Weird switching transformer noise, Audio attached

MacIntoshCZ · Apr 13, 2021

Hi,
When tl494 is regulating voltage on output of switching transformer weird sound appear.
This only happen when positive pin of error amplifier is higher than negative pin (limiting output voltage / current).
In rar you can see recorded sound.
With higher regulation, sound increase intensivity.
Sorry for anoying beeping. That is unit wattmeter detecting current peaks.
Any ideas how to solve it?

MacIntoshCZ · Apr 13, 2021

I scope primary winding and probably reason is core is vibrating becouse there are like five 50khz pulses and then huge "gap" that makes with next wave 450Hz.
So i need to lower duty cycle for lower loads to have lower "gap". Hmm

MacIntoshCZ · Apr 13, 2021

Also there is huge noise when sound comes.
This is sensed on current transformer

MacIntoshCZ · Apr 13, 2021

There is softstart option in tl494 that i missed. Maybe it will help

Nigel Goodwin · Apr 13, 2021

Is it perhaps squegging?, a common cause of audible noises from switch-mode supplies.

MacIntoshCZ · Apr 13, 2021

I Have never Heard about this term. It could be that. It sounds similar as sparking. Thanks

Nigel Goodwin · Apr 13, 2021

MacIntoshCZ said:
I Have never Heard about this term. It could be that. It sounds similar as sparking. Thanks

It's a very old radio term

In a switch-mode power supply what 'should' happen is the pulse width should change to regulate the output, and the frequency should be well above audible range.

But what sometimes happens is instead of the pulse width varying properly, the PSU shuts down totally, then restarts, then shuts down again - and repeats over and over. This can be caused by too little load on the power supply, and it's doing it's best to maintain the required output voltage. Because it's pulsing ON and OFF it's at a much lower frequency, hence it's audible.

This is known as 'squegging'.

Some power supplies are designed to work in that way - in fact the MicroChip PicKit2 (and possible the 3 & 4?) do exactly that to generate VPP. The PWM generates constant width pulses, and the output voltage is monitored by an interrupt driven ADC routine - if it's above a certain voltage then the PWM is shut down, and if it's below a certain voltage then the PWM is turned back on.

ChrisP58 · Apr 13, 2021

Please post the full schematic of the circuit and winding details of the transformer.

MacIntoshCZ · Apr 13, 2021

Nigel Goodwin said:
It's a very old radio term

In a switch-mode power supply what 'should' happen is the pulse width should change to regulate the output, and the frequency should be well above audible range.

But what sometimes happens is instead of the pulse width varying properly, the PSU shuts down totally, then restarts, then shuts down again - and repeats over and over. This can be caused by too little load on the power supply, and it's doing it's best to maintain the required output voltage. Because it's pulsing ON and OFF it's at a much lower frequency, hence it's audible.

This is known as 'squegging'.

Some power supplies are designed to work in that way - in fact the MicroChip PicKit2 (and possible the 3 & 4?) do exactly that to generate VPP. The PWM generates constant width pulses, and the output voltage is monitored by an interrupt driven ADC routine - if it's above a certain voltage then the PWM is shut down, and if it's below a certain voltage then the PWM is turned back on.

Than this is exactly what happen.

MacIntoshCZ · Apr 13, 2021

ChrisP58 said:
Please post the full schematic of the circuit and winding details of the transformer?

That will be problem.
Power part ( I dont have a schematic yet. Its connected with wires.)
Q - irf460 / D3,4 - Mur880 / T1 39 turn 15x0,1mm primary and 16 turn 30x0,1mm secondary / D1,2 Mur6060PT / Lo around 30uH
Also there is rdc for mosfets - uf5408, 1nF and 100R.
I connected output with tl494 with driver input with RC to make sure DTC stay around 34% ( I burned GDT once =D)
There was changes in DTC voltage. I changed voltage divider to 270R and 2k2
I will try to connect soft start RC. PWM change duty cycle on higher load. On light load there is few preset DTC pulses and then huge gap.
**broken link removed**

MacIntoshCZ · Apr 14, 2021

My Circuit with tl494 is badly designed.
I found several errors that needs to be fixed. I already fixed it manually when prototyping. And now its working fine. No crappy buzzing anymore.
Pin 13 needs to be connected to Vref
Resistors need to be conected after emittor of Q1/2.
recalculate RT and CT for push pull mode (two times higher RC constant)

MacIntoshCZ · Apr 17, 2021

I found out that when load is current limited no buzzing / squezing occurs. But when its not squezing occurs.
Output voltage attached. When its not current limited there is 500mV ripple at 2,47kHz.
When its current limited there is 200mV ripple and 47khz

Weird switching transformer noise, Audio attached

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