Hello Artjom,

Good day. Thank you for reaching out to us.

We would like to ask for the turn ratio of your transformer to evaluate the exact value of the Zener. For the time being, you may refer to the device design guide on page 8 under 'The Zener diode voltage.' It states the Zener diode to be selected is approximately 6 V above the bias winding voltage.

Primary winding 100 N

Auxiliary winding 10 N

Secondary winding 20 N

Hi Artjom,

Thank you for your response. We suggest a Zener diode of 25V.

Thank you for the response. There is one thing I don't understand. At al load of approximately 0.6 Amps, the auxiliary voltage is around 38V. According to the datasheet, the Zener diode voltage should be about 6V above the bias winding. For this reason, I would have chosen a 44V Zener diode. However, if the Zener diode has a voltage of 25V, it will conduct, and the current Isd will increase. Or am I misunderstanding something? 

Thanks

Hi Artjom,

Thank you for your prompt response. You are correct; I apologize for the confusion.

From our understanding, since the voltage of the auxiliary is 12V at no load, we conclude that your output is 24V as per your turn ratio.

Our suggested calculation for the zener diode at the primary OVP is equal to the overvoltage of the output minus the BP/M voltage. And the overvoltage is 125% of the output voltage. So, our initial computation is as follows: (24*1.25)-VSHUNT = 25 V 

Now, considering the bias voltage at full load, the calculation must be the one presented on the attached.

Hence, your understanding of using 44V is correct.

I hope this works for you.
 

Thank you very much for your response. You have helped me a lot. I have one more question: I selected the resistor R8 with 22 kΩ as specified in the datasheet. This works well so far. However, PI Expert suggests a value of about 47 kΩ. Which value is better? In idle mode, the current  Isd (BP/M) is about 800 µA, and from 0.1 Amps, the Isd current is constant at about 1.23 mA at maximum load. How high should the minimum  Isd  current be? Is the idle current of 800 µA too low?

Thanks 

Hi Artjom,

Good day. Could you please provide us with the PI Expert you've generated so that we can review it further?