MOSFET selection for boost converter problem

On 2016-11-16 23:33, John Nagle wrote:
>    I've been working on a power supply for antique Teletype
> selector magnets.  These need 120VDC 60mA for the first
> 2ms or so of each bit time, but the sustain voltage needed
> is only 3-4V.  So I've built an isolated boost supply and
> interface board, all powered from a USB port.
> The whole project is on Github:
>
> https://github.com/John-Nagle/ttyloopdriver
>
> where there's an explanation of how it works which
> doesn't require downloading the whole KiCAD project.
>
> Schematic as image:
> http://www.aetherltd.com/public/misc/loopdriverv1.0.png
>
> LTSpice model:
> http://www.aetherltd.com/public/misc/ttydriver22.asc
>
>     I've had a board made, and it produced 120VDC
> briefly, until the IRLB8721 MOSFET blew out. I just
> used that because it's a common MOSFET.
> Not surprising - the circuit shows 40V across source
> and drain, and that's a 30V part.
>

Others have pointed out potential problems, including the lack of a 
snubber. In the interest of getting this going on the bench fast put a 
zener-based snubber in there, like here:

http://www.eetimes.com/author.asp?doc_id=1280601

A 7-12V zener to your 5V supply, fast Schottky diode in series, from the 
drain. Use a big zener of at least 1/2 watt if you have one. That should 
prevent it from blowing up. Of course, a 555 is too wimpy to drive this 
FET but this should essentially get the circuit going. I never use 555 
but maybe you can find a CMOS version that will at least swing all the 
way to 5V? Longterm I'd get rid of the 555 as was already suggested.


>     So what to put in there?  I've been trying various
> parts in LTSpice. The AP9465GEM works great in LTSpice,
> but it's a surface mount part and only 40V, so there's
> no headroom on voltage.  Almost nothing with a rating
> of 50V or better works in simulation. Turn-off is too
> slow, or capacitance is too high, and the resulting
> output voltage peak on the primary is too low.  Try
> loading up the LTSpice model and using different MOSFETs
> to see this.
>
>     I don't really know what I'm doing here.


We were all there once and many of us have the scars to prove it :-)


>                                           ...  I'm
> aware that this MOSFET application depends strongly
> on the transient properties of the device.  Turning
> off that transformer primary to get the inductive kick
> is tricky.


When you set the coupling coefficient in SPICE to, say, 0.98 you'll get 
the leakage inductance. Or just put a small non-couple inductance in 
series with the primary. The sim will now slow way down but you'll see 
the ugly stuff brewing at the drain node. Lightning, peals of thunder, 
some smoke.

[...]

-- 
Regards, Joerg

http://www.analogconsultants.com/

I found a major error in your simulation. The Coilcraft DA3022 has a ratio 
of 10:1, so with a primary inductance of 10 uH, the secondary is 100 times 
that, or 1000 uH. I also replaced your MOSFET with a commonly available 
TO-220 logic level device rated 100V and 15A IRL530NS_L

http://www.mouser.com/ds/2/427/91299-106665.pdf

So here is the simulation:

http://enginuitysystems.com/pix/electronics/ttydriver22b.png

Note that the peak drain voltage on the MOSFET is less than 20V, and the 
output now easily reaches 120V. But something else that may be a problem is 
that the voltage on the selector coil drops to -120V when turned off. This 
might be corrected by using a standard diode in place of D7 (but this will 
cause current to flow for a longer time). Other methods as discussed in the 
previous thread may point to a better solution.

Thru-hole components are becoming scarce for some devices, especially newer 
ones. But it is really quite easy to solder larger packages such as the 
power flat pack, and even SOIC-8 and SOT parts are not bad, even on the 
usual proto-boards. Don't let SMT keep you from choosing better components.

Paul 

John Nagle wrote:
>    I've been working on a power supply for antique Teletype
> selector magnets.  These need 120VDC 60mA for the first
> 2ms or so of each bit time, but the sustain voltage needed
> is only 3-4V.  So I've built an isolated boost supply and
> interface board, all powered from a USB port.
> The whole project is on Github:


    Why not dig up a '70s edition of the ARRL Radio Amateur's Handbook, 
and see what they did for an interface?


-- 
Never piss off an Engineer!

They don't get mad.

They don't get even.

They go for over unity! ;-)

In message <qgmr2ctl7k3m0hnanrraasf7rhhfkta7na@4ax.com>
          John Larkin <jjlarkin@highlandtechnology.com> wrote:

> On Wed, 16 Nov 2016 23:33:57 -0800, John Nagle <nagle@animats.com>
> wrote:

>>
>>(I was asking previously about low duty cycle
>>555 timer circuits. That part is working fine now.
>>Thanks.)
>>
>>                                John Nagle

555 reasons to not use this anymore ;-)


> There's no flyback clamping or snubbing on that fet. The drain
> probably spikes up a hundred volts or more. Your sim has no leakage
> inductance, so doesn't model the drain spikes.

> Use a higher-voltage fet and add some sort of flyback absorber, an R-C
> or a zener or something.

> And isn't the fet switching, basically, into a shorted load most of
> the time?
Good point, that's why first initial chops on an empty
cap load should be very short.......

The snubber alone will not be enough, the selection of the transformer
is OK. What is missing is gate drive and THE-control.

When MOSFETs had been poor some spikes above the max drain voltage
instantly killed them - like the old BUK Philips ones, what a crap
this was....

You can sense and regulate by the flyback pulse the output and limit
thereby also the flyback pulse down to non dangerous amplitudes.
Used this method in my TL594 controlled analogue ballasts
many years ago.

Now even available in one chip from LT:
http://www.linear.com/docs/10239

Regards

Joerg Ni


-- 
Joerg Niggemeyer on RISCOS Raspberry Pi2

http://www.led-temperature-protection.com
http://www.nucon.de

On 11/17/2016 5:00 PM, P E Schoen wrote:
> "John Nagle"  wrote in message news:o0jmg3$gj6$1@dont-email.me...
> 
>>  I've been working on a power supply for antique Teletype selector
>> magnets.  These need 120VDC 60mA for the first 2ms or so of each bit
>> time, but the sustain voltage needed is only 3-4V.  So I've built an
>> isolated boost supply and interface board, all powered from a USB port.
>> The whole project is on Github:
> 
>> https://github.com/John-Nagle/ttyloopdriver
> 
>> where there's an explanation of how it works which doesn't require
>> downloading the whole KiCAD project.
> 
>> Schematic as image:
>> http://www.aetherltd.com/public/misc/loopdriverv1.0.png
> 
>> LTSpice model:
>> http://www.aetherltd.com/public/misc/ttydriver22.asc
> 
>>   I've had a board made, and it produced 120VDC briefly, until the
>> IRLB8721 MOSFET blew out. I just
> used that because it's a common MOSFET. Not surprising - the circuit
> shows 40V across source and drain, and that's a 30V part.
> 
>>   So what to put in there?  I've been trying various parts in LTSpice.
>> The AP9465GEM works great in LTSpice,
> but it's a surface mount part and only 40V, so there's no headroom on
> voltage.  Almost nothing with a rating of 50V or better works in
> simulation. Turn-off is too slow, or capacitance is too high, and the
> resulting output voltage peak on the primary is too low.  Try loading up
> the LTSpice model and using different MOSFETs to see this.
> 
>>   I don't really know what I'm doing here.  I'm aware that this MOSFET
>> application depends strongly on the transient properties of the
>> device. Turning off that transformer primary to get the inductive kick
>> is tricky. I'm trying to struggle through the Art of Electronics
>> chapter on MOSFETs, while hammering on the problem with LTSpice.
> 
>> (I'd like to use a TO-220 form factor, or at least a through-hole
>> part, and get this first board working. After that, I can get a new
>> board made.)
> 
>> (I was asking previously about low duty cycle 555 timer circuits. That
>> part is working fine now. Thanks.)
> 
> I made some suggestions in your previous post, so I am familiar with
> what you want to do. So, where to begin?
> 
> As a reference, here is the LTSpice simulation you provided:
> 
> http://enginuitysystems.com/pix/electronics/ttydriver22.png
> 
> Some issues:
> (1) D1, D2, and D3 have no model defined (probably not a problem)
> (2) Your source is 4.6V with 5 ohms resistance - that severely limits
> performance

     That's a worst case test. USB guarantees only 4.5V, although it's
nominally 5V.

> (3) As mentioned, no leakage inductance for L1-L2 is specified.
> (4) Also as mentioned, 100 kHz is very high, and the 555 is not a very
> good gate driver under best circumstances
> (5) The AP9465GEM is not commonly available.
> (6) The IRLB8721 is only a 30V part, and it is really characterized for
> gate drive of at least 3 volts and preferably 4.5V. Your power supply is
> already crippled with 4.6 volts, and the NE555 has a typical output high
> voltage of only 3.3V at 100mA. Even at 1 mA the output will be 1 volt
> below the rail, so just barely 3.6 volts with a 4.6 volt supply.

    Now using the IRL530, which is a 100V part.  The 555 timer
doesn't have enough output power to drive it properly. Here's the
scope trace at the gate of the IRL530.

http://www.aetherltd.com/public/misc/mosfetgate1.jpg

It's only reaching 2V.  The overall result is that the circuit runs
but the final output is only 45V instead of 120V.

    http://www.aetherltd.com/public/misc/unloadedoutput.jpg

So the big problem is getting enough drive for the power MOSFET's gate.

> (7) I don't see any bypass capacitors on your actual schematic on the
> nominal 5V supply.

    Good point.  I'm relying on the 220uF filter cap too much.

> (8) You have a TPS2030 USB switch for the enable. This part may be
> obsolete, but probably not a problem.

    It's dated, but available in DIP.

> (9) There is U5 that looks like "IEQ524S" but I can't find it. Looks
> like an interface to the optos.

    IE0524S.


http://www.digikey.com/product-detail/en/xp-power/IE0524S/1470-1407-5-ND/4487788

    That's a 5VDC to 24VDC converter used to provide voltage for the
keyboard side of the system. That's independent of all the custom
switching stuff.

    Thanks.

				John Nagle

On 11/17/2016 6:29 PM, P E Schoen wrote:
> I found a major error in your simulation. The Coilcraft DA3022 has a
> ratio of 10:1, so with a primary inductance of 10 uH, the secondary is
> 100 times that, or 1000 uH. I also replaced your MOSFET with a commonly
> available TO-220 logic level device rated 100V and 15A IRL530NS_L
> 
> http://www.mouser.com/ds/2/427/91299-106665.pdf
> 
> So here is the simulation:
> 
> http://enginuitysystems.com/pix/electronics/ttydriver22b.png
> 
> Note that the peak drain voltage on the MOSFET is less than 20V, and the
> output now easily reaches 120V. But something else that may be a problem
> is that the voltage on the selector coil drops to -120V when turned off.
> This might be corrected by using a standard diode in place of D7 (but
> this will cause current to flow for a longer time). Other methods as
> discussed in the previous thread may point to a better solution.

   I tried that.  Then I put in a tiny resistor (0.01 ohm) between
the output of the 555 timer and the gate of the MOSFET so I
could get LTSpice to measure current.   That
shows a peak current of 350mA for about 200ns.  The 555 timer
can output a maximum of 200mA.  I'm using the "idealized 555
timer" model, and it can apparently deliver more current
with no voltage drop.

   In the real world, the output from the 555 timer drops
to about 2V and stays there for the entire ON portion of
the cycle.   I'm not sure how the gate of a MOSFET behaves
when you underdrive it. I thought the high current draw
was just during turn-on.  Is the problem that, without
enough drive, the gate current remains high?  Or
is something else wrong?

   Here's the scope trace again:

   http://www.aetherltd.com/public/misc/mosfetgate1.jpg

Incidentally, the other waveform

   http://www.aetherltd.com/public/misc/unloadedoutput.jpg

shows data being sent through the board at 45 baud.
Functionally, roughly the right stuff is happening.

   Someone suggested using an MC34063 instead of a
555 timer, and I'll have to look at that option.
But it's a lower switching frequency.  More later.

			John Nagle

"John Nagle"  wrote in message news:o0ma68$jd6$1@dont-email.me...

>   I tried that.  Then I put in a tiny resistor (0.01 ohm) between the 
> output of the 555 timer and the gate of the MOSFET so I could get LTSpice 
> to measure current.   That shows a peak current of 350mA for about 200ns. 
> The 555 timer can output a maximum of 200mA.  I'm using the "idealized 555 
> timer" model, and it can apparently deliver more current with no voltage 
> drop.

>   In the real world, the output from the 555 timer drops to about 2V and 
> stays there for the entire ON portion of
the cycle.   I'm not sure how the gate of a MOSFET behaves when you 
underdrive it. I thought the high current draw
was just during turn-on.  Is the problem that, without enough drive, the 
gate current remains high?  Or is something else wrong?

>   Here's the scope trace again:

>   http://www.aetherltd.com/public/misc/mosfetgate1.jpg

> Incidentally, the other waveform

>   http://www.aetherltd.com/public/misc/unloadedoutput.jpg

> shows data being sent through the board at 45 baud.
> Functionally, roughly the right stuff is happening.

>   Someone suggested using an MC34063 instead of a 555 timer, and I'll have 
> to look at that option. But it's a lower switching frequency.  More later.

I could try to analyze what's going on with your most recent efforts, but I 
found more problems that are more serious. First off, it's not going to work 
with a 5 ohm power source resistance. A USB will supply close to 5VDC (maybe 
as low as 4.8) and it will do so until it approaches the standard 500 mA 
limit, at which it may throttle back or shut down completely. So I used your 
4.6V with a 0.5 ohm resistance. That will still mean a drop to about 4.3 
volts at 500 mA, but still workable.

BTW, you can read current without adding a resistor. Hover the cursor on the 
pin (such as the gate of M2). It will show an icon for a black clamp-on 
meter and in this case it will measure Ig(M2). You may also choose it from a 
list of available data by right-clicking on the plot, choosing "Add Trace 
(Ctrl-A)", and a list will appear. You may also manually define what you 
want to plot, such as a voltage ratio, or voltage times current (power).

Next, I changed the input diode to a Schottky, which drops only about 0.3 
volts rather than almost 1 volt. That was a huge improvement. I also found a 
better MOSFET, the IRL3915. It seems that it is actually an IRLR3915. There 
are probably even better ones available, but this is only a dollar and is 
easily available:

http://www.mouser.com/ds/2/196/irlr3915pbf-936699.pdf

It is 55V, 30A, with 14 mOhm RdsOn, and will work with as little as 2V on 
the gate. I also cleaned up your simulation, getting rid of parts not 
needed, and added a commutating snubber that totally eliminates negative 
spikes and ringing. I dropped the frequency to about 40 kHz. Here is the 
result:

http://enginuitysystems.com/pix/electronics/ttydriver22c.png

Note that this seems to draw only about 300 mA from the supply, or about 1.6 
watts. Fine for a USB source.

Here is the netlist:

http://enginuitysystems.com/pix/electronics/ttydriver22c.asc

Hope this helps.

Paul

I thought you might like to se the details of the waveform, particularly the 
gate current Ig(M2), drain current Id(M2), and the drain voltage Vd, 
actually V(n004).

http://enginuitysystems.com/pix/electronics/ttydriver22c-Ig-Id-Vd.png

Paul

On 11/18/2016 1:22 AM, John Nagle wrote:
> On 11/17/2016 6:29 PM, P E Schoen wrote:
>> I found a major error in your simulation. The Coilcraft DA3022 has a
>> ratio of 10:1, so with a primary inductance of 10 uH, the secondary is
>> 100 times that, or 1000 uH. I also replaced your MOSFET with a commonly
>> available TO-220 logic level device rated 100V and 15A IRL530NS_L
>>
>> http://www.mouser.com/ds/2/427/91299-106665.pdf
>>
>> So here is the simulation:
>>
>> http://enginuitysystems.com/pix/electronics/ttydriver22b.png
>>
>> Note that the peak drain voltage on the MOSFET is less than 20V, and the
>> output now easily reaches 120V. But something else that may be a problem
>> is that the voltage on the selector coil drops to -120V when turned off.
>> This might be corrected by using a standard diode in place of D7 (but
>> this will cause current to flow for a longer time). Other methods as
>> discussed in the previous thread may point to a better solution.
>
>    I tried that.  Then I put in a tiny resistor (0.01 ohm) between
> the output of the 555 timer and the gate of the MOSFET so I
> could get LTSpice to measure current.   That
> shows a peak current of 350mA for about 200ns.  The 555 timer
> can output a maximum of 200mA.  I'm using the "idealized 555
> timer" model, and it can apparently deliver more current
> with no voltage drop.
>
>    In the real world, the output from the 555 timer drops
> to about 2V and stays there for the entire ON portion of
> the cycle.   I'm not sure how the gate of a MOSFET behaves
> when you underdrive it. I thought the high current draw
> was just during turn-on.  Is the problem that, without
> enough drive, the gate current remains high?  Or
> is something else wrong?
>
>    Here's the scope trace again:
>
>    http://www.aetherltd.com/public/misc/mosfetgate1.jpg

What is the FET P/N that you are measuring?

On 11/18/2016 02:22 AM, John Nagle wrote:

>> (8) You have a TPS2030 USB switch for the enable. This part may be
>> obsolete, but probably not a problem.
>
>     It's dated, but available in DIP.
>

If I'm reading the schematic correctly it looks like you're feeding the 
flyback from the '2030 high-side switch, which the datasheet has a 
continuous current limit of 300mA.

Looking at the waveforms here:

http://enginuitysystems.com/pix/electronics/ttydriver22c-Ig-Id-Vd.png

it looks like the mean MOSFET drain current may be over that.