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From: Daku on 20 Jun 2010 12:30
Dear Sir,
I have considered using PFCs before, but there is one point that is
not very clear. The scheme you suggest is that the PFC senses the grid
line, and forces the boost converter(input DC from solar panel) output
current synchronize with the grid AC voltage - am I right ? So then
comes the actual inverter. In this case who is supplying the PWM unit
with its high frequency carrier and low frequency modulation signal. I
guess the grid AC voltage could serve as the low frequency modulation
signal ?? Maybe I am thinking too much in analog terms.

On Jun 19, 11:41 pm, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:

> Yes, you'd have to sense the line voltage to sync to it, and probably
> the inverter-line current.
>
> You could do what the PFC power supply chips do, namely sense the AC
> line voltage waveform in real time and make a boost converter's output
> current track that. A slow control loop wraps around all that to get
> the average, in this case optimum, power transfer. Maybe you could use
> one of the PFC controller chips; they're cheap.
>
> The problem is basically to present a negative resistance to the AC
> power line.
>
> Get anything wrong and it goes boom. Expect a shoe box full of
> exploded fets if you do this from scratch. Wear eye protection.
>
> ftp://jjlarkin.lmi.net/ExFets.jpg
>
> John

From: Cydrome Leader on 20 Jun 2010 17:00
Phil Allison <phil_a(a)tpg.com.au> wrote:
>
> "Cydrome Leader"
>
>>
>> I had a hard time recently locating a specific power transistor in a
>> TO-218 case, with the metal tab and not the lump of plastic with a metal
>> back like everything seems to be now.
>
>
> ** Why bother ??
>
> TO-218 is virtually the same pak and construction as TO-3P except you no
> longer need to use an insulating bush with the latter.

to not have to modify existing heatsink clamps. The setup in the product
was a bit weird with a clamp holding the metal tab onto the heatsink. I'm
not sure why the transistor wasn't just screwed down. There was a screw
and nut for the clamp anyways.

> TO-3 is a whole nuther animal with many advantages over plastic paks.

the TO-218 is actually somewhat compatible with the TO-3.

If you have an old SK or EGC cross book you might find a section where
they show you how to wire the only available replacements for some TO-3
transistors using the TO-218 package.

If the original setup didn't use sockets, it wasn't a big deal.
From: John Larkin on 20 Jun 2010 20:38
On Sun, 20 Jun 2010 09:30:06 -0700 (PDT), Daku <dakupoto(a)gmail.com>
wrote:

>Dear Sir,
>I have considered using PFCs before, but there is one point that is
>not very clear. The scheme you suggest is that the PFC senses the grid
>line, and forces the boost converter(input DC from solar panel) output
>current synchronize with the grid AC voltage - am I right ? So then
>comes the actual inverter. In this case who is supplying the PWM unit
>with its high frequency carrier and low frequency modulation signal. I
>guess the grid AC voltage could serve as the low frequency modulation
>signal ?? Maybe I am thinking too much in analog terms.

The HF carrier is any old oscillator. The modulation signal (which
controls boost converter duty cycle in a PFC-type power supply) is
created by analog-multiplying the line voltage waveform by a loop
error signal. It would probably be easier, nowadays, to do all this
digitally, in a little PIC or ARM or something.

Look up some PFC power supply control chip datasheets and appnotes.
It's a similar, or sort of reverse, problem.

John

From: Phil Allison on 20 Jun 2010 23:12

"Cydrome Leader"
> Phil Allison
>> "Cydrome Leader"
>
>>> I had a hard time recently locating a specific power transistor in a
>>> TO-218 case, with the metal tab and not the lump of plastic with a metal
>>> back like everything seems to be now.
>>
>>
>> ** Why bother ??
>>
>> TO-218 is virtually the same pak and construction as TO-3P except you no
>> longer need to use an insulating bush with the latter.
>
> to not have to modify existing heatsink clamps. The setup in the product
> was a bit weird with a clamp holding the metal tab onto the heatsink. I'm
> not sure why the transistor wasn't just screwed down. There was a screw
> and nut for the clamp anyways.

** A well designed clamp does a far better job with TO-3P and similar single
hole packs than using a bolt does. Normally the clamp supplies high pressure
along the CENTRE line of the plastic body of the device. The body spreads
the pressure evenly over the tab and there is then no tendency for it to
lift up at the lead end as there is with a bolt.


>> TO-3 is a whole nuther animal with many advantages over plastic paks.
>
> the TO-218 is actually somewhat compatible with the TO-3.

** The main advantage of the TO3 pak is that there is no plastic in contact
with the chip which sits in a hermetically sealed environment - means
chips have a much longer service life even when running near max temps.

Internal feed wires can be sized to fuse immediately if a device fails - a
valuable feature not seen in plastic paks.

Two bolt mounting is easy to get right - long as you avoid those soft
plastic ( Nylon ? ) insulating bushes that crush under heat and pressure.


..... Phil


From: Cydrome Leader on 21 Jun 2010 11:50
Phil Allison <phil_a(a)tpg.com.au> wrote:
>
> "Cydrome Leader"
>> Phil Allison
>>> "Cydrome Leader"
>>
>>>> I had a hard time recently locating a specific power transistor in a
>>>> TO-218 case, with the metal tab and not the lump of plastic with a metal
>>>> back like everything seems to be now.
>>>
>>>
>>> ** Why bother ??
>>>
>>> TO-218 is virtually the same pak and construction as TO-3P except you no
>>> longer need to use an insulating bush with the latter.
>>
>> to not have to modify existing heatsink clamps. The setup in the product
>> was a bit weird with a clamp holding the metal tab onto the heatsink. I'm
>> not sure why the transistor wasn't just screwed down. There was a screw
>> and nut for the clamp anyways.
>
> ** A well designed clamp does a far better job with TO-3P and similar single
> hole packs than using a bolt does. Normally the clamp supplies high pressure
> along the CENTRE line of the plastic body of the device. The body spreads
> the pressure evenly over the tab and there is then no tendency for it to
> lift up at the lead end as there is with a bolt.
>
>
>>> TO-3 is a whole nuther animal with many advantages over plastic paks.
>>
>> the TO-218 is actually somewhat compatible with the TO-3.
>
> ** The main advantage of the TO3 pak is that there is no plastic in contact
> with the chip which sits in a hermetically sealed environment - means
> chips have a much longer service life even when running near max temps.

I'm not arguing that a TO-3 is better all around- it's just that fewer and
fewer semiconductors are packaged as nicely these days so that's what you
have to work with most of the time. The hermetically sealed and welded
metal can obviously costs more than some plastic resin molded around a
sliver of metal.

Sanken uses to have the most exciting TO-3 cases. The base was
surprisingly thick and they felt like they were made of gold.

> Internal feed wires can be sized to fuse immediately if a device fails - a
> valuable feature not seen in plastic paks.

But the plastic ones explode and burn.

> Two bolt mounting is easy to get right - long as you avoid those soft
> plastic ( Nylon ? ) insulating bushes that crush under heat and pressure.

I think they were a glass filled plastic. I may still have some mounting
kits with the mica washer, nuts, washer and lug.

I've seen lots of power supplies now have the power semiconductors glued
to the heatsinks, or the thermal pad is sort of like a really hard two
sided tape. I can't figure out if there is any repair method for that type
of stuff.
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