SMPS nearly finished, not stable

[cjk2]

Is there a secret to making these things stable? I believe I designed my circuit correctly. I am currently trying to get my supply stable in constant voltage mode only, Ill fool with the current control only after the voltage mode is working.

I have played with the value of R4 and found that smaller seems better. The value of R40 seems critical to getting the supply to stop hissing and chirping. R40 is currenly a 100K pot set to about 90K ohms. Much increase or decrease at all seems to make it hiss or "crackle" at various loads and voltage settings. The supply seems to regulate fine, the output voltage stays constant regardless of load. The Hissing and crackling sound also changes with the load and I hate this! Can't I have a supply that is stable with any load I connect?

How do you guys manage to build these quiet, stable, SMPS's?

[Sch3mat1c]

Try a capacitor across R10, or maybe R+C to avoid injecting HF ripple. Or I suppose since those go to gain amps, you might do this across R5/R6. But they'll need to be much bigger, like 100k, to get useful cap values. Which means C5-C8, R3 and R4 will be different, too.

You should only need C6/C8 to keep the error amps from amplifying ripple, and an R+C across R5/R6 to set some derivative gain.

Tim

[zkaiser]

Having same problem, and I came to conclusion that the problem is when pwm controller needs to be near zero duty either with low voltage and/or low current. When these condition are met controller starts to work rather erratic and it is possible to hear noise at audible range.

So far I am seeing couple of solutions on a horizon, to increase idle load to PS which is not very practical if you trying to make hi-eff. ps, or use some crude tricks and make transformer more uniform by vacuum varnishing or possibly the most advanced solution to use constant on time variable freq. at low voltage and/or load, but this requires different controller or some workaround existing one.....

[Electrone]

Try more carefully controlling the minimum duty cycle of the pulse width modulator. See if you can make the feedback circuit just barely, if at all, be capable of achieving a completely off-state of the PWM.

[switchmodepower]

Current mode with a half bridge is inherently unstable unless you have a balancing winding or some other way to counter the effects of volt-second asymetry. This is due to the series cap AND trying to run current mode.

Go to voltage mode by removing the current feedback and injecting a ramp (ct) directly to the ramp pin. Compensation should be type III- an RC across R5 or on the diff amp.

[warmen]

ive looked at your schematic and i have suspicion that C11,C12 have way too low capacitance

[cjk2]

Thanks for all the help! I did some reading on instability in current mode smps using half bridges. I was able to stabilize my power supply using a balance winding.

[switchmodepower]

Good job. Can you post an update to your schematic?

[cjk2]

Here is my final and correct circuit. I ask that if you build this PLEASE post here so I can see your work! I would love to see my design used by someone else!

[zkaiser]

So, how does ps performs now ? Is there audible noise at low load or high current and low voltage ?