Negative output characteristic

Posted by: fla on 03/26/2021

Hello.
For a new project, we have the following requirements:
Vin = 55 - 253 Vac 45-75 Hz
Vo = 24 V +/- 3%
Po = 36 W
We have to implement a negative output characteristic, so that the output voltage is:
24.4 V at off-load
24.0 V at 50 % load
23.6 V at full-load

The regulator has not been selected yet. The possibility to implement / add the negative output characteristic is a key criterium.
Could this be done using a PI regulator? and which one would you recommend for this application?
If application notes or reference designs are available, I would like to study them.

Thank you.

Comments

Hi fla,

The first solution that comes to mind is to use the InnoSwitch3-Pro family of converters. The InnoSwitch3-Pro output voltage can be adjusted thru the chip's I2C interface. This will require a microcontroller with I2C to serve as the master (Arduino is OK) but you will be able to specify any arbitrary V-I curve you desire.

If the use of a microcontroller is not acceptable, you can use other InnoSwitch3 variants and just modify the feedback resistor network to achieve your desired V-I requirement. One idea that comes to mind is thru the use of an Op-amp and a voltage divider which will add or subtract current to the feedback network depending on the load current.

Of course, if high efficiency is not critical you can just set the output to 24.4V and place a resistor that will cause a 0.8V drop at full load.

We do not have reference designs exactly for your output V-I requirements but the DERs listed in the InnoSwitch3 page might be of help.

Thanks

Hello.

Thank you for your reply.
I will consider the solution with an op-amp for the output characteristic.

1. Considering the InnoSwitch3-family, which variant would be best suited for our application?
The board will be inside a rack. The maximium ambient temperature is 85 °C.

2. Do you see any problem to operate the circuit at 55 Vac input?

3. The availability of a demo-board would be great to speed up our design.

Thank you.

1) You can check out the InnoSwitch3-CP family of ICs for your application. The following is a reference design that is similar to the one you are trying to build.

https://ac-dc.power.com/design-support/reference-designs/design-examples/der-815-45-w-24-v-output-isolated-flyback-power-supply-smart-speakers-…

You might need to use a chip with a higher power rating (size) due to your 55Vac input requirement. You can use out PIExpert tool to modify the design as needed.

https://piexpertonline.power.com/site/login

85 degree ambient is no problem as long as you implement proper thermal management.

2) As stated above, you can use the InnoSwitch3 at 55Vac but you might need to use a variant with a higher power rating. If overall unit size is not an issue, increasing the bulk capacitance will help maintain high efficiency and lower heat dissipation. You might also need to adjust the UV threshold of the design or disable UVLO.

3) The DER link above contains all the information needed to get you started. Gerber files are also available. As for the demo board, I'll have to get back to you for this although there are PI RDKs available in Digikey.

Thanks