Flyback Transformer output being limited

Hi,

Please provide your schematics or circuit so we can have a look and also to understand what exactly you would like to have.

Here is a picture of the XFMR circuitry:

Hi,

Please provide your schematics including the flyback power stage since the issue you want to resolve is the power stage so only control stage cannot provide enough detail for analysis.

Hi,

Your schematics does not show the flyback power stage. If you think it is there, then please circle the power stage. I need to know which part if flyback power stage in your schematics to understand what the issue is.

Please reply to this thread and do not use other thread or platform to avoid possible missing to your reply. 

Here is the power stage. 

Hi,

Which TI PWM flyback control device is used in this design?

I have not implemented a PWM flyback controller in this design yet. I have been trying to drive it directly from my microcontroller. If I need a PWM flyback controller I have been looking at UCC28C50-Q1. 

Hi,

Usually in a flyback, the main MOSFET should have a snubber. But I am not sure which part of your circuit is functioned as the snubber. Can you circle it?

I do not currently have a snubber on this schematic. I have found that the snubber removes the ripple but does not help with the magnitude of the output voltage.

Hi,

You need to put the snubber then send to me the test results of MOSFET Vds and the issue you are referring. Without the snubber the flyback may not run suppostedly.

Here is a screenshot of the MOSFET Vds using an RCD snubber across the flyback input. The issue I am referring to is why my voltage is only spiking to 15V. I currently don't have thing in my circuit that should be limiting that peak. 

Hi,

Can you provide your schematics and show where the snubber is located and where did you measure the MOSFET Vds? Your waveform is strange to me so I need to see exactly these above to understand why the waveform is like that.

Here is my updated schematic:

I probed at pin 2 of IRF740PBF. 

Hi,

Your snubber capacitor value seems too large, can you change it to smaller to try?

What is your transformer turns rations?

Also, can you probe the waveforms of Vgs and Vds at the same time?

My TTR is 1:100. 

Here is another scope shot. Purple = Vgs Blue = Vds

I will try some smaller capacitor values. 

Hi,

It looks the Vds cannot stay at flat while Vgs is still high - what is Vds value when Vgs is high?

Also, can you show your original concern of the below - where did you measure this below waveform, and how it looks now with the snubber?

Vds rises to approximately 2.3V while Vgs is high. 

I am getting a higher output voltage, but the output is still plateauing. 

Hi,

It looks like your MOSFET with 400V rating, while its work voltage somewhere 9V as input. Can you change your MOSFET to 30V to 40V rated?

2.3V for 400V MOSFET is small, but your voltage only at about 9V so 2.3V is big compared to 9V input.

Here is a scope shot with a 30V rated MOSFET. 

So what is your output voltage now?

My output voltage is still plateauing around 800V. I am going to try playing around with the output capacitor to see if I can smooth out the signal.  

Do you have any idea where this slight ripple on the Vds is coming from?

Another observation:

Vds hits a max value in the first impulse sent to the transformer and then decays down to an equilibrium point. My ideal operation would be to have the Vds spikes gradually grow in magnitude. 

Hi,

It looks all these are the behavior of the transformer and the other parasitic.

One likely help is to change IN4007 to a faster speed diode. IN4007 speed is very slow and also with relative high junction capacitance. So the "slight" ripple is likely due to the junction capacitance resonant to the leakage inductance.

The very first high Vds spike is likely due to the leakage inductance. So the pulse width gradually increase (soft start) may help - but need to try to find out.

C13 is another possible factor. You may use a bigger value capacitor of electrolytic with a high frequency small capacitor (ceramic X7R) to see if any improvement 

I've replaced the diode with S4D02120E which is a diode with negligible reverse recovery time, but the junction capacitance is 116pF (Saw that the 1N4007 had a capacitance of 15pF). I am still seeing the ripple. I will look for more diodes with fast switching and low junction capacitances. 

I will try a soft start to try to get rid of the high Vds spike and play around with C13. 

Hi,

For test purpose, you can try SiC diode which has no reverse recovery and very small capacitance.