MOSFET selection for boost converter problem

The selection of TO-220 parts is pretty scant these days. I suspect you'll have a much better time of it if you buy DPAK-2 parts and solder pigtails onto the lands to make them into sorta-TO-220 parts. Or just bite the bullet and start using surface-mount parts throughout.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott
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I haven't seen much of an availability problem there.

In this case no problem but I think John wants to have the board through-hole. TO220 are still widely available and will be for a long time because bigger supplies than this need heat sinks. You can't reliably sink more than low single-digit watts into a PCB. I design in SMT since 1986, was an early adopter but had to use many TO220 parts for power stages.

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Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

....

Makes sense. I'd changed that in some of my own simulations.

That's a big help. It's IRLU3915 in through-hole form.

That simulation is still using the idealized 555 timer, which seems to have unlimited output current. The MOSFET gate current spikes to about 350mA, which won't happen in the real world. So I added a 10 ohm resistor to the sim between the 555 output and the MOSFET input. That limits the current to about 200mA (the real 555 limit) and the switcher still works. Probably don't need that resistor in the real world.

Sim says 25V, 2.7A max at the MOSFET, so that's within limits.

Too much snubbing - the magnet won't release fast enough. Look at the current through I3. It's above 60mA for the whole OFF period. The energy in the magnet needs to be dumped in about 2ms or the mechanism won't work. The idea is to use just enough snubbing to avoid component damage. The original 1926-1930 equipment used mechanical contacts which arced when the circuit opened. Around 1940, RF suppression was added, as a small RC circuit.

I can tell the USB interface to ask for 300mA. No problem.

What's with the note "Draws about 1.7 amps or 6.2 watts?"

I'm going to try substituting a IRLU3915 for now and take measurements, then do a new board design. Thanks.

John Nagle

Reply to
John Nagle

Often in the commercial products from that era I've looked at, like synthesizers, etc. it's all through-hole except for one or two big chips with many pins, like a custom gate array or display controller, which look surprisingly modern by comparison.

Reply to
bitrex

Don't worry too much about the MOSFET gate current spikes. Driving a capacitance with a square wave does that. The 555 is not an ideal gate driver, but probably adequate. What is important is for the ON voltage to be well above the linear threshold. This was not the case previous to fixing the 5V supply problems.

That's my error, and today I saw that the single Zener was acting as a commutating diode and my snubber was actually doing nothing. I was looking at Vselector and not I(L6). I added a silicon diode in series to block the commutating action and now the snubber works as expected. Actually, D6 and the added D8 are probably not needed at all and can be removed. My simulation seems to run awfully slow so I'm not going to run again to verify, but it looks much better now!

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I also changed the PWM duty cycle to about 10%. Now the input draws only about 150 mA or 800 mW.

I also updated the ASCII file:

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About half that now ;)

That was for one of the previous simulations. Probably when the MOSFET was running in linear mode due to the supply voltage crashing.

I'd be happy to review your new design before you send off for new boards. This has been an interesting project. I am not particularly "into" antique electronics but I do find them interesting, and I also enjoy working on electro-mechanical contrivances. I have lots of old electronic components and equipment, and I recently toured a local computer museum that has old mechanical computers and various examples of the transition from iron core to solid state memory, relays and vacuum tubes to transistors and ICs, and other technology. They are even restoring a big old linotype machine, and they probably have some teletypes as well.

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They may benefit from your work and you are of course welcome to visit. Let me know. I also have a friend who specializes in old electronic stuff, mostly ham radio gear, as well as juke boxes, pinball machines, and scopes and such. He also volunteers at the National Electronics Museum at BWI:

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Paul

Reply to
P E Schoen

I did a bit more on this. I removed the 555 and replaced it with a simple pulse generator with 4V output. I replaced the MOSFET with a simple 2N4401 BJT. I added some parallel capacitance to the transformer, and I tweaked the output snubber. Results seem pretty good:

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I'm sure many more improvements can be made. There is still a lot of high frequency ringing on the transformer that perhaps could be reduced. I might use a PIC for the PWM, and possibly also for the USB-serial conversion rather than the CP2102 module.

Paul

Reply to
P E Schoen

Well, I substituted that MOSFET, and it doesn't help much. It will not work with 2V on the gate. It takes 3V in the real world, and that's consistent with the data sheet. See Fig. 3 on

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and note that at 2V, it's starting to turn on, at

3V it's passing 10X as much current, and at 7V on the gate, it's fully turned on and passing 100X as much current as at 2V. This is not a 2V device.

As a test, I replaced the 555 timer signal with a bench waveform generator. This can go to 3V, but not to 5V, and at 3V, things sort of start to work. But not very well. There's no inductive kick from the transformer at turn-off. See this waveform, which is MOSFET drain to ground, with a 10x probe. This is with gate drive at 100KHz, 25% duty cycle.

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The comparable simulation works much better than the real world. Much faster rise time at MOSFET turn-off, and to a higher voltage.

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(screen)

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(LTspice)

The blue line is the drain of the MOSFET; the green line is the gate. The output voltage in the sim ramps up much faster than in the real world. Both make it to 120VDC eventually, but it's about 12ms in sim and about 500ms real world (which is too slow for the application.) I don't understand why it's so much less effective in the sim.

The model of the Coilcraft DA2032-AL transformer is probably inadequate. Here's its data sheet:

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Suggestions?

John Nagle

Reply to
John Nagle

Have you fixed the power supply voltage problem? You should be able to get

4.5 volts on the 555 and that should provide at least 2.5-3 volts gate drive. You might be able to tweak the output a bit higher with a pull-up resistor to the 5V supply.

It's probably about as good as you're going to get in a TO-220 package. There are many devices in SOIC-8 and SOT-23 packages with 1.8, 1.5, and even lower gate voltages, although the Vce may not be more than 30V. Try IRLML6344, which will conduct 3A with 0.2V drop using a gate voltage of 1.7 volts.

It appears that the supply voltage is only about 3 volts. That is much too low for the circuit to work. It also appears that the signal generator does not have much drive current which is indicated by the slow ON time and the very visible Miller plateau. The delay provides a much reduced effective duty cycle.

The simulation does not adequately model your real-world circuit. The transformer model is probably OK. You should use actual component modules for the diodes. Also, the TPS2030 shown in your original schematic is current-limited to 300 mA. It would help if you could update the schematic, and also perhaps add a proper low-side MOSFET driver. It is also possible to use a BJT such as a MPSA06 or 2N4401. With a 50 ohm base resistor, you will get about 25 mA and with hfe >= 100 you can easily drive 500 mA.

Paul

Reply to
P E Schoen

As suggested before a CMOS version of the 555 will probably go higher (check first though) but would have to be ordered.

For a quick fix hang a bus driver between 555 and FET gate. Maybe with several sections paralleled. That is usually something that can be found in the parts bins.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

The real problem appears to be the low voltage from the USB power lines. The drain voltage on the MOSFET when it is OFF is only about 3.5 volts. I think the USB switch chosen for this circuit limits current to 300 mA by becoming a current source when overloaded.

A CMOS 555 might provide a voltage close to the rail, but the output current drive is much less than the ~200 mA of the standard part. The ICM7555 CMOS part has a typical output high of 4.3 volts with 5 volt supply, at 0.8 mA. It is actually a CMOS inverter with a rated load of 2 TTL gates. For an 8 mA load, the voltage drops to 2 volts below supply rail. This is equivalent to a 250 ohm source impedance. The IRL3915 MOSFET has an input capacitance of almost 2000 pF, and this corresponds to a TC of about 500 nSec. With a 100 kHz PWM and about 10% duty cycle, the ON pulse is 1000 nSec, so the TC effectively cuts that in half. This is also seen in the scope trace.

Paul

Reply to
P E Schoen

That's why I suggested a bus driver. They provide low tens of ohms and you can usually parallel sections inside the same IC.

Also, you don't need a FET with 2000pF gate capacitance for this job.

--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

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