MC33063A: 5V-400V Step up Converter

Guiyong Yang

Part Number: MC33063A
Other Parts Discussed in Thread: LM3478

Hi,

I have the question like below, could you please tell me the reason and how to fix the isse? Thanks.

quote:

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I have used approximately 3mH inductor and achieved voltage upto 270V. But problem is the voltage is gradually decreasing (Voltage drops down to 2V it puts continuous on) and the power Mosfet is heating too much.

As per datasheet Vout= 1.25(1+R2/R1), which indicates it is depending on resistor values. But during testing the output voltage is changing with change in Inductor value. With (10-100) uH inductors voltage is ~100V. When used (1-3) mH Inductors voltage rise to ~ 270V.

Please suggest how should I achieve this much of stable voltage without heating the components.

over 5 years ago

0 Jasper Li over 5 years ago

TI__Mastermind 46905 points

i think the circuit work for a while, but the Mosfet is damaged, so it is very hot. could you share the schematic and layout?

0 Guiyong Yang over 5 years ago in reply to Jasper Li

Prodigy 130 points

The schematic is shown as below. The ciruit is work well at TA=25 degree centigrade. However, When work at TA=60 degree centigrade, the output waveform distort. Besides, when we change the coil (L1)from 470uH(1A) to 150uH(3A), the output waveform does not distort, but the amplitude of the output is lower than before. So, I want to know why? thanks for reply.

0 Jasper Li over 5 years ago in reply to Guiyong Yang

TI__Mastermind 46905 points

Hi Guiyong

What is the output current in your application? For 5V to 400V with a boost solution, the duty cycle would be very small and trigger the maximum duty limitation. So I would suggest using non-isolation flyback solution. Thy duty cycle can be controlled by the wind ratio of the transformer. LM3478 is a good solution of this flyback converter.

0 Guiyong Yang over 5 years ago in reply to Jasper Li

Prodigy 130 points

The output current of the application is about 1.5A max. so maybe not use the recommended device. I am curious about the influence of the inductor, not only about the inductor value, but also the sheild type or open type. the open type indcutor is better than the sheild type. For the calculate formula is Vout=(R1/R2+1)VFB, have nothing to do with the inductor. how to explain this?

0 Jasper Li over 5 years ago in reply to Guiyong Yang

TI__Mastermind 46905 points

Hi Guiyong

The inductor is importance for the normal operation of a switching converter.
1. What is the inductor current ripple, Is the device in CCM or DCM?
2. What is the inductor current rating, is it possible that the inductor becomes saturation?

if IOUT=1.5A, the output power is 600W. could you confirm this?

0 Guiyong Yang over 5 years ago in reply to Jasper Li

Prodigy 130 points

Hi Japser LI,
I knew the inductor was important. But output waveform amplitude changed as the inductor, not decided by the formula. That is what I want to know. the inductor rated vale is more or less 2A and do not become to saturation. that should not be the reason. The max current is 1.5A is bacause of the device peak. usaully operate at about 0.2A and work at DCM, the output power may be about 20W.

thanks for the reply.

0 Jasper Li over 5 years ago in reply to Guiyong Yang

TI__Mastermind 46905 points

Hi Guiyong

Could you share waveform to share the behavior you described? I would like to have the VIN, SW and VOUT waveform. It is easier for us to analyze the root cause with operating waveform.

0 Guiyong Yang over 5 years ago in reply to Jasper Li

Prodigy 130 points

Hi Jasper,

The waveform is shown as below. The nominal output VOUT is about 95V, however the actual output VOUT is about 70V with fluctuation.

0 Jasper Li over 5 years ago in reply to Guiyong Yang

TI__Mastermind 46905 points

Hi Guiyong

What is the output current in this measurement?
Could you also measure the voltage in IPK (PIN 7)? I'm afraid the voltage drop between the 0.3ohm resistor is higher than 300mV and trigger current limitation. We can try to reduce this resistor if current limit is triggered.

0 Guiyong Yang over 5 years ago in reply to Jasper Li

Prodigy 130 points

Hi Jasper Li,

The current is sawtooth and the max current is over the rated value of the inductor. I also checked the voltage in IPK, and the voltage drop is about 0.1mV. but this change imporve the phenomenon that the output amplitude decrease as the inductor value change to smaller.

0 Jasper Li over 5 years ago in reply to Guiyong Yang

TI__Mastermind 46905 points

Hi GuiYong

How did you measure the voltage drop of the R2 (0.3R)? The current is only 0.3mA through the resistor if the voltage drop is 0.1mV.

0 Guiyong Yang over 5 years ago in reply to Jasper Li

Prodigy 130 points

Hi Jasper,

Sorry for the mistake, the voltage drop is 0.1V, not 0.1mV, so the current of the calculation is 0.3A. Thanks.

0 Jasper Li over 5 years ago in reply to Guiyong Yang

TI__Mastermind 46905 points

Hi Guiyong

Could you measure the waveform through the resistor?

0 Guiyong Yang over 5 years ago in reply to Jasper Li

Prodigy 130 points

Hi Jasper,

I measured the waverform of the resistor and found that the voltage is about DC 24V nearby.

0 Jasper Li over 5 years ago in reply to Guiyong Yang

TI__Mastermind 46905 points

Hi Guiyong

No sure about your meaning. I want to know the waveform of the resistor, then we can calculate the current through the resistor. And also check if the voltage of the resistor is higher than 0.3A

0 Guiyong Yang over 5 years ago in reply to Jasper Li

Prodigy 130 points

Hi Jasper,

I checked the voltage and found that the voltage between of the resistor was about 0.1V. so the current of flowing into the resistor is 1A peak. Besides, it take effect when I have a resistor in parallel and the voltage between of the resistor was about 0.085V. It seems that release the current limit. Thanks.

0 Jasper Li over 5 years ago in reply to Guiyong Yang

TI__Mastermind 46905 points

Hi Guiyong

May I know the 0.1V is the average voltage or the peak voltage? The peak voltage can only be measure with oscilloscope. please share me what does the waveform across the resistor looks like.

0 Jasper Li over 5 years ago in reply to Jasper Li

TI__Mastermind 46905 points

Hi Guiyong

The post is closed as no further feedback. It opens again if you reply.

0 Guiyong Yang over 5 years ago in reply to Jasper Li

Prodigy 130 points

Hi Jasper，
Sorry for reply late. I checked the waveform of the resistor two sides and found that the voltage had a enveloping line approximate sine but the amplitude of the wavform is too small and had some noise. because of the high input voltage. these seem to horizontal line if I did not scale the time to small. Does this have other influence? The current limit seems released after I change the resistor to smaller. The peak voltage of the device is 0.3V(Typ.), 0.25V(Min.) and 0.35V(Max) in datasheet.

0 Jasper Li over 5 years ago in reply to Guiyong Yang

TI__Mastermind 46905 points

Hi Guiyong
It would be great if you can share me the waveform.
Please change to resistor to small value and check if this helps to solve your problem.

0 Guiyong Yang over 5 years ago in reply to Jasper Li

Prodigy 130 points

Hi jasper,

thanks for the reply.The waveform is just like the picture as below. I confirmed that changed the resisotr to smaller may solve the problem. thanks again.

0 Guiyong Yang over 5 years ago in reply to Guiyong Yang

Prodigy 130 points

0 Jasper Li over 5 years ago in reply to Guiyong Yang

TI__Mastermind 46905 points

Hi Guiyong

Good to know small resistor solve the issue. is there any further problem?

0 Guiyong Yang over 5 years ago in reply to Jasper Li

Prodigy 130 points

Hi Japser，

I have another quesiotn about the circuit. We have a 33uF/160V output capcitor at now. But I think the output capcitor value is too small. How to calculate the output capcitance and what the remcommended value of the ouput capcator. I confused when I lookup the datasheet.

0 Jasper Li over 5 years ago in reply to Guiyong Yang

TI__Mastermind 46905 points

Hi Guiyong

The output capacitor depend on the output ripple and load transient performance. there is not limitation. you can increase the output cap until the ripple is acceptable.

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MC33063A: 5V-400V Step up Converter