Is it possible to use capacitor for isolation instead of optocoupler?

Question

I designed a Buck converter which drives several LEDs. The PWM signal, which has a certain fixed duty cycle, is generated by a 555 timer IC. The circuit which drives the Buck MOSFET, including the 555, is boot-strapped and floating between 0V and 310V (rectified 220V AC). A current sensor measures the LED current and is supposed to send a pulse to the reset pin of the 555 to delay the next pulse.

The problem is, I can't find a cheap optocoupler which is guarantied to respond in less than one period of the PWM. If the current is not stopped within one period, the current through the Buck inductor would rise above the critical limit and damage the LEDs.

I decided to use a capacitor for isolation. I built up the circuit below to simulate it. However, I couldn't get a good response out of it regardless of what I tried. I would like to know if this method would actually work in theory and application. If it can work, what am I doing wrong here? And also, I would like to see if there is an example design.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

A capacitor can be used to block a large DC offset between two circuits, but it cannot be used to isolate two circuits whose reference levels swing widely with respect to each other. The AC voltage difference causes a large AC current to flow through the capacitor.

Have you considered using a pulse transformer instead?

Answer 2

It can be done, at least in controlled conditions (on a chip or hybrid). Two very low value (less than 0.2pF) in a differential arrangement. See this: http://www.eetasia.com/STATIC/PDF/201007/EEOL_2010JUL16_POW_TA_01.pdf?SOURCES=DOWNLOAD

The way you're trying it-- no.

Answer 3

It depends on what type of noise you wanted to eliminate since both devices are different in working principle.Capacitor is usually used for DC offset elimination,while it can't block out the biased AC current that leaks out to the entire system somehow.While the photoisolator can,it only pick up the differential signal that goes into one terminal of the diode to the other.