MC34063A: MC34063A Circuit audible noise issue

Could you (1) provide your schematic, and (2) show us the waveform of switch-node (the switching side of the inductor), for us to review?

Please find attached schematics for your reference

1. Input= 18V to 30V DC and Output =+15V @80mA.

2.  Waveforms at Pin no 2 and pin 3 attached below , Waveform in Blue is waveform at Pin 3 and  Green is waveform at pin no 2.

3. Please suggest solution for removing audible noise.

the audible noise is caused by the intermittent switching. The L-C-L can cause loop instability. Can you remove C16 and C17, making the two inductors in direct series and behave like one, and re-run the test?

Hello,  As per previous discussion I removed  C16 and C17 from circuit. also to increase switching frequency C21 ( i.e. Ct changed to 220pF from 470pF).

With these modifications humming noise (Audible noise) reduced , audible sound is still present but intensity of sound reduced.

Modified schematics attached below:-

There is one doubt in above changes that L20 has current rating of 80mA and power supply current consumption is 60mA. if maximum ( i.e. 60mA) current consumption continues through power supply , will it be any heating issue occur with L20.

Waveform at PIn 2 and Pin 3 after making above changes as follows:-

Hello,  As per previous discussion I removed  C16 and C17 from circuit. also to increase switching frequency C21 ( i.e. Ct changed to 220pF from 470pF).

With these modifications humming noise (Audible noise) reduced , audible sound is still present but intensity of sound reduced.

Modified schematics attached below:-

There is one doubt in above changes that L20 has current rating of 80mA and power supply current consumption is 60mA. if maximum ( i.e. 60mA) current consumption continues through power supply , will it be any heating issue occur with L20.

Waveform at PIn 2 and Pin 3 after making above changes as follows:-

The circuit is still not stable. The audible noise will disappear once the circuit switches each cycle in steady state.

Your current L20 may saturate at 60mA. Short L20, and replace L19 with a 220uH or so, Isat>0.5A inductor, for example Coilcraft MSS1038, and see if the switching can become stable.

As per our previous discussion we removed C16 and C17  and checked output voltage regulation . but, it is observed that by removing C16 and C17 circuit voltage drop less than 15V for 18V to 20V input and 50mA load.

So, modified design as per attached below:-

In This design we replaced L20 with ferrite bead with 200mA DC current rating (part number:- BLM31AJ601SH1L , Make :- Murata)

And changed Ct with 100pF capacitor.

Switching waveforms as per above modifications are as follows:-

With these modifications audible noise eliminated.  But, there are two points over your suggestion required

1. With above modification (Ct-100pF and ferrite bead) switching frequency i.e. frequency at Pin no 3 is 123KHz . which is above 100KHz. Is it Ok to increase switching frequency above 100KHz ??

2. Is it Ok to use Selected ferrite bead 200mA DC current rating (part number:- BLM31AJ601SH1L , Make :- Murata) in this design. Where, input voltage 18V to 30V and Output voltage 15V @ 60mA.

Please resolve there two queries

Dear Sir,

Please reply to previous query.

Also reply below query:

1) We removed C16 and C17  and checked output voltage regulation . but, it is observed that by removing C16 and C17 circuit    voltage drop less than 15V for 18V to 20V input and 50mA load.  What will be the reason.

Gentle reminder ,

Please reply to above queries

Can we continue  as per mentioned design.

 

Dear Naveen San,
Yes, tried with increasing L19 from 56uH to 68uH and shorted L20. But, with this Humming noise issue still present.

L19 can not increase above 68uH due to inductor size and other mechanical constrains.