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Author Topic: PWM chip with constant impulse time ?  (Read 8078 times)

forest

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PWM chip with constant impulse time ?
« on: April 19, 2015, 04:43:00 PM »
Hi


I have a question for experienced electronic gurus here :


Do you know any chip which is working similiar to SG3525 or any other PWM , but with fixed (which can be set by user) ON time , while managing output voltage (like in DC-DC converters using voltage divider feedback) but with variable OFF time (and thus variable frequency) ?
Is there a known method of such kind of building DC converters ? How is that kind called ?


MarkE ? TinselKoala ? Somebody ?
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picowatt

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Re: PWM chip with constant impulse time ?
« Reply #1 on: April 19, 2015, 04:53:41 PM »
Quote from: forest on April 19, 2015, 04:43:00 PM
Hi


I have a question for experienced electronic gurus here :


Do you know any chip which is working similiar to SG3525 or any other PWM , but with fixed (which can be set by user) ON time , while managing output voltage (like in DC-DC converters using voltage divider feedback) but with variable OFF time (and thus variable frequency) ?
Is there a known method of such kind of building DC converters ? How is that kind called ?


MarkE ? TinselKoala ? Somebody ?


Pulse Frequency Modulation  (PFM)

http://www.digikey.com/en/articles/techzone/2014/mar/the-advantages-of-pulse-frequency-modulation-for-dc-dc-switching-voltage-converters
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forest

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Re: PWM chip with constant impulse time ?
« Reply #2 on: April 19, 2015, 05:58:09 PM »
Thanks picowatt


Do you know any simple popular pure PFM chip ?
Do you know if voltage regulation feedback loop in  PFM mode can be made using software code inside such microcontrollers as Attiny or Atmega ? Any code for that ? Mainly I wonder how to implement load regulation in case somebody switched off/on most of load and overvoltage on output capacitor should be prevented.
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Drak

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Re: PWM chip with constant impulse time ?
« Reply #3 on: April 19, 2015, 06:19:51 PM »
Hi forest,

I'm building a switching power supply using AVR's and other helper chips (mostly out of boredom). The ATtiny26 can PWM out at 250khz. I'm using an ATmega168p to control the ATtiny26's on time 0-255. If you want faster pwm than that you will either have to sacrifice resolution or choose a different chip.
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forest

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Re: PWM chip with constant impulse time ?
« Reply #4 on: April 19, 2015, 06:33:43 PM »
Drak


Maybe, but I'm not using PWM nor plan to use it (Well, maybe if I investigate it deeper to find the way to limit ON time width).
Thanks.PFM is exacly what I want with analog pin watching output capacitor voltage. Seems a lot of hacking before me to implement correct code :-)
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Drak

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Re: PWM chip with constant impulse time ?
« Reply #5 on: April 19, 2015, 06:47:41 PM »
forest,

Ah I see. I'm only familiar with the AVR family, If I were wanting PFM I'm not sure how I could get high frequency PFM but I would use an adjustable external clock source to run the MCU (such as an ad9850). Its what I'm doing to run the sinewave to the ATTiny.

http://draksplace.com/PowerSupply.png

Sorry I can't be of much more help.
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forest

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Re: PWM chip with constant impulse time ?
« Reply #6 on: April 19, 2015, 08:02:57 PM »
Well...if you need pure sinewave , there is cheap ready to go H-bridge driver with all functionality to build DC-AC pure sinewave inverter on ebay . Below 10$ with free shipping. But of course doing it own way is a fun too.
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MarkE

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Re: PWM chip with constant impulse time ?
« Reply #7 on: April 20, 2015, 03:16:23 AM »
Quote from: forest on April 19, 2015, 04:43:00 PM
Hi


I have a question for experienced electronic gurus here :


Do you know any chip which is working similiar to SG3525 or any other PWM , but with fixed (which can be set by user) ON time , while managing output voltage (like in DC-DC converters using voltage divider feedback) but with variable OFF time (and thus variable frequency) ?
Is there a known method of such kind of building DC converters ? How is that kind called ?


MarkE ? TinselKoala ? Somebody ?
There are lots of PWM chips designed to have constant on time or constant off time.  TI in particular offers a bunch to the automotive market.  Many of those converters use hysteretic control.  One buck converter example that works very nicely is the LM25017.  http://www.ti.com/lit/ds/symlink/lm25017.pdf
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MarkE

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Re: PWM chip with constant impulse time ?
« Reply #8 on: April 20, 2015, 03:22:58 AM »
Quote from: forest on April 19, 2015, 05:58:09 PM
Thanks picowatt


Do you know any simple popular pure PFM chip ?
Do you know if voltage regulation feedback loop in  PFM mode can be made using software code inside such microcontrollers as Attiny or Atmega ? Any code for that ? Mainly I wonder how to implement load regulation in case somebody switched off/on most of load and overvoltage on output capacitor should be prevented.
PFM can easily be implemented with a counter timer in AVR's.  I would use an external comparator though, because the comparator in the AVRs is not very good.  The main problem with using a uC based device for DC-DC is that the dedicated IC's are really, really good and some are very cheap.  The limited counter frequency in the parts without PLLs and the power consumption in the parts with PLLs makes an external design superior in most cases.  I have done some pretty tricky designs in AVRs including designs that transition from fixed on time to fixed off time to minimize ripple.  I have found that the savings in absolute cost and real estate of the energy storage components favor dedicated PWM hardware.
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forest

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Re: PWM chip with constant impulse time ?
« Reply #9 on: April 20, 2015, 12:32:06 PM »
Thanks MarkE


Can I use output of this lm25017 for driving external mosfet ? Seems that output can go below zero ?  Any other chip dedicated for external mosfet switch using constant ON-time, variable off- time with integrated comaparator and maybe hysteresis loop ?
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forest

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Re: PWM chip with constant impulse time ?
« Reply #10 on: April 20, 2015, 12:52:35 PM »
Found some Ti chip here http://www.ti.com/lit/ds/symlink/lm25085-q1.pdf but it has PFET gate driver. Is the output reversed then ? Can I connect a N Channel FET with a mosfet driver or totem pole or whatever to drive nfet in low side mode here ?
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forest

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Re: PWM chip with constant impulse time ?
« Reply #11 on: April 20, 2015, 01:21:06 PM »
This one looks nice http://www.ti.com/lit/ds/symlink/lm5006.pdf What do you think ?
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MarkE

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Re: PWM chip with constant impulse time ?
« Reply #12 on: April 20, 2015, 01:23:16 PM »
Quote from: forest on April 20, 2015, 12:52:35 PM
Found some Ti chip here http://www.ti.com/lit/ds/symlink/lm25085-q1.pdf but it has PFET gate driver. Is the output reversed then ? Can I connect a N Channel FET with a mosfet driver or totem pole or whatever to drive nfet in low side mode here ?
It would be easier to help you if you would describe what you are trying to build:  Buck or boost, positive or negative input voltage, positive or negative output voltage.
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forest

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Re: PWM chip with constant impulse time ?
« Reply #13 on: April 21, 2015, 10:19:20 AM »
It looks more like a simple flyback converter. Can't describe precisely . All I need is the chip with PFM and output voltage loop to control voltage on output capacitor. Chip cannot be used directly as per datasheet but only to drive external N-Channel mosfet, because  I need to switch 300-400VDC. The output should also be 300-400VDC. :-) and that's why   feedback is required to change OFF-time.
The most important is constant ON-time, variable OFF time.
I will try to adapt my Attiny45 code for that. So far I learned how to merge two timers to work together  to get constant ON-time pulse and a period between, the problem I have is how to change timer0 period on the fly without glitch (timer0 is working in simple CTC mode while timer1 is doing the ON time pulse).
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MarkE

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Re: PWM chip with constant impulse time ?
« Reply #14 on: April 21, 2015, 10:37:03 AM »
Quote from: forest on April 21, 2015, 10:19:20 AM
It looks more like a simple flyback converter. Can't describe precisely . All I need is the chip with PFM and output voltage loop to control voltage on output capacitor. Chip cannot be used directly as per datasheet but only to drive external N-Channel mosfet, because  I need to switch 300-400VDC. The output should also be 300-400VDC. :-) and that's why   feedback is required to change OFF-time.
The most important is constant ON-time, variable OFF time.
I will try to adapt my Attiny45 code for that. So far I learned how to merge two timers to work together  to get constant ON-time pulse and a period between, the problem I have is how to change timer0 period on the fly without glitch (timer0 is working in simple CTC mode while timer1 is doing the ON time pulse).
To use a part like an ATTINY25/45/85 use timer1 in pwm mode.  Use either OC1A or OC1B for your output using the COM1xy bits and the associated compare register for the fixed pulse width.  That will give you hardware controlled timing for the on pulse.  Use OC1C to set the period.  If you use an AVR with a 16 bit timer1 make sure that you update the 16 bit registers atomically.
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