Low power PWM controller, possibly the 555 (flame war coming)

On a sunny day (Thu, 30 Aug 2012 04:05:39 -0700 (PDT)) it happened Klaus Kragelund wrote in :

control.

PWM chips out there with low operating current.

I know it is an old song, but I am using PICs (PWM output, build in 2 comparators, calibration in EEPOM) for all that. Why go looking for chips when I have one in the box that can do almost anything, given some software capabilities.

wer PWM control.

there a PWM chips out there with low operating current.

parators,

ything, given some software capabilities.

Well, I have a Cortex M3 running at 4MHz, but the sw guys don't like me using that in a SW loop.

I could generate a PWM to set the frequency, add F/F to set the output, and a comparator to reset the output (current mode control). Then add an voltage error amp to control the peak current level.

Going further, I could use the internal comparator for the reset of the PWM for peak current control without CPU resources used, but the comparator has 2-4us propagation delay, seriously lowering the sensible frequency the converter can run at. I guess the delay is just a level shift of the peak current level and that is regulated back by the error amp anyway, so maybe it could work. Would however have a minimum duty cycle of 20% (for 100kHz and 2us propagation delay)

For that solution I need to add a circuit to monitor the current and reset the CPU if it hangs. That takes some more parts in the wrong direction.

But, I would like to see if there is a PWM controller that has low enough current anyway.

Regards

Klaus

control.

PWM chips out there with low operating current. Sofar I found the TPS5110 with sub mA current consumption:

That's a buck controller, technically not what you'd use in a forward converter. Are you actually doing a non-isolated buck?

What are your input and output voltages?

PWM control.

a PWM chips out there with low operating current. Sofar I found the TPS5110 with sub mA current consumption:

Yes, its a buck, and it has to large max duty cycle. Just an example of a low current PWM.

My input voltage is from about 8V to 20V, output 3.3V. Transformer will probably be 1:1 so I can use off the shelf types approved for EN69650.

Regards

Klaus

PWM control.

there a PWM chips out there with low operating current. Sofar I found the TPS5110 with sub mA current consumption:

current PWM.

probably be 1:1 so I can use off the shelf types approved for EN69650.

Transformer? A buck uses an inductor!

What is the topology?

Well, a forward, isolated Buck. Sorry if that was not clear.

Use a 6-pin PIC, program it yourself, and pass it off as an analog component.

Do something like this, but configured for forward operation instead:

You can save the bias current wasted in the switching transistor by using a positive feedback winding, making the circuit a basic blocking oscillator instead.

Tim

-- Deep Friar: a very philosophical monk. Website:

low power PWM control.

isn't there a PWM chips out there with low operating current. Sofar I = found the TPS5110 with sub mA current consumption:

a low current PWM.

probably be 1:1 so I can use off the shelf types approved for EN69650.

You clearly missed the isolation requirement implied by EN69650.

?-)

power PWM control.

there a PWM chips out there with low operating current. Sofar I found the TPS5110 with sub mA current consumption:

current PWM.

probably be 1:1 so I can use off the shelf types approved for EN69650.

EN69650? I don't have that.

The problem wasn't clearly stated. It still isn't.

Well, I originally wrote it was a Forward, and that is commonly known as an isolated buck.....

Regards

Klaus

isolated buck.....

Got a basic schematic of your topology? Is load regulation needed? Is there isolated feedback?

--

John Larkin Highland Technology Inc

Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators

snipped-for-privacy@gmail.com a écrit :

isolated buck.....

The question being: why do you want a forward at this low power level?

an isolated buck.....

Very basic schematics here:

Please note that it is simplified as I am not allowed to post details to th= e web.

Single switch forward with extra winding reset. Chosen since the input volt= age is not steady 20V, but can drop very fast, which would pose a problem w= ith half-bridge solution (capacitor midpoint slewing slowly) and since acce= ss to the current is easy (sense resistor)

I need high efficiency, so will probably convert this to ZVS topology later= on. The FET is controlled by microcontroller PWM. This PWM is setup as high dut= y cycle (45%), but peak current is detected by the comparator which in turn= resets the PWM (input to PWM module) before reaching 45%.

On the output I have classical TL431 regulation with optoisolated feedback = to the primary side. (not shown). Larger caps needs to be placed on the out= put to ease the response time of the regulation loop (microcontroller measu= red error voltage)

I have a need for an extra voltage on the primary side for internal supply,= but have not yet found off-the-shelf magnetics that facilitates this.

Thanks

Klaus

isolated buck.....

web.

is not steady 20V, but can drop very fast, which would pose a problem with half-bridge solution (capacitor midpoint slewing slowly) and since access to the current is easy (sense resistor)

cycle (45%), but peak current is detected by the comparator which in turn resets the PWM (input to PWM module) before reaching 45%.

the primary side. (not shown). Larger caps needs to be placed on the output to ease the response time of the regulation loop (microcontroller measured error voltage)

have not yet found off-the-shelf magnetics that facilitates this.

That seems awfully complex for a 100 mW converter.

I've done similar things with ISDN line transformers, but much simpler circuits.

A flyback converter would be simpler, too.

as an isolated buck.....

the web.

oltage is not steady 20V, but can drop very fast, which would pose a proble= m with half-bridge solution (capacitor midpoint slewing slowly) and since a= ccess to the current is easy (sense resistor)

ter on.

duty cycle (45%), but peak current is detected by the comparator which in t= urn resets the PWM (input to PWM module) before reaching 45%.

ck to the primary side. (not shown). Larger caps needs to be placed on the = output to ease the response time of the regulation loop (microcontroller me= asured error voltage)

ly, but have not yet found off-the-shelf magnetics that facilitates this.

Sounds interesting, how can you make it much simpler? (notice that either t= he diodes or the FETs are mounted on the output, the FETs are for syncronou= s rectification)

Yes, that would be simpler, but a flyback must be gapped, so no off-the-she= lf magnetics in small sizes are available.

Regards

Klaus

an isolated buck.....

web.

voltage is not steady 20V, but can drop very fast, which would pose a problem with half-bridge solution (capacitor midpoint slewing slowly) and since access to the current is easy (sense resistor)

on.

cycle (45%), but peak current is detected by the comparator which in turn resets the PWM (input to PWM module) before reaching 45%.

to the primary side. (not shown). Larger caps needs to be placed on the output to ease the response time of the regulation loop (microcontroller measured error voltage)

but have not yet found off-the-shelf magnetics that facilitates this.

diodes or the FETs are mounted on the output, the FETs are for syncronous rectification)

magnetics in small sizes are available.

How about something like this?

wn as an isolated buck.....

to the web.

voltage is not steady 20V, but can drop very fast, which would pose a prob= lem with half-bridge solution (capacitor midpoint slewing slowly) and since= access to the current is easy (sense resistor)

later on.

h duty cycle (45%), but peak current is detected by the comparator which in= turn resets the PWM (input to PWM module) before reaching 45%.

back to the primary side. (not shown). Larger caps needs to be placed on th= e output to ease the response time of the regulation loop (microcontroller = measured error voltage)

pply, but have not yet found off-the-shelf magnetics that facilitates this.

the diodes or the FETs are mounted on the output, the FETs are for syncron= ous rectification)

helf magnetics in small sizes are available.

considered push-pull something like a MAX845 ?

you must have a 3.3V available for the MCU, can you use that? probably a lot more to choose from at that voltage, 20V is a bit to high for most of the low power parts aimed at battery supply

-Lasse

an isolated buck.....

the web.

voltage is not steady 20V, but can drop very fast, which would pose a problem with half-bridge solution (capacitor midpoint slewing slowly) and since access to the current is easy (sense resistor)

on.

duty cycle (45%), but peak current is detected by the comparator which in turn resets the PWM (input to PWM module) before reaching 45%.

to the primary side. (not shown). Larger caps needs to be placed on the output to ease the response time of the regulation loop (microcontroller measured error voltage)

but have not yet found off-the-shelf magnetics that facilitates this.

diodes or the FETs are mounted on the output, the FETs are for syncronous rectification)

magnetics in small sizes are available.

That works, as does this (basic) equivalent that I've used for automobile ignition systems...