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[SOLVED] UCC3808 12-24V 3A DC-DC problem

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Newbie level 4
May 10, 2010
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Hi, 2 months ago I started designing my own SMPS based on Texas Instruments UCC3808 Current-mode PWM controller. It's one of the rare PWM controllers (current-mode ones) that have overcurrent protection.

Schematics is in attachment.

My problem is, everything is working, i have 23.9V @ 0A, and voltage stay at that value until i reach 1.5A, then voltage start dropping. Power supply is working at costant power mode. Short circuit current is 280 mA.

As you can see, I added slope compensation becouse duty cycle is above 50% (2×25%).

I wanted to increase current limit, but i cant get out more than 2A, and i tried to modify shunt, then i tried voltage divider to lower the current sense signal (I was carefull not to scramble RC constant on pin 3). Nothing, i just make power supply unstable and noisy.

I tried to modify 20k resistor going from VCC to 2N2905 emiter. Nothing. I tried add aditional shunt resistor. Nothing. I shorted pin 3 to ground, 23.9V even at 5A and no noise at all, MOSFETs are at 28°C

I have no idea what is wrong.


  • SMPS.png
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To get 2A at 24V out...

you need to send 4A at 12V in.

48 watts in, 48 watts out (not counting losses).

Can you verify that your power input loop has low enough resistance to pass the required current?

Can you verify your coil is not reaching saturation at a high current level?

my power supply is capable of 12V and 50A, also, as i said, with grouded pin 3, i draw 5A from power supply without any problems and i didn't noticed any noise from SMPS. Voltage was 23.9V at 0A and at 5A load.

The switch-On time must also be long enough so current can reach the necessary peak (4 A or more). Then you will switch it off and allow the coil to feed energy to the output stage.

Your schematic shows a transformer with a center tap on both the primary and secondary. This is a sophisticated topology compared to the plain single coil type. Evidently you can get yours to work at lower power levels. What I have observed is that the secondary can respond at different times in the power cycle.

While you are in the developing phase, can you operate it as a simple flyback transformer?

You need to verify that your diodes are oriented in the proper direction to pass high current from the secondary. (Try installing a full-wave diode bridge, to make sure.)

You need to verify that sufficient current flows in the primary during every switch-On period.

Power supply is already on PCB, but i think i could operate in flyback mode, there is small gap vhere i glued core and i have spare UC3842 so i could built simple flyback circuit and tested it.

Diodes are orientated propertly, i don't have any bridge rectifier that fast.

I guess i have enough current, at 5 amps on output i have about 11.5 amps on input

I guess i have enough current, at 5 amps on output i have about 11.5 amps on input

Then your target power output should be in reach. It could be a simple thing that is preventing ample current flow in the primary windings.

* Can you verify that your mosfets turn off completely during Off-time?

* Can you verify that you are not in continuous conduction mode (CCM)? A transformer is fine with discontinuous mode (DCM).

* Have you tried reducing the operating frequency? (Duty cycle may or may not need to change.)

Tried to connect emitter to output voltage, but nothing... I think this is too much. I should choose voltage mode controler, they are easier for first project.

Problem solved, i added 1N4007 and 47ohm 2W resistor between VCC of the power supply and VCC of the PWM controller. Power supply is now fully functional. R12 is increased to 3.9k, short circuit current is about 4.6amps, before it was 0.28 amps. Max power i manage to get out is 87 watts, for this shunt configuration, i can increase R12 to about 4.6k, above that, power supply decrease voltage under load to 19V, and start to make continous sound, very quietly. Below that, there is no sound at all, across whole output current range, and voltage is steady, at 3A (rated current) i have about 35-40 mV of voltage drop. That is very good stabilized.

Good work.

It seems like there is some obstacle to success in every project, doesn't it?

Sometimes we have to try a number of things before we hit on a solution.

That's right, It's important not to give up when we hit one of the obstacle :)

Now, when power supply is over, i will upgrade it, there is always room for improvement, next stop feedback using optocoupler and TL431

I spent 2 hours reading various books trying to find solution, and it paid off

I simply separate chip VCC from main VCC using diode and resistor, two simpe components made so much diference.

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