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.) John Nagle
MOSFET selection for boost converter problem
Started by ●November 17, 2016
Reply by ●November 17, 20162016-11-17
On 11/17/2016 02:33 AM, 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. > > 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.) > > John Nagle >One thing I notice is that it looks like you're using a bipolar 555 from a 5 volt supply to drive the '8721, which has a max Vth of ~2.4 volts. However the bipolar 555's output high voltage is going to be quite a bit lower than the supply voltage - I've seen it as much as 2 volts lower than Vcc in practice. And maybe I'm reading the schematic wrong, but it looks like you've got a 680 ohm resistor in the 555's power line. That's not going to help things. Unfortunately I think that using a 555 as a gate driver from a 5v supply is a poor choice here. Needs to be driven harder.
Reply by ●November 17, 20162016-11-17
On 11/17/2016 08:05 AM, bitrex wrote:> On 11/17/2016 02:33 AM, 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. >> >> 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.) >> >> John Nagle >> > > One thing I notice is that it looks like you're using a bipolar 555 from > a 5 volt supply to drive the '8721, which has a max Vth of ~2.4 volts. > However the bipolar 555's output high voltage is going to be quite a bit > lower than the supply voltage - I've seen it as much as 2 volts lower > than Vcc in practice. And maybe I'm reading the schematic wrong, but it > looks like you've got a 680 ohm resistor in the 555's power line. That's > not going to help things. > > Unfortunately I think that using a 555 as a gate driver from a 5v supply > is a poor choice here. Needs to be driven harder.Also I think 100kHz is a pretty high frequency to run a standard 555 at and expect it to source/sink much current from a load during the switching cycles (which needs to be done.) The output BJTs don't turn on and off too quick.
Reply by ●November 17, 20162016-11-17
On Wed, 16 Nov 2016 23:33:57 -0800, John Nagle <nagle@animats.com> 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. > > 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.) > > John NagleThere'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? Why all that isolation? Aren't the keyboard and magnet isolated already? -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●November 17, 20162016-11-17
On 11/17/2016 11:36 AM, John Larkin wrote:> On Wed, 16 Nov 2016 23:33:57 -0800, John Nagle <nagle@animats.com> > 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. >> >> 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.) >> >> John Nagle > > 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? > > Why all that isolation? Aren't the keyboard and magnet isolated > already? > >Looks like power to the flyback is coming from the TPS2030 high-side switch, OP says he needs 120V @ 60 mA for a few mS, but from the '2030 data sheet it says that the switch current-limits at 300mA. Could be a problem
Reply by ●November 17, 20162016-11-17
On 11/17/2016 12:09 PM, bitrex wrote:> On 11/17/2016 11:36 AM, John Larkin wrote: >> On Wed, 16 Nov 2016 23:33:57 -0800, John Nagle <nagle@animats.com> >> 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. >>> >>> 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.) >>> >>> John Nagle >> >> 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? >> >> Why all that isolation? Aren't the keyboard and magnet isolated >> already? >> >> > > Looks like power to the flyback is coming from the TPS2030 high-side > switch, OP says he needs 120V @ 60 mA for a few mS, but from the '2030 > data sheet it says that the switch current-limits at 300mA. Could be a > problemSeems unnecessary, could just use a gate driver IC with an enable pin.
Reply by ●November 17, 20162016-11-17
On Thu, 17 Nov 2016 12:12:56 -0500, bitrex <bitrex@de.lete.earthlink.net> wrote:>On 11/17/2016 12:09 PM, bitrex wrote: >> On 11/17/2016 11:36 AM, John Larkin wrote: >>> On Wed, 16 Nov 2016 23:33:57 -0800, John Nagle <nagle@animats.com> >>> 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. >>>> >>>> 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.) >>>> >>>> John Nagle >>> >>> 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? >>> >>> Why all that isolation? Aren't the keyboard and magnet isolated >>> already? >>> >>> >> >> Looks like power to the flyback is coming from the TPS2030 high-side >> switch, OP says he needs 120V @ 60 mA for a few mS, but from the '2030 >> data sheet it says that the switch current-limits at 300mA. Could be a >> problem > >Seems unnecessary, could just use a gate driver IC with an enable pin.It all looks very complex to me. More stuff to go wrong. -- John Larkin Highland Technology, Inc lunatic fringe electronics
Reply by ●November 17, 20162016-11-17
On 11/17/2016 12:50 PM, John Larkin wrote:> On Thu, 17 Nov 2016 12:12:56 -0500, bitrex > <bitrex@de.lete.earthlink.net> wrote: > >> On 11/17/2016 12:09 PM, bitrex wrote: >>> On 11/17/2016 11:36 AM, John Larkin wrote: >>>> On Wed, 16 Nov 2016 23:33:57 -0800, John Nagle <nagle@animats.com> >>>> 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. >>>>> >>>>> 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.) >>>>> >>>>> John Nagle >>>> >>>> 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? >>>> >>>> Why all that isolation? Aren't the keyboard and magnet isolated >>>> already? >>>> >>>> >>> >>> Looks like power to the flyback is coming from the TPS2030 high-side >>> switch, OP says he needs 120V @ 60 mA for a few mS, but from the '2030 >>> data sheet it says that the switch current-limits at 300mA. Could be a >>> problem >> >> Seems unnecessary, could just use a gate driver IC with an enable pin. > > It all looks very complex to me. More stuff to go wrong.The Iron Rule of the 555: It's a very useful IC, which will always be the wrong choice for whatever application you have in mind.
Reply by ●November 17, 20162016-11-17
On 11/17/2016 02:14 PM, bitrex wrote:> On 11/17/2016 12:50 PM, John Larkin wrote: >> On Thu, 17 Nov 2016 12:12:56 -0500, bitrex >> <bitrex@de.lete.earthlink.net> wrote: >> >>> On 11/17/2016 12:09 PM, bitrex wrote: >>>> On 11/17/2016 11:36 AM, John Larkin wrote: >>>>> On Wed, 16 Nov 2016 23:33:57 -0800, John Nagle <nagle@animats.com> >>>>> 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. >>>>>> >>>>>> 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.) >>>>>> >>>>>> John Nagle >>>>> >>>>> 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? >>>>> >>>>> Why all that isolation? Aren't the keyboard and magnet isolated >>>>> already? >>>>> >>>>> >>>> >>>> Looks like power to the flyback is coming from the TPS2030 high-side >>>> switch, OP says he needs 120V @ 60 mA for a few mS, but from the '2030 >>>> data sheet it says that the switch current-limits at 300mA. Could be a >>>> problem >>> >>> Seems unnecessary, could just use a gate driver IC with an enable pin. >> >> It all looks very complex to me. More stuff to go wrong. > > The Iron Rule of the 555: It's a very useful IC, which will always be > the wrong choice for whatever application you have in mind. >Well, given that an MC34063 is 18 cents and an LM555 is 10, the 555 is wrong for this one, sure. But CMOS 555s are useful sometimes, e.g. for chopping LEDs. I use them all the time in protos, though not recently in a finished design. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Reply by ●November 17, 20162016-11-17
"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 justused 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 (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. (7) I don't see any bypass capacitors on your actual schematic on the nominal 5V supply. (8) You have a TPS2030 USB switch for the enable. This part may be obsolete, but probably not a problem. (9) There is U5 that looks like "IEQ524S" but I can't find it. Looks like an interface to the optos. Enough for now. Paul
