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/AAAAAAAAVM4/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, still well below the limit.) Second, I was planning on running the PWM at 200kHz because it allows the boost to run in continuous mode with a much smaller load. This way I can test with a single nixie (around 1.5mA). I have worked through the boost calculations and the PWM register values in a spreadsheet here <https://docs.google.com/spreadsheets/d/1AlgrgIQSByZBakUH5zwjJ94VcO_23aLYUsACbMPxswc/edit?usp=sharing>. Please take a look if you are curious. I may be off base with my desired values. One concern I had based on your feedback is the ratio of the A to B register. This ratio sets the duty cycle of the PWM output, right? To boost 24V to 200V, I need to duty cycle of 88%, which exceeds your recommendation of 80% max. Can you talk a little bit about why this is important? I know the inductor needs some time to discharge, but isn't controlling that on/off time the only way to set the output voltage? I also had a question about driving the FET gate. Why set the pin to an input and rely on the pulldown rather than driving the pin low? I really like the soft start idea and the guidance about using the ADC interrupt routine. Plus I appreciate your advice on ramping the voltage up slowly, I will make sure I take care when testing (and especially debugging). Good to know about the HV5530 at 5V too! -Brian On Wednesday, June 10, 2015 at 5:34:19 AM UTC-4, petehand wrote: > > I've used the main CPU as the switching regulator in just about all my HV > projects, and I can tell you exactly how to go about it. A word of warning, > though - the STM chip is 3.3V VCC. That is exceedingly marginal for turning > on even a logic level FET of the rating you need. You should probably > choose a different processor. Anyway, to the technique. > > Choose a timer for PWM. It needs two compare registers, one for setting > the maximum count (A) and one for toggling an output (B). Most CPUs have > this. Set it up for a free running frequency of at least 30kHz - that is, > it runs from 0 to max count and resets in 30us. 40kHz is better (25us). It > doesn't need many bits - 8 bits is fine for this job. Example, if you have > an 8 bit counter clocked at 8MHz, the modulus (maximum count, 'A' compare > register) would be 200 for 40kHz. Set this value in the A register and a > low value, like 1, in the B register. This gives you a soft start. > > The pin toggled by the B comparator should be settable as input or output. > Connect this pin to the FET gate with a 2.2k pulldown resistor. When the > pin is set as input, this should pull down close to ground to keep the FET > off. After reset, start off with this pin as input. > > Choose an ADC channel and input, and connect through a resistor divider to > the HV output. Calculate what the ADC reading will be at the desired > voltage - this will depend on the reference voltage, the resistor values, > etc. Set the ADC going in free run, with an interrupt on conversion > complete. > > In your ADC interrupt routine, compare the ADC reading to the setpoint you > calculated. If it is below, increment the counter 'B' compare register by > 1, up to about 80% of the top value you set in the 'A' compare register. > Don't go past that point! The FET needs some off time to discharge the > inductor. Then set the PWM pin as an output. If the ADC reading is above > the setpoint, decrement the 'B' compare by 1 and set the PWM pin as input, > to turn off the FET. If it is equal, do nothing. When starting out from > nothing you may need to limit the B register to less than 80% of A, to > limit the current. I start mine out with a 25% limit and change that to 80% > when the voltage comes up above 75%. At low voltage there's no load so it > comes up quickly even with a low duty cycle. > > For 20-30kHz, you need about a 220 to 270 microhenry inductor with a > saturation current of 1A or more. The IRF640 is a suitable driver. You'll > find there's a limited choice of high voltage FETs with a logic level gate. > A technique I've been using for some time is to use a center tapped > inductor with the FET attached at the center tap. That way it only > experiences half the HV pulse. > > The only problem I have with this system is when I'm debugging and stop at > a break point. Because the PWM is completely asynchronous there's a good > chance the processor halts with the FET turned on, and things quickly get > quite warm. I find it expedient to put a jumper in the FET gate that I can > remove to shut down the HV. Most of the time when I'm debugging I don't > need the tubes lit anyway. For the finished clock no doubt you'll have a > watchdog timer, and the first thing that will do when it trips is turn off > the PWM port. > > If you had a 5V processor you could use the HV5530 with a VCC of 5V and no > level shifting. It works perfectly well at 5V for nixie currents, but not > at full speed - the clock timing is about 4 times longer. I've tried it at > 3.3V though and it's no go. > > > On Tuesday, June 9, 2015 at 5:54:56 PM UTC-7, Brian wrote: >> >> I do have a scope, via work. I know a little about switching converter >> design, but I've never had to design one at this level. Getting the >> converter right is the main reason I am doing this small board before >> jumping fully into the project. Hopefully I can lean on you guys a little >> until I get it right! >> >> -Brian > > -- 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 [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/e6adc750-12e6-4060-813e-11d208f13b32%40googlegroups.com. 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