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LT1249 not turning on

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KMN1

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I've built a boost PFC circuit on a PCB using the LT1249 controller, using the reference design guide provided by Analog Devices here: https://www.analog.com/media/en/technical-documentation/data-sheets/1249fbs.pdf

I've scaled the reference design to achieve an output voltage of 60V with a 24VAC input: enter image description here

The VCC is provided by a bench power supply which has the ground connected to the same ground as the resistor Rload.

However when I start the circuit with the bench power supply turned on, the output is essentially the input voltage filtered by the output capacitor meaning the IC is not switching on and operating as in the simulation.

I've checked every connection for potential faulty components or short circuit but regardless of if the IC is on or off the output will sit at roughly 33V. Can anyone help me understand where I could be going wrong
 

Hi,

On a quick view...
I miss the VCC capacitors .. recommended in the "pin functions" section.

Why no diode at GTDR pin?
Why no current sense resistor?
There are so many modifications to the datasheet circuit. Why?

Klaus
 

Hi,

On a quick view...
I miss the VCC capacitors .. recommended in the "pin functions" section.

Why no diode at GTDR pin?
Why no current sense resistor?
There are so many modifications to the datasheet circuit. Why?

Klaus
Hi Klaus thanks for the response.

-I do have VCC cap in place on the pcb but not on the simulation and same for GTDR pin diode.
-the current sense resistor is R4 in the schematic
-the circuit is the same as the sample circuit on LTspice and in the datasheet just scaled to fit my application
 

Simulation showing waveforms you can expect to see.
25 kHz appears reasonable for 100 uH coil.

Notice about 12A must be drawn from 24 VDC supply if you wish to provide 60V to 22 ohm load.

Your 1800 uF smoothing cap is likely to draw extreme startup current while it charges. It may disrupt normal running of your system. A much smaller Farad value may be sufficient.

boost conv clk-driv 24V mosfet 60V load 22 ohms.png
 

Simulation showing waveforms you can expect to see.
25 kHz appears reasonable for 100 uH coil.

Notice about 12A must be drawn from 24 VDC supply if you wish to provide 60V to 22 ohm load.

Your 1800 uF smoothing cap is likely to draw extreme startup current while it charges. It may disrupt normal running of your system. A much smaller Farad value may be sufficient.

View attachment 183275
Would placing a ntc resistor at the output help solve this issue?
 

Simulation showing waveforms you can expect to see.
25 kHz appears reasonable for 100 uH coil.

Notice about 12A must be drawn from 24 VDC supply if you wish to provide 60V to 22 ohm load.

Your 1800 uF smoothing cap is likely to draw extreme startup current while it charges. It may disrupt normal running of your system. A much smaller Farad value may be sufficient.

View attachment 183275
Brad pls include a shortcut to your sims in future. Thanks
also try AC /w diode bridge
--- Updated ---

Your design differs greatly from the datasheet reference design.

e.g. you have a huge cap that will conduct over current even without switching.
1686591695730.png
 
Last edited:

This design will never work. With no series current limiting, you will saturate your series inductor and it will just become a power resistor with massive overtemp, overcurrent and other issues if it hasn't burnt out already.

Start with better specs on source impedance, inductor current limit and choose a dual switch topology with series and shunt control. Perhaps an AC SCR pre-regulator to DC then a DC-DC regulator will be easier than this grossly mismatched impedances from AC to boost DC
--- Updated ---

If you had a cap before a series FET Switch to enable a Vdc min > 6V then you might be able to use something like the LM5022. This was generated used a wide input DC tolerance by logging into TI.com and useg the design tool WEBBENCH for power supplies. you must follow the Bill of Mat'ls and PCB Layout exactly, unless you know the what you are doing. Notice this example also does not have inrush PWM control , so run the simulations and understand the behavior before building one.

1686598579290.png
 
Last edited:

Brad pls include a shortcut to your sims in future. Thanks
also try AC /w diode bridge

My revised simulation. The link below opens my latest circuit in the website: falstad.com/circuit

tinyurl.com/28bzq6ar

Toggle full screen (in File menu). Enlarge scope traces by dragging mouse upward on the top of the scope region (hover mouse to make blue dragline appear).

Having added a 24 VAC supply and diode bridge, I find the waveform has 32 A peaks. This is very different from the waveforms taken from a stable 24 VDC supply. My waveforms now are wandering up and down at 2x mains frequency.

boost conv clk-driv 24VAC supply diode-bridge 10mF cap 25kHz.png
 
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