UCC28950: What is the best way to limit the current on the output?

Hi Jason,

I am asking my colleagues if they have a circuit to share for this application.

Regards

Peter

Thank you, I would appreciate that

Hello Jason

SO - the current sensor at the top of the H bridge is used to control the peak current on each switching cycle. It's the current sensor used for Peak Current Mode control. It's on the Webench schematic and is also present on the PMP8740 schematic - (T1 on sheet 1)

The UCC28950 will limit the peak current signal to a maximum of 2V at the CS pin so that provides a 'safety net' which the INPUT current cannot exceed.

However, you want to have an adjustable OUTPUT current limit and for that you need two output error amplifiers - one sensing Vout (U1A) and the other sensing Iout (U1B). This arrangement is basically the same as the one in this document

/cfs-file/__key/communityserver-discussions-components-files/196/CI_5F00_CV.pptx.

The way to turn this into an adjustable constant current is to change the reference voltage used by the I_out Error amplifier

Regards

Colin

Also can I use this transformer:

750343382

https://www.we-online.com/web/en/electronic_components/produkte_pb/products_pbcm/product_spotlight/tida00951.php

for my application? Because I don't really know how to make a centertap transformer myself. 

Hello Feng Lin

I've had to ask the designer of the circuit to explain the reason for the two small signal MOSFETs - I hope to have an answer in a few days.

The PMP8740 uses a TL431 to generate a 5V reference for the error amplifier - this is sometimes necessary if the 1.5% accuracy of the on-board reference is not good enough. You can see that the +5VREF signal comes from the cathode of U8 on sheet 3. In your case, I'd suggest you use the on-board reference and the 2 x 2.37k resistors to generate the 2.5V reference for your circuit.

R13 and R19 have the same function as R6 and R7. There are two small differences - R12 is a small resistor added in series with the voltage sensing loop  as a convenient point to inject a signal during loop gain/phase testing - that's the reason for the 'Voltage Loop' note. R39 does the same for the current loop.

The designer added R14 as a way to modify the output voltage slightly using the Parallel Bias signal. You won't need this resistor unless you are planning to operate two or more of your design in parallel .

Regards

Colin

Hello Feng Lin

I got this answer from the designer -

"Both MOSFETs Q2 and Q4 form actually a “poor man” DAC: when the microcontroller provides a PWM signal (digital level) to PWM_Vref and PWM_Iref, these two square waveforms are translated to a waveform on each drain with the same frequency (but with duty cycle 1-D) and with a peak value = the reference voltage +5Vref. After low pass filtering of this waveform through the double RC filter (for example R24, C7 and R23, C6), we get a DC reference proportional to +5Vref and the duty cycle of the PWM."

So - they are basically a way to modify the reference into the error amplifier. If you don't need to do this then you don't need the MOSFETs.

Regards
Colin

Dear Colin,

Thank you for the informative insight, can I send you my total PCB schematics for a small review? Maybe I am missing something or maybe I am

doing something wrong I would appreciate it.

My total schematic:

Full Bridge converter and Control Unit.pdf  

I am planning to order the PCB on wednesday, I am just not 100% sure about the transformer can you also check the if I can use this transformer:

https://www.we-online.com/web/en/electronic_components/produkte_pb/products_pbcm/product_spotlight/tida00951.php

750343382 it is used as an bi-directional transformer. Can I also use this transformer as an stepdown transformer instead of a stepup?

With kind regards,

Xiao Feng Lin

Hello Feng-lin

I should be able to do that tomorrow - I'm based in Ireland so there's a time difference.

Regards

Colin