TOP270 100 watt Offline Flyback Design - Output overshoot

Posted by: nmit.201 on 01/18/2016

Hi,

Have used PIExpert too to design offline SMPS of about 100 watts consisting of 2 outputs.

1. 24V, 4A

2. 9V, 500mA

Schematic is as attached. On varyting i/p voltage from 85V, till about 130V, both outputs are over shooting.

Expected 24V is shooting to 34.6 V after startup and remaining there and 9V output is shooting to 14V and

remaining there.

Have double checked feedback circuit components, and they look fine. In my first run had used TLV431 instead

of TL431 and subsequently corrected it. Also a resistor mounted was wrong in the feedback circuit which I subsequently

corrected, but voltage is always overshooting.

Point me on how to debug this circuit.

Regards,

Manju

Files

Motor Driver Card V1.2 - Schematic.pdf (185.07 KB)

Comments

One more observation is that the bias voltage is measuring 20.6V, while it was designed was 12V.

This makes all output voltages along with bias voltage to be greater than designed(more than 40%).

What can the reason be for this behavior? Should I suspect the transformer or the feedback circuit?

Hi,

The value of R14 is too big, Can you change the value of R14 to about 1Kohm and veirfy. This is for debugging purpose only.

Could you also please send me your PIEpert design file.

Regards,

PI-NANO

Dear PI-NANO,

Please find design sheet attached. Will check about R14. How do you logically select values of these resistors?

Regards,

Manju

Attachment	Size
Motor Driver Card V1.2 - Design Sheet - 2.pdf (801.97 KB)	801.97 KB

Dear PI,

As suggested changed R14 to about 1050E and removed the phase boost network(R15,C17) and voltages

are regulated to epected levels.

Can you explain this behavior?

Regards,

Manju

Hi,

If the value of resistor R14 is too high, it will limit the current flowing into the control pin and effects control loop operation.

Since the current flowing into the control pin is limited by R14, the control loop will think that it needs to deliver more energy to the output in order to regulate. However it results in higer oput volatge than the required voltage level.

I would recommend you to use PIExpert to design your power supply, so that you can get the schematic directly from PIExpert.

Regards,

PI-NANO

Hi,

Ok. What is the use of the Phase Boost Network in the circuit?

Regards,

Manju

Phase Boost Network is used for Control loop. It provides fast transient response to the System.