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Making a Constant_off_time controller for an SMPS?

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Flyback

Well-Known Member
Hello,
We wish to make a constant off time controller for an SMPS. We wish to do this by using an MCP1630 PWM controller and a LTC6993 Monostable.
We will simply feed the gate drive output of the MCP1630 into the LTC6993, as well as driving the FET …we will then feed the output of the LTC6993 back to the OSC_IN pin of the MCP1630.
Can you see any potential problems with this?

LTC6993 monostable
http://cds.linear.com/docs/en/datasheet/69931234fc.pdf

MCP1630 PWM controller
http://ww1.microchip.com/downloads/e...Doc/21896C.pdf

The PIC16F785 has a fast internal comparator that could do the source current sense function, but it has loads of pins which we do not need….it also does not have an on-chip gate driver.
PIC16F785 Datasheet...
http://ww1.microchip.com/downloads/e...Doc/41249E.pdf
 

ronsimpson

Well-Known Member
Most Helpful Member
constant off time controller for an SMPS
http://www.electro-tech-online.com/threads/help-with-voltage-regulator.149949/page-2#post-1285253
Go to post #33.
This is how I make a constant off time SMPS.
When the MOSFET is on the oscillator is locked up. When the current reaches some point the MOSFET is turned off and the osc is allowed to run (for the off time). The off time is set by RC and is constant. The on time is set by the current ramping up in the inductor.
 

Flyback

Well-Known Member
http://www.electro-tech-online.com/threads/help-with-voltage-regulator.149949/page-2#post-1285253
Go to post #33.
This is how I make a constant off time SMPS.
When the MOSFET is on the oscillator is locked up. When the current reaches some point the MOSFET is turned off and the osc is allowed to run (for the off time). The off time is set by RC and is constant. The on time is set by the current ramping up in the inductor.
Thanks, i have used that before, with the pin-for-pin ucc38c4x series, but the problem is the high current sense reference voltage of those chips.

It amazes me that nowhere in the world do they make a constant off time control chip with a lower current sense reference voltage than the LT124X/UCC38C4X series.

I am just wondering what is the smallest pic micro that has a fast comparator in it. As you know, it needs to really be faster than 100ns with a few 10's of millivolts of overdrive.

Thanks for NCL30105, again , this has a high current sense reference voltage of 1V....I WOULD LIKE MORE LIKE 300-400Mv.......ie not too high and not so low that noise can corrupt it.
(sorry about capitals, ive cut my finger and cant type properly)
As you know, the disadvantage with constant off time is that on time can be very long which isnt great for using current sense transformers , which as you know, can help reduce the sense resistor loss.
 
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tomizett

Active Member
I know that it's not really what the OP asked, but the topic reminded me of something so I thought I'd share it in case it's of interest.

The attached circuit is a constant-off-time controller designed with generic parts. In this case it's configured as a simple boost converter, but the output coil could of course easily become a flyback transformer. I built it and it worked (I've got no record of how it performed though) although I seem to remember having some trouble with EMI...
This isn't a true "constant" off-time design, as the off time is decreased as the output voltage rises, in order to keep the output ripple roughly constant. Off-time could easily be fixed by returning R1 to the input rather than the output.

To be honest I'd forgotten all about it, but it seems that I also did some maths on the circuit - I've uploaded that too, but (as the contributors here know) my maths is poor so don't rely on it!
 

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