l6566bh based flyback converter ground short together

[Kaleeswaran]

hi all,

     i am designing flyback converter based on the l6566bh. i have 2 outputs on the secondary side, both are separate windings (different ground potential) in transformer. can i short together for shared ground reference if isolation is not necessary.

[Peter BENSCH]

It would be best if you could add a sketch as a picture so that people can immediately recognise what you mean.

If you don't need any insulation on the secondary side, you can of course connect the GND there together.

Does it answer your question?

Regards
/Peter

[Kaleeswaran]

 can i short RTN and GND_iso(transformer has separate winding for two outputs), both are isolated from primary side.

[Peter BENSCH]

Yes, the two secondary windings can have a common GND, so you can connect GND_ISO and RTN together.

However, 15V_ISO is then also galvanically (not directly) connected to 12V, so a current can flow from 15V-ISO to 12V if there is something in between.

Regards
/Peter

[Kaleeswaran]

Dear Sir,

The 15V and 12V rails have different ground potentials (in both the transformer windings and the PCB). Can I connect these two grounds externally using a jumper or wire? Will this cause voltage fluctuations?

[Peter BENSCH]

You have not made any statement about what your connected loads look like. If there is a reason for the 15V and 12V rails to have different GND potentials, you can of course not simply connect the GND.

[Kaleeswaran]

The 12VDC supply is used to energize the relay coil, which controls the AC load (e.g., a pump). The 15V supply is converted to 3.3V, which powers the STM32 microcontroller. The microcontroller activates the relay through a ULN2003A driver. Initially, to prevent electrical arcing from the AC load, I separated the grounds and used an optocoupler to isolate the STM32 from the ULN2003A. However, using an optocoupler increases the circuit cost. To reduce costs, I decided to connect the two different grounds together.

[Peter BENSCH · Accepted solution]

OK, in this case you can connect both GNDs together.

You can even reduce the costs even further by eliminating the second winding for 15V including D5, R14, C14 and C2, replacing L1 with an inductor with e.g. 1mH and connecting its left side (formerly at cathode D5) to 12V. L1, together with C23 and C22 (which you can make smaller or remove if necessary), forms a low-pass filter that further removes remaining ripple from the 12V rail.

Regards
/Peter

[Kaleeswaran]

Hi sir,

Our PCB design using the L6566BH controller has been completed. The design includes two outputs: 15V and 12V. When measured with a multimeter, both outputs show correct voltage levels. However, when observed on an oscilloscope, significant ripple (up to 2V) is present on both outputs.

Additionally, we are deriving a 5V output from the 15V rail using a step-down converter(L7805). This 5V output also shows noticeable ripple on the oscilloscope, although the multimeter still reads it as 5V.

I have attached the VDS waveform and the 15V output waveform for your reference.

Could you please advise on how to reduce the ripple in the outputs?

[Peter BENSCH]

Ripple can have various causes, for whose assessment you have to look at the layout, among other things.

An L780x is decades old, extremely inexpensive and well suited to generating a lower voltage from DC, but it is the most unfavourable device imaginable for filtering high-frequency input voltages that come from the switching regulator as ripple. When using such linear regulators for filtering, the value of the PSRR must be specified at your switching frequency and be as high as possible. However, before you use an expensive special LDO, you should consider passive filtering with an LC element.

BTW: although it is very easy to take a quick photo with your mobile phone, it is much better to take screenshots that your Rigol DHO924 can save directly to a USB stick that you can insert underneath the screen.
It also makes it easier to get help from other community members if you insert such images as images using the camera button rather than attaching them, because then they don't have to be downloaded first.