Does PI Expert simulate the secondary Zener part?
Good afternoon,
I am using PI Expert online using TNY286PG to design a supply for 12V/1A. On the optocoupler side, I use the TLP385(GB-TPL,E for its small height and since it's suggested in the tool.
I would like to know if the optocoupler itself is being taken into account and that the message of "Design Passed (No optimization)" is true for that part as well? I am trying to better understand the EN pin, I have been through the application notes but I didn't fully grasp it. I am currently using the 1kOhm parallel resistor, TZMB10-GS08 zener diode (10V) and 100Ohms series resistor and the design passes. This should rougly provide 1.2V to the LED @ 6mA which should be sufficient for current to flow in the BJT.
Apologies if this question should be directed to the optocoupler manufacturer, but as it couples with the TNY286 I want to understand the limitations and what current to set in the LED, to make sure that I am operating it correctly.
Thank you,
Chris
Hi Chris,
The octocoupler equations are taken into account when deciding whether the design has passed or not. So you can assure that the parts suggested by PIExpert were chosen to allow proper operation of the controller. You may also want to check Table 13 of AN-82 for the typical component values of the zener feedback circuit. The values suggested by PIExpert are close to the typical values stated in the table.
To help you understand better, the EN pin is used to decide whether the primary switch should be enabled or not. MOSFET switching is terminated when a current greater than a threshold current is drawn from this pin through the photo transistor, and switching resumes when the current being pulled from the pin drops to less than the threshold current.
The threshold current, which is the collector current of the phototransistor, is between 75 μA and 115 μA. This collector current can be controlled by the current flowing through the LED side of the optocoupler (I_C = CTR x I_LED) by properly selecting the components of the zener feedback circuit. The goal of this circuit is to turn on the phototransistor (to sink current from the EN pin) when the output voltage exceeds the target regulation voltage level (optocoupler LED voltage drop + zener voltage + series resistor drop) which should be close to the target output voltage. When this happen, the zener diode conducts and current will start to flow in the LED, which then turns on the phototransistor, which then sinks current from the EN pin disabling MOSFET switching.
Should you have any further questions, please do not hesitate to ask. :)