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From: Hammy on 23 Apr 2010 12:31
On Fri, 23 Apr 2010 09:07:11 -0700, Joerg <invalid(a)invalid.invalid>
wrote:

>John Larkin wrote:
>> On Fri, 23 Apr 2010 08:06:40 -0700, Joerg <invalid(a)invalid.invalid>
>> wrote:
>>
>>> Hammy wrote:
>>>> On Fri, 23 Apr 2010 09:23:59 GMT, Jan Panteltje
>>>> <pNaonStpealmtje(a)yahoo.com> wrote:
>>>>
>>>>
>>>>> Use optos, and the transformer to generate a floating voltage
>>>>> for a driver directly connected to the MOSFET?
>>>> It's an option I have some fast Avago gate drive opto's.
>>>
>>> Optocouplers aren't very well suited to drive FETs. They can't muscle a
>>> gate capacitance around within tens of nanoseconds and at a couple of
>>> amps. Transformers are the better choice.
>>>
>>> Size the caps right, and maybe optimize the transformer coupling some
>>> more (twisting the wires before winding etc. unless you did that
>>> already). Burp mode in a converter isn't that great with transformers
>>> but there aren't many alternatives. In ultrasound we do this a lot,
>>> transmit a sequence of fast power pulses across a transformer, followed
>>> by a major pause (to wait for echoes).
>>
>> There are some fast optocoupled mosfet gate drivers.
>>
>> The home-brew way to do this is to use a cheap (ca $4) potted DC-DC
>> converter to make a supply that floats on the fet source. Then use a
>> fast logic-type optocoupler and a real gate driver chip up there.
>>
>
>That's a lot of stuff for a 50W-100W converter :-)
>
>
>> The advantage of optos is DC coupling without duty-cycle problems. And
>> not having to wind transformers.
>>
>
>Another option is to use a chip with proper bootstrap circuitry.

Yes but I think you might have problems as well with burst mode.

>So far transformers have been good to me. The only problem is that
>off-the-shelf gate transformers are IMHO way overpriced and the
>reasonably priced ones from China are custom runs and have MOQs. Which
>for a product design doesn't really matter. It seems the usual COTS
>vendors don't really realize how much of their lunch is being eaten.
>Maybe time to invest in Rinminbi, if I just knew where ;-)

My input is North American only it's my supply; my use only. I've made
small floating supplies with a cheap PIC10F controlling a small pulse
transformer and floated a driver on it for an SSR. I used the PIC for
zero cross detect and to drive an opto for controlling the floating
driver. The supply just ran open loop the output was clamped with a
zener so it never exceeded the Vgs of the FET's. The footprint was
small and I could vertically mount it I guess.

The whole thing only consumed a couple of mA, it wasn't switching
though just on/off.

Switching at 100kHz with a PIC and fet driver I'm getting about 6mA
quiescent current 3mA or so is probably due to the 78l05 for the PIC.

Avago does make some opto's with totem outputs sink/source couple of
amps at least. Depending on what you want to spend 1uS propagation
delay's down to the low nS.
From: Joerg on 23 Apr 2010 12:38
Hammy wrote:
> On Fri, 23 Apr 2010 09:07:11 -0700, Joerg <invalid(a)invalid.invalid>
> wrote:
>
>> John Larkin wrote:
>>> On Fri, 23 Apr 2010 08:06:40 -0700, Joerg <invalid(a)invalid.invalid>
>>> wrote:
>>>
>>>> Hammy wrote:
>>>>> On Fri, 23 Apr 2010 09:23:59 GMT, Jan Panteltje
>>>>> <pNaonStpealmtje(a)yahoo.com> wrote:
>>>>>
>>>>>
>>>>>> Use optos, and the transformer to generate a floating voltage
>>>>>> for a driver directly connected to the MOSFET?
>>>>> It's an option I have some fast Avago gate drive opto's.
>>>> Optocouplers aren't very well suited to drive FETs. They can't muscle a
>>>> gate capacitance around within tens of nanoseconds and at a couple of
>>>> amps. Transformers are the better choice.
>>>>
>>>> Size the caps right, and maybe optimize the transformer coupling some
>>>> more (twisting the wires before winding etc. unless you did that
>>>> already). Burp mode in a converter isn't that great with transformers
>>>> but there aren't many alternatives. In ultrasound we do this a lot,
>>>> transmit a sequence of fast power pulses across a transformer, followed
>>>> by a major pause (to wait for echoes).
>>> There are some fast optocoupled mosfet gate drivers.
>>>
>>> The home-brew way to do this is to use a cheap (ca $4) potted DC-DC
>>> converter to make a supply that floats on the fet source. Then use a
>>> fast logic-type optocoupler and a real gate driver chip up there.
>>>
>> That's a lot of stuff for a 50W-100W converter :-)
>>
>>
>>> The advantage of optos is DC coupling without duty-cycle problems. And
>>> not having to wind transformers.
>>>
>> Another option is to use a chip with proper bootstrap circuitry.
>
> Yes but I think you might have problems as well with burst mode.
>

Not really, just make sure that the bootstrap voltage remains 10V or so
above the source at all times.


>> So far transformers have been good to me. The only problem is that
>> off-the-shelf gate transformers are IMHO way overpriced and the
>> reasonably priced ones from China are custom runs and have MOQs. Which
>> for a product design doesn't really matter. It seems the usual COTS
>> vendors don't really realize how much of their lunch is being eaten.
>> Maybe time to invest in Rinminbi, if I just knew where ;-)
>
> My input is North American only it's my supply; my use only. I've made
> small floating supplies with a cheap PIC10F controlling a small pulse
> transformer and floated a driver on it for an SSR. I used the PIC for
> zero cross detect and to drive an opto for controlling the floating
> driver. The supply just ran open loop the output was clamped with a
> zener so it never exceeded the Vgs of the FET's. The footprint was
> small and I could vertically mount it I guess.
>
> The whole thing only consumed a couple of mA, it wasn't switching
> though just on/off.
>
> Switching at 100kHz with a PIC and fet driver I'm getting about 6mA
> quiescent current 3mA or so is probably due to the 78l05 for the PIC.
>

Sound about normal.


> Avago does make some opto's with totem outputs sink/source couple of
> amps at least. Depending on what you want to spend 1uS propagation
> delay's down to the low nS.


If you want to go opto you'd probably be better off with John's
suggestion. Get a fast but run-of-the-mills weak optocoupler, then
follow it with a staunch MIC4421 or some other driver for a little over
a buck.

The fancy chips will always be expensive because us guys never use them
in product designs. We only would if they'd be cheap and 2nd-sourced,
sort of the chicken and egg problem.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: Hammy on 23 Apr 2010 13:10
On Fri, 23 Apr 2010 09:24:53 -0700, Joerg <invalid(a)invalid.invalid>
wrote:


>With a flying source that's a problem because the Instek connects the
>probe shield to PE. So you get ground loops and tons of ringing. Plus
>you neighbors listening to AM radio won't be very happy about that. You
>can get battery packs for many Instek models but it's very expensive. I
>never checked whether one can hack into this and connect one's own
>battery. Even then you'd at least need to wrap the probe through a nice
>big ferrite a few times.

Yep they defiantly aren't giving the battery option away.

>Maybe at your frequency range you could try a diff measurement with two
>channels.
>
>
>> I don't think its possible to do this without a floating supply and
>> using an opto to send the HS control signal to a driver. Even if I
>> disable the burst mode the controller could still go into burst mode
>> under fault condition like over current. To many chances something
>> could go wrong.
>>
>
>With properly size caps it should be possible.

I haven't given up yet I'll play around with what I have part wise and
see if I cant get something reliable.

>> I suppose I could use an old UC384X it has none of these fancy
>> features and a 0.5 duty cycle clamp. Other then the unpredictability
>> of the burst mode it does work well.
>>
>
>Question: Since this is a flyback why is the source flopping about?
>What's it doing? Maybe post the whole schematic if it isn't confidential.

No not confidential I haven't made up a full schematic yet. :-)

Here's my actual test schematic. I'm not showing Bypass caps and the
lm317 for the driver and the 78l05 for the PIC both regulators get
their input from the 20 V supply V1.

http://i39.tinypic.com/1zf4gpl.png

That is just a test set-up to test the high side drive for a active
clamp flyback shown below.

http://i39.tinypic.com/10s7ybl.png

I don't have a schematic for the flyback because I just started
considering it yesterday. I'm just working out how to drive the
High-side FET.


>> I was also planning on using synchronous rectification a shottky
>> dissipates 4W but this FET would have less then 1W conduction losses.
>> I thought I would try something different;-)
>>
>> http://www.nxp.com/documents/data_sheet/PSMN8R2-80YS.pdf
>>
>> They are coming out with some nice FETS.
>
>
>But you have to drive over 3500pF of gate capacitance. Make sure the
>Rdson versus gate capacitance deal is somewhat optimized for the chosen
>frequency and power level. Ideally the losses due to both effects would
>be about on par.

A two amp driver should take care of that. I've just been using a
spreadsheet from On semi for rough estimates for now.

>It's pretty brazen that they advertize this as a 130W device. I wonder
>what it's lifetime will be under that sort of load (in milliseconds,
>from turn-on to a sharp popping noise).

They all do that it seems worse now because of the high 100 plus amps
they also quote for the part.
From: Joerg on 23 Apr 2010 13:48
Hammy wrote:
> On Fri, 23 Apr 2010 09:24:53 -0700, Joerg <invalid(a)invalid.invalid>
> wrote:
>
>
>> With a flying source that's a problem because the Instek connects the
>> probe shield to PE. So you get ground loops and tons of ringing. Plus
>> you neighbors listening to AM radio won't be very happy about that. You
>> can get battery packs for many Instek models but it's very expensive. I
>> never checked whether one can hack into this and connect one's own
>> battery. Even then you'd at least need to wrap the probe through a nice
>> big ferrite a few times.
>
> Yep they defiantly aren't giving the battery option away.
>

Another old trick is to use a car battery and inverter. Mine just
croaked. Couldn't believe it, half a year ago it was showing slight
signs of weakness but would still crank a 2.6l gasoline engine alright.
So I replaced the one in the car. When I needed it for isolation
purposes a month ago ... nada, zip, zilch. It was totally hi-Z, charger
went to 17V and shut off, and the battery wouldn't even light a 1W bulb.
Puzzling how it could have gone so fast.


>> Maybe at your frequency range you could try a diff measurement with two
>> channels.
>>
>>
>>> I don't think its possible to do this without a floating supply and
>>> using an opto to send the HS control signal to a driver. Even if I
>>> disable the burst mode the controller could still go into burst mode
>>> under fault condition like over current. To many chances something
>>> could go wrong.
>>>
>> With properly size caps it should be possible.
>
> I haven't given up yet I'll play around with what I have part wise and
> see if I cant get something reliable.
>

Try to find some smaller ones, maybe from EMC filters :-)


>>> I suppose I could use an old UC384X it has none of these fancy
>>> features and a 0.5 duty cycle clamp. Other then the unpredictability
>>> of the burst mode it does work well.
>>>
>> Question: Since this is a flyback why is the source flopping about?
>> What's it doing? Maybe post the whole schematic if it isn't confidential.
>
> No not confidential I haven't made up a full schematic yet. :-)
>
> Here's my actual test schematic. I'm not showing Bypass caps and the
> lm317 for the driver and the 78l05 for the PIC both regulators get
> their input from the 20 V supply V1.
>
> http://i39.tinypic.com/1zf4gpl.png
>
> That is just a test set-up to test the high side drive for a active
> clamp flyback shown below.
>
> http://i39.tinypic.com/10s7ybl.png
>

Since it can't bang past either rail you cold probably bootstrap that.
But transformers are a good option here, just not with 0.1uF in series.


> I don't have a schematic for the flyback because I just started
> considering it yesterday. I'm just working out how to drive the
> High-side FET.
>

No chance just to do a half-bridge and be done with it? It would also be
a bit easier EMI-wise. And no air gap.

>
>>> I was also planning on using synchronous rectification a shottky
>>> dissipates 4W but this FET would have less then 1W conduction losses.
>>> I thought I would try something different;-)
>>>
>>> http://www.nxp.com/documents/data_sheet/PSMN8R2-80YS.pdf
>>>
>>> They are coming out with some nice FETS.
>>
>> But you have to drive over 3500pF of gate capacitance. Make sure the
>> Rdson versus gate capacitance deal is somewhat optimized for the chosen
>> frequency and power level. Ideally the losses due to both effects would
>> be about on par.
>
> A two amp driver should take care of that. I've just been using a
> spreadsheet from On semi for rough estimates for now.
>

Yes, 2A would be plenty.


>> It's pretty brazen that they advertize this as a 130W device. I wonder
>> what it's lifetime will be under that sort of load (in milliseconds,
>>from turn-on to a sharp popping noise).
>
> They all do that it seems worse now because of the high 100 plus amps
> they also quote for the part.


The topper is what I saw in some discount store aisle. An audio
amplifier in a blister pack, very ritzy packaging. It claimed 1000 (!)
watts PMPO and the power supply was <gasp> a wall wart. Yeah, right ...

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: Hammy on 23 Apr 2010 14:32
On Fri, 23 Apr 2010 10:48:28 -0700, Joerg <invalid(a)invalid.invalid>
wrote:


>>> With properly size caps it should be possible.
>>
>> I haven't given up yet I'll play around with what I have part wise and
>> see if I cant get something reliable.
>>
>
>Try to find some smaller ones, maybe from EMC filters :-)

Yea I'm digging through my parts. I tried a 10 nf and the oscillations
weren't as bad but my Vgs decreased by about a volt. I'll try some
other values. I don't think I can go below 10nf without adjusting the
inductance though.

>No chance just to do a half-bridge and be done with it? It would also be
>a bit easier EMI-wise. And no air gap.

That's my next PSU project;-). Coil Craft was kind enough to give me
some samples of flyback transformers. I used up all the small 10W ones
on projects but I still have some of the bigger ones left.

>The topper is what I saw in some discount store aisle. An audio
>amplifier in a blister pack, very ritzy packaging. It claimed 1000 (!)
>watts PMPO and the power supply was <gasp> a wall wart. Yeah, right ...

Haven't you heard the Chinese can stick a small reactor in a wall wart
enclosure. That's not the really amazing part though ,they can do it
for $1.00 or less depending on quantities. ;-)
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