Re: [neonixie-l] Re: First Nixie Project Schematic Review
I was looking at my posts again and see I missed out a necessary tip. The ADC conversion period *must be longer* than the PWM rate, otherwise it may go crazy making multiple changes to the period in the same cycle. The ADC and the PWM don't need to be synchronized. I find making the conversion time between three and four times the PWM period gives a good balance between transient response and good regulation. -- You received this message because you are subscribed to the Google Groups neonixie-l group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/28d30327-3f8f-4cfd-a2c5-ce192077b1b1%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
Re: [neonixie-l] Re: First Nixie Project Schematic Review
The FET current is governed not by the output power, but by the inductance and its time constant. The output voltage is immaterial. Once the magnetic energy is stored in the inductor and the current cuts off, that energy WILL be dumped, by hook or by crook, even if it takes a million volts to do it. If you don't have an output capacitor it will rise until something breaks down, which is usually the FET. So it's meaningless to say it needs a duty cycle of X to achieve Y voltage, since it can achieve any Y with any X in a single cycle. With the capacitor in circuit, it will charge it higher by Q/C volts every cycle (where Q = It, the charge generated by the collapsing magnetic field). The calculation you need to apply is how much input power at 24V is needed to maintain the capacitor output voltage at the output current you desire. If it's 200v at 35mA, 7000mW, then that's the power you have to put in (plus losses) at 24V - say 350mA. That's 350mW dissipation at the optimistic typical Rds(on) for 100mA. But 350mA is an average, delivered in pulses of much higher current, and until the capacitor is charged and achieves regulation those pulses will be longer and heavier. The FET will be drawing over an amp in pulses, will have Rds(on) more in the region of 10 ohms and will rapidly heat up, and as it heats up its Rds(on) increases, so it gets hotter, and then it unsolders itself. Believe me, I've seen it happen and still have the holes burned in my fingertips to remind me. You should be looking for an FET with an Rds(on) in the 100 milliohm range at 1 amp. You can experiment and find out for yourself but I strongly recommend the big fat IRL640 - it's threshold is in the 2V range so it should work. There's another reason for this, which is the inductive time constant, L/R. As the on resistance increases the time constant gets smaller, so you need higher inductance to keep it from saturating during the on period. Controlling the on time is the *only* thing that affects the output voltage. Less on time means less magnetic energy stored in the inductor, so less charge dumped into the capacitor each cycle. But you must always have *some* off time, for that energy to be dumped, as if you don't limit it the PWM can run up to the point where the inductor doesn't have time to discharge fully and still retains some flux. Then next cycle it ramps to a higher flux density, and higher every time, until it saturates and your FET goes up in smoke. My rule of thumb is 20% of the cycle time. You can have less, but the longer the on time, the higher the FET current. I set the PWM pin to an input because that can be done in a single instruction, whereas to drive it low you first have to disable the PWM, and then later it has to be enabled again, which is an unnecessary complication. I just let the PWM continue to run with its output disabled and let the resistor hold the FET off. If you can run the PWM at 200kHz, good luck. With AVRs at a modest clock rate that doesn't allow enough counts for regulation, but with a gigahertz ARM I guess that's not a problem. However, the gate capacitance of the FET has to be driven, and that too has a time constant. The turn on time is unimportant but the turn off time is of the highest importance, since during the interval from where the gate starts to pinch until it shuts off completely, all that stored magnetic energy is being used to heat up the FET, not being dumped into the output. There is always some turn-off loss, every cycle. When the cycle time is 25us you only pay this fare 40k times a second. At 5us you'll pay five times as much. It takes a lot of skill to reduce that loss and it really only became practical to run converters above 100kHz when the switches could be integrated. 200kHz is a worthy aim but a very long drive, across water with deep bunkers on the other bank. On Wednesday, June 10, 2015 at 12:45:41 PM UTC-7, Brian wrote: Thank you very much, Pete, for the insights! It will be great to have someone that's done this before as a resource. I have a few questions. First, I did think about driving the FET from 3.3V, so I selected it carefully. I am using the Vishay TN2404K, and assuming a Vgs of 3.3V, it looks like Rds(on) is about 2.3 ohms: https://lh3.googleusercontent.com/-C8b3V6gWFgY/VXhR1u8tLeI/VM4/xpHLMYM7F0s/s1600/TN2404K_rdson.png Assuming 35mA max current draw (ballparking 4 IN-18 tubes as worst case), the voltage drop across the FET would be about 81mV, and the power dissipation about 3mW. The datasheet says the FET is capable of 360mW of dissipation (at room temp), so I felt pretty good about using them. Is there something I am missing there? (As a cheaper alternative I may use the BSS131, which I am already using to drive the nixie channels. Making the same assumptions, Rds(on) is about 10.3 ohms, which a voltage drop of 361 mV, and power dissipation of about 13mW,
Re: [neonixie-l] Re: First Nixie Project Schematic Review
If you need a good MOSFET with a low Vgs(on), consider the DMN6040SVT. It has a low Rds(on), around 60 milliohms. My wristwatch uses this for the DC-DC converter for these reasons, and also because it has very low leakage. I drive the gate at 3.2V . BUT.if you use this device, you must use a transformer (basically 2 coupled inductors), to reduce the kickback spike that occurs while the HV cap is charging. The DMN6040SVT will only tolerate 60V, so if you want to generate 180V, you will want a transformer that is at least a 4:1 turns ratio. Mine is 10:1 . Finding low Vgs(on) devices with a high Vds is difficult; there are not very many and this was the best one I could find for my application. You may want to look into circuit changes that will give you a higher Vgs. -- You received this message because you are subscribed to the Google Groups neonixie-l group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/075bfa73-0dee-43fe-bd62-138960054193%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
[neonixie-l] Re: First Nixie Project Schematic Review
Others have used the HV5522 (I'm designing with the HV5530) because it combines the shift-register with the HV driver. The only drawback is that you have to level-shift the inputs to the HV55xx to 12V. I'm using comparators for that; have yet to run spice simulations to optimize for noise-rejection, etc. -- You received this message because you are subscribed to the Google Groups neonixie-l group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/0315fe32-f22d-4762-91d6-7f970aa2cd6b%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
[neonixie-l] Re: First Nixie Project Schematic Review
Thanks for letting me know that parts exists. I didn't know they were making combo parts like that. I will probably stick with the design I have for now...Newark has the BSS131 on sale for $0.063, and the 74HC595 on sale for $0.066, which means 32-bits of output costs $2.28. The lowest price I could find for the HV5522 was $5.53, though I guess the cost is made up in solder time :-). (I also already have the 24V adapter and no 12V rail.) Thanks for the comments, you should post your preliminary design too! -Brian On Tuesday, June 9, 2015 at 4:52:53 PM UTC-4, gregebert wrote: Others have used the HV5522 (I'm designing with the HV5530) because it combines the shift-register with the HV driver. The only drawback is that you have to level-shift the inputs to the HV55xx to 12V. I'm using comparators for that; have yet to run spice simulations to optimize for noise-rejection, etc. -- You received this message because you are subscribed to the Google Groups neonixie-l group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/fe0f4ee1-e23a-4984-9726-ff39910ab8d1%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
Re: [neonixie-l] Re: First Nixie Project Schematic Review
Brian, There are people on this list who have actually built such power supplies. They would know better than I would, what's the best protection against wayward boost drivers. On 6/9/2015 2:55 PM, Brian wrote: Ha, that's a good point, thanks! Do you think I should throw a pulldown on the gate of Q1 to solve that? On Tuesday, June 9, 2015 at 5:46:52 PM UTC-4, nixiebunny wrote: Brian, Other than that, it looks like it will work, as long as you have a guarantee that your boost converter FET gate will not be enabled for longer than a few microseconds incase the CPU goes stupid. -- You received this message because you are subscribed to the Google Groups neonixie-l group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/55776ED7.8080003%40dakotacom.net. For more options, visit https://groups.google.com/d/optout.
Re: [neonixie-l] Re: First Nixie Project Schematic Review
Ha, that's a good point, thanks! Do you think I should throw a pulldown on the gate of Q1 to solve that? On Tuesday, June 9, 2015 at 5:46:52 PM UTC-4, nixiebunny wrote: Brian, A lower soldering time option for the transistors is the TD62084, which has eight 50V transistors in a 18 pin DIP or TSSOP. The SN75468 has seven 100V transistors in a 16 pin DIP. Other than that, it looks like it will work, as long as you have a guarantee that your boost converter FET gate will not be enabled for longer than a few microseconds incase the CPU goes stupid. There is a story about that - the folks who beat the roulette wheel using physics (read The Eudaemonic Pie) built a computer in a bra, but the tapping solenoids that communicated to the wearer would stay engaged when the CPU crashed. Hot! Ouch! A series capacitor and a pulldown resistor on the transistor's input solved that problem. On 6/9/2015 2:23 PM, Brian wrote: Thanks for letting me know that parts exists. I didn't know they were making combo parts like that. I will probably stick with the design I have for now...Newark has the BSS131 on sale for $0.063, and the 74HC595 on sale for $0.066, which means 32-bits of output costs $2.28. The lowest price I could find for the HV5522 was $5.53, though I guess the cost is made up in solder time :-). (I also already have the 24V adapter and no 12V rail.) Thanks for the comments, you should post your preliminary design too! -Brian On Tuesday, June 9, 2015 at 4:52:53 PM UTC-4, gregebert wrote: Others have used the HV5522 (I'm designing with the HV5530) because it combines the shift-register with the HV driver. The only drawback is that you have to level-shift the inputs to the HV55xx to 12V. I'm using comparators for that; have yet to run spice simulations to optimize for noise-rejection, etc. -- You received this message because you are subscribed to the Google Groups neonixie-l group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/228d47af-59ad-463d-9791-41321a70faca%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
Re: [neonixie-l] Re: First Nixie Project Schematic Review
Brian, A lower soldering time option for the transistors is the TD62084, which has eight 50V transistors in a 18 pin DIP or TSSOP. The SN75468 has seven 100V transistors in a 16 pin DIP. Other than that, it looks like it will work, as long as you have a guarantee that your boost converter FET gate will not be enabled for longer than a few microseconds incase the CPU goes stupid. There is a story about that - the folks who beat the roulette wheel using physics (read The Eudaemonic Pie) built a computer in a bra, but the tapping solenoids that communicated to the wearer would stay engaged when the CPU crashed. Hot! Ouch! A series capacitor and a pulldown resistor on the transistor's input solved that problem. On 6/9/2015 2:23 PM, Brian wrote: Thanks for letting me know that parts exists. I didn't know they were making combo parts like that. I will probably stick with the design I have for now...Newark has the BSS131 on sale for $0.063, and the 74HC595 on sale for $0.066, which means 32-bits of output costs $2.28. The lowest price I could find for the HV5522 was $5.53, though I guess the cost is made up in solder time :-). (I also already have the 24V adapter and no 12V rail.) Thanks for the comments, you should post your preliminary design too! -Brian On Tuesday, June 9, 2015 at 4:52:53 PM UTC-4, gregebert wrote: Others have used the HV5522 (I'm designing with the HV5530) because it combines the shift-register with the HV driver. The only drawback is that you have to level-shift the inputs to the HV55xx to 12V. I'm using comparators for that; have yet to run spice simulations to optimize for noise-rejection, etc. -- You received this message because you are subscribed to the Google Groups neonixie-l group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/55775ED7.5090409%40dakotacom.net. For more options, visit https://groups.google.com/d/optout.
