ucc2893 duty ratio problem

10053055

Prodigy 50 points

Other Parts Discussed in Thread: UCC2893, TL431

I design specifications of DC to DC isolate forward and used high-side N Channel is as follow:

input voltage: 100V
output voltage: 12V
output current: 0A~10A
switch frequency: 100kHz
duty ratio maximun : 0.45

When i testing , load is used Electronic load and set to constant current mode from 0A Gradually increase to 10 A.

Start increase input voltage to 111V and Observe output voltage is 12.xx V and current is 0 A.

then output current from 0A Gradually increase to 10 A.

Question 1: Why output(pin13) duty ratio has been keep 0.09 and not increase to 0.45 ?

Question 2: When output current increase but output voltage drop. Why?

When output current increase to 0.1072A and Observe output voltage is 7.13V , other pin voltage as follow:

V_DD(rms)(UCC2893 pin 14) = 11.4V
V_cs(rms)(UCC2893 pin 7) = 0.5V
V_FB(rms)(UCC2893 pin 9) = 4.9V
V_{ss(rms)(UCC2893 pin 10) = 2.8V}V_{LUV(rms)(UCC2893 pin 15) =1.5V}V_in(UCC2893 pin 16) =111V

show figure yellow waveform is V_DD(UCC2893 pin 14) and purple waveform is V_out(UCC2893 pin 13)

over 12 years ago

0 John Bottrill over 12 years ago

TI__Mastermind 25130 points

You need to provide a schematic with values to be able to get this question answered.

John

0 10053055 over 12 years ago in reply to John Bottrill

Prodigy 50 points

Can I send schematic to your email:noreply@e2e.ti.com ?

Please help me, thanks.

0 John Bottrill over 12 years ago in reply to 10053055

TI__Mastermind 25130 points

Attach it to the posting as a PDF file.

0 10053055 over 12 years ago in reply to John Bottrill

Prodigy 50 points

OK ! But i Implementation of control circuit and driver circuit is used Breadboard. input ac voltage is 80 V . after rectifier is 111V

forward active clamp.pdf

0 John Bottrill over 12 years ago in reply to 10053055

TI__Mastermind 25130 points

i took a look at the circuit and noticed a couple of things immediately.

Since the circuit is acually working I am ging to assume that you have the diode bridge connected properly not as shown in the schematic.

Do you really have the SYNC pin connected to the 5k feedback resistor which is connected to the optocoupler??? That resistor should be connected to the Vref pin and the sync pin left open.

I think you have noise issues. Read the linked article

http://www.wirelessdesignmag.com/ShowPR.aspx?PUBCODE=055&ACCT=0032090&ISSUE=0902&RELTYPE=pwr&PRODCODE=Q0140&PRODLETT=A&CommonCount=0

Connect PGND and GND pins on the IC right at the IC, and then connect the CS pin capacitor directly to the GND pin. This might take care of the noise issue.

Because of the resonance set up between the primary side inductance of the transformer and the reset capacitor there is a notch i the control loop using current mde control that is at a fairly low frequency.

Your feedback around the TL431 is designed to flatten at about 2 kHz and then start to roll off again at about 20 kHz. This is close to what you will get but be aware that there is a sneek path for feedback with the design you have choosen. see link below for an explanation

http://www.controleng.com/index.php?id=483&cHash=081010&tx_ttnews[tt_news]=68909

This frequency might well be in the region where that notch is present and it will dominate.