TL3844 based SMPS design

Atul,

I used the free power stage designer tool from TI
http://www.ti.com/tool/powerstage-designer

It suggested a 59uH primary and 3:1 turns ratio using a flyback design..

Hi,

How to calculate the value of the start-up resistor for TL3844 or similar PWM controllers ?

Regards

Atul Bhakay

Hi,

The max start-up current for TL3844 is 1mA. So for min input voltage of 20VDC, if the start-up resistor is 20K, then I get the start-up current of 1mA. But at high line of 150V, with 20K as start-up resistor, I get 7.5mA. Is this OK ?

Regards

Atul Bhakay

0841.TL3844_3 output_EE2005_Transformer.doc

Hi, Kindly verify theattached  transformer details of this TL3844 based SMPS

Regards

Atul Bhakay

Hi Ron,

Thanks for yr reply.

At the beginning, when Vcc is 'zero' volts and the Vcc capacitor is being charged thro the start-up resistor, then with a input of 20V and a max start-up current of 1mA, should not the value of start-up resistor be 20V / 1mA = 20K ohms ?

Also, since the minimu operating threshold voltage , after start-up is typically 10V for TL3844, the bias winding ahs to be designed for 10V - correct ?

These are the 2 queries I have.

I am waiting for a reply on how to select the resistors connected from VFB pin when an opto-coupler is used in the TL3844 SMPS

Regards

Atul bhakay

Atul,

My knowledge in transformer construction is not sufficient to answer the transformer spec question. You can contact the transformer manufacturer or post a new thread.

Atul Bhakay said:

At the beginning, when Vcc is 'zero' volts and the Vcc capacitor is being charged thro the start-up resistor, then with a input of 20V and a max start-up current of 1mA, should not the value of start-up resistor be 20V / 1mA = 20K ohms ?

Also, since the minimum operating threshold voltage , after start-up is typically 10V for TL3844, the bias winding has to be designed for 10V - correct ?

The data sheet parameters describe the TL3844 operation, not the application operation. They don't need to match, they need to be satified. 
The current in to the TL3844 will be a maximum of 1mA per data sheet. This means less than 1mA of the resistor's current flows into TL3844 and the rest charges the capacitor. There must still be at least 1mA of resistor current flow even if the capacitor voltage is 17.5V. 

At 10V VCC half of the TL3844 population will be shutdown. VCC must remain greater than 11.5V to ensure all TL3844 devices remain on.

Hi Ron,

Thanks for the suggestion on calculating the start-up resistor. With the suggested value of 2.5K, I was able to get the SMPS 'working' and delivering the reqd power, too.

However, I noticed that at 20VDC input the VCC pin voltage is around 10.8V and at 120VDC input, the VCC pin voltage is 12.2 V. Why this variation , hwen I have a VCC winding ?

Regards

Atul Bhakay

Atul,

Check the voltage waveform on anode of the diode for the VCC winding rectifier. If you have the correct winding polarity, then the positive peaks should be about the same. What is that peak voltage?

Hi Ron,

Thanks for this important tip. Yes, I will check the anode voltage waveform of the VCC winding diode. However, prima facie, will the VCC voltage change as Vin increases ?

Regards

Atul Bhakay

Hello Atul,

in flyback please consider ever: the wave form at secondaries is the opposite of the primary wave form; that's to say: with a DC about 0.3 at primary winding (Vgate about: __--__--__), you will get positive pulses -measured on diode anodes, against GND- with DC=0.7 (approx.: ----_----_). The positive value of them will be your VCC output values, while the negative ones could be very high if you have a high Vin value together with a low n.

n=3 is quite ideal for most applications.

Alejandro Sincich