Isolated SMPS and zero crossing detection

[eepty]

I have a isolated switch mode power supply (SMPS) circuit. The output is good for my requirement. However, I need AC line zero crossing detection for my product. Since the DC reference is at different potential with the AC Live or Neutral, I can not use the general zero crossing detection circuit.

The only way I can think of is to make another non-isolated simple voltage drop circuit (eg. capacitor voltage divider), and use the common zero crossing detection circuit.

However, by this way, first, the circuit is not isolated. Second, it appears to me that the cost of the capacitor voltage divider circuit is wasted.

Can anyone suggest another way to do the zero crossing detection?
Thank you.

 

[karesz]

Hi,
What about a zero crossing triggered optomos/triac? coupler, MOCxxxx?
K.

 

[FvM]

The optimal solution depends on the required zero crossing accuracy, that hasn't been said yet. A simple solution is a sensitive
optocoupler with a reverse polarity protection diode for the LED, driven by a series resistor from the mains voltage with e.g.
1 to 5 mA.

 

[dick_freebird]

"Sniffer" winding near / on one of the line chokes (?) might
give you a "good enough" transformer; resistor to ground
and a comparator?

I once had a technician job, building SCR motor controls, and
we took the housekeeping supply's transformer secondary
voltage, divided down, for this purpose (firing angle reference
pulse).