help needed on current control SMPS

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
hi all,

attached is a current controller circuitry that i have build using UC 3846, but with no success..

the problem here is i guess with the current sensing part as for an open loop circuit without feedback i would get +/- 40V with no noisy coming from the switchimg transformer, with a nice pulse.

but when the feedback is given im getting only +/- 17V with a noisy sound coming from the switching transformer and the mosfet gets heated up quite fast.

Is it the rite way to sense the current. please advise.
 

Attachments

  • cmc.jpg
    cmc.jpg
    60.5 KB · Views: 464
just few comments:
- the way you sense the output current is compromising the isolation barrier.
- the voltage error amplifier output pin (COMP) is floating!!!:hot: . You have no compensation network for EA. The system is oscillating.
- No current limit protection is implemented (pin 1 is floating)
- the optocoupler and series zener diodes are connected in a strange way
- you have no duty cycle control. Most probably the two outputs are generating the maximum duty cycle value, internal T type flip-flop doing its job and toggling the outputs.

Search for UC3846 app notes and read them carefully...
 
Except for the obvious, that you have connected primary mains and secondary side together in a most lethal way - you miss the most advantageous part of current mode which is to detect and avoid flux imbalance in the transformer.

If you instead sense the current (in an isolated fashion), you will get the advantage that the transformer doesn't walk into saturation.
 
mflorin said:
just few comments:
- the way you sense the output current is compromising the isolation barrier.
- the voltage error amplifier output pin (COMP) is floating!!!:hot: . You have no compensation network for EA. The system is oscillating.
- No current limit protection is implemented (pin 1 is floating)
- the optocoupler and series zener diodes are connected in a strange way
- you have no duty cycle control. Most probably the two outputs are generating the maximum duty cycle value, internal T type flip-flop doing its job and toggling the outputs.

Search for UC3846 app notes and read them carefully...


hi,

This is my 1st design for current control...

as you said that the way i sense the output current is compromising the isolation barrier, that means for an isolated SMPS is it we should only sense the current from the primary part and not from the secondary part... because the common ground for the IC is from the primary ground..

The optocoupler and zener diode, is actually a diagram mistake.
 
The UC3846 is a peak current mode chip as it responses to the peak current rather than the average current. What this means is that the chip is expecting to see a ramp of current during the on time. Therefore, it will probably not work well if you sense the output current.

Take a look at some of the applications notes to understand how to use the UC3846.
 
And also keep in mind that half bridge with peak current mode control doesn't work without special balancing circuits.

That's the probably the "more involved" part switchmodepower is referring to.

I recommend two switch forward instead if you want to use peak current mode control.
 
Judging by the kind of mistakes that your schematic contains (it's mostly a no-brain copy-paste from others) you lack the electronics knowledge required for safe experimentation with off-line switching power supplies.

Please learn to handle isolated low voltages first. Once you succeed a few times, you may try offline circuits. This may probably take a long time.
 
Dear all,

As megajoke and switchmode said, for half-bridge topology employing peak current mode control, a balancing circuit is required..

An auxialiary balancing winding of same turn as the primary winding is connected between the center points of coupling capacitors (330uF) and the junction point of diodes. Is this method sufficient enough to overcome the stability problem of hal-bridge topology using peak current controller.

Dear Eva,

Thanks for your comments. Anyway im still a newbie in the mode of learning by mistakes and studying in the area of SMPS all my own with no help here..I agree i have no experiance on SMPS, but your comment saying " no -brains copy-paste ..." is a little bit degrading a newbie who studies SMPS on his own, to be honest i felt a little bit discouraged.

thank you
 

Attachments

  • hepps balancing circuit.jpg
    hepps balancing circuit.jpg
    21.1 KB · Views: 276
Don't blame me, I'm just translating your schematic into words. The schematic itself is degrading.

In fact, the no-brain way in which you connected the current sense amplifier is so dangerous that forum moderators may choose to delete the schematic or close the thread at any time. It goes against forum rules.

Then we have the rest of the mistakes, I mean the ones not involving risk of death...

You are far from ready for such a project. Perhaps you could succeed after a year or two of learning. Try simpler things.
 
Do take in what Eva said. I have been experimenting with SMPS for over a year and I still am not ready to do an off line design.

You can still learn about current sensing, feedback circuits, compensation, magnetics design and other important SMPS topics with low voltage supplies.

If you are not interested in SMPS enough to consider learning that sort of stuff, go and buy a commercial dual rail SMPS from hypex or someone else.

I know that isn't DIY but it will be safer and cheaper in the long run.
 
Hi Jegandren,

If you are not very clear about current sensing in SMPS, for the time being, you can use fast blow fuse at the output or can use resettable fuse, which will protect your circuit from overcurrent. Please try to sense current from the primary side (possibly with a current transformer) which I think is preferable.

In SMPS, nobody should follow anyone's schematic blindly and foolishly without having thorough knowledge of each segments of the circuit, as SMPS circuit is always lethal.

As I found, no company in this world is taking responsibility for any accident for their products/design and for that, at the beginning/end of the datasheet/booklet, they usually give a warning - what to say about the circuit/schematic of an individual. So, one should be very careful in using parts/design/schematic in practical smps use, keeping in mind their own security.
Thanks.
 
Dear all,

Here the 240ac supply is actually not coming from the direct mains supply, but instead its coming from a variable ac power supply in which the maximum AC current is 1.6A....so its much safer.

Any comments on the half-bridge balancing winding as i attached earlier would be much appreciated.
 
If you are using a 'variac' (variable autotransformer), please realise that it is NOT isolated from the mains.

Using a balancing winding in your circuit isn't really going to improve things unless you learn more about SMPS design. I've seen voltage mode and current mode half bridge designs happily still working after ten years of service.
 
That variable supply probably still isn't 100% isolated. Get a *real*, 2.5KV hipot-tested, isolation transformer before you start on any offline supply.

Anyway, the first thing you must do is buy three things:

- Get an isolation transformer. I seriously mean this. It can save your life, and it also allows you to hook test equipment (oscilloscopes, etc) to the primary side of the SMPS.
- Get a variac also. Lots of power supplies will work fine at 60VAC but explode at 120VAC due to overshoot and other things. A variac is an immensely useful tool for figuring out why things go bang, and also for testing your supply at its full voltage input range.
- Pick up the Abraham Pressman "Switching Power Supplies" book. This book is basically the bible of SMPS design. Read it and make sure it sinks in, don't just skim it.

Then, Rip up your current design and start fresh. Build a single transistor forward converter based on a UC3845 chip with TL431+opto feedback. Use a separate bench supply to power the chip.

Once you've understood how to make this supply work and know how to design compensation for this kind of supply, then you can move into more advanced full-bridge, push-pull, phase-shift, etc. topologies like the one you've started with here.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.