on the PCB was a new job to me - everything I had read suggested it required solder paste & liquid flux that I did not have.

In the end, I used 1.2mm Savbit 60/40 wire solder, a Hakko FX-888 iron set to 330 C and acted damn quick.

..... Phil

Reply to
Phil Allison

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** Yep - most of those kind of SMPSs are know as "PFC corrected" .

The incoming AC is rectified and converted to 400V DC by a switching converter that tracks the incoming AC voltage. Makes the current wave follow the voltage wave ( both sine) so the "power factor" is close to unity.

Has advantages in terms of how many units can be run of the same power circuit and is often mandated in regulations for lighting and computer products.

But not for domestic or entertainment audio.

..... Phil

Reply to
Phil Allison

Once I had a defective PSU in a Dell desktop PC, made a case at the local Dell phone number, got sent a PSU from a European parts center, and it was set to 120V. BANGGG!! (OF COURSE I did not check that! All newly delivered Dell PCs come with the switch correctly set for our region! and there was no "please be advised that you need to check the setting" note packed with it either...)

Called them again, got sent another PSU, again set to 120V. Of course this time I switched it before plugging it in.

I think this is crazy. When you manufacture PSUs and have to have a default setting, then at least SET IT TO 240V!!! When the local line is 120V then at least it will not fail, and when it does not work you can still change the setting.

Reply to
Rob

PFC is mandated for domestic goods above 20w here. PFC correction & the ability to run at 120 & 240 without changing a switch are of course 2 separate things.

NT

Reply to
tabbypurr

Before the PFC solution there were SMPS that used the standard rectifier/doubler setup with a TRIAC in place of the jumper that normally selects 120V/240V operation.

Reply to
Rob

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** Yeah - the cct starts off in the 240V position and jumps down to the 120V setting if the output voltage is much too low.

Think makers called it "auto-select" or something like that.

Few examples were ever made so I suspect their were some issue with it like when the AC supply was dodgy - ie brown outs and such like.

It's safer to have the setting locked to the AC voltage where you live - and not have it jump to voltage doubling cos of a short term voltage drop.

The PFC solution normally works brilliantly, long as the AC supply impedance is reasonably low. If there are hundreds of metres of 15amp cable between the load and the supply - all bets are off.

Then, if the AC voltage drops, the load draws more current causing the voltage to drop more - and well you get the idea.

.... Phil

Reply to
Phil Allison

on.

20V setting if the output voltage is much too low.

ke when the AC supply was dodgy - ie brown outs and such like.

and not have it jump to voltage doubling cos of a short term voltage drop.

nce is reasonably low. If there are hundreds of metres of 15amp cable betwe en the load and the supply - all bets are off.

ltage to drop more - and well you get the idea.

Safest is to simply design the psu to output the correct voltage with eithe r 240 or 120 in. It's not a big problem to design an smpsu to cope with tha t from the get go, so there's no need for any bolt-ons to try to get round an inadequate working range. And that's exactly what most do.

NT

Reply to
tabbypurr

The typical PFC circuit is a boost regulator placed between the rectifier and the reservoir cap. In this way, the reservoir cap can charge during the entire AC cycle instead of only during the peaks, improving the power factor. But this also has the effect that the PWM circuit will see the same input voltage regardless of the line voltage. So, wide input range often comes along with PFC.

-- Adam

Reply to
Adam

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** Most PFC circuits have damn near sine wave current draw, so the PF is around 0.9 to 0.95. However, with the vast majority of domestic, electronic appliances this is of no advantage to the user or the supply system. Only lighting and computers are required to have good PFs in places like Europe and the US while the main reason for that is cos it allows twice as many of these to run from the same AC supply, compared to non PFC versions.

Many buildings have hundreds of light fittings installed and large numbers of mains powered PCs for workers - and that is where it matters!

FYI: domestic fridges, microwave ovens plus most washers and clothes driers have long had near sine wave current draw and so good power factors.

** Correct.

..... Phil

Reply to
Phil Allison

That's what I thought but being in a 120 volt country never had to deal with it.

That makes perfect sense in a 240 country.

Here, it can't really hurt itself.

There are some high power amps here that need 240. They just can't run on 120. Those don't switch down.

Anyway yes, you are definitely better off with a switch.

Reply to
Jeff Urban

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