david.cano90
Junior Member level 3
Hello everybody,
I am having troubles with a 500W Push-Pull converter (Vo=200V, Vin=24V). I think that the slope compensation circuit or the current sense resistor is wrongly sized. There are four questions below... if you can help me with any one or all of them I would be really thankful.
The chip that I am using is the UCC3808AD-2. I followed this guide to size the current sense resistor but there it is used an UCC28083 instead. Here you can see its differences, which are not too much:
Shorty, from that last guide, I've got that the controller would measure a peak current of 41.5A (from eq. 11) and for instance (taking the Overcurrent threshold (OCT) of 0.75 V) the current sense resistor Rs would be (from eq. 12):
Rs=OCT/41.5A=18mOhm
In the last calculation, the magnetization current value was not included (figure 3 from the guide). If so:
Rs=OCT/(41.5A+3.25A)=16.8mOhm *This one I have used in the implemented circuit.
An scheme that simplifies part of the primary side of the converter is shown:
My questions are going to be listed so that you guys can answer easily
a) Should I take in the numerator the Overcurrent threshold value (red) or the Maximum Input value (green)?
b) What is the effect of the Gain parameter shown in the datasheets? Should I include it in the calculations?
c) Is it better to include the magnetization current value in the calculations?
Additionally, I am going to share a capture of the behavior of the measured current and the output voltage of the converter. The converter squeals (high-pitch noise) once the controller is activated. I am afraid that there are subharmonic oscillations (correct me please if I am wrong), which means that there is not enough slope compensation.
d) Am I wrong with my last comment?
As always, thank you for your help.
Best regards,
David.
I am having troubles with a 500W Push-Pull converter (Vo=200V, Vin=24V). I think that the slope compensation circuit or the current sense resistor is wrongly sized. There are four questions below... if you can help me with any one or all of them I would be really thankful.
The chip that I am using is the UCC3808AD-2. I followed this guide to size the current sense resistor but there it is used an UCC28083 instead. Here you can see its differences, which are not too much:
Shorty, from that last guide, I've got that the controller would measure a peak current of 41.5A (from eq. 11) and for instance (taking the Overcurrent threshold (OCT) of 0.75 V) the current sense resistor Rs would be (from eq. 12):
Rs=OCT/41.5A=18mOhm
In the last calculation, the magnetization current value was not included (figure 3 from the guide). If so:
Rs=OCT/(41.5A+3.25A)=16.8mOhm *This one I have used in the implemented circuit.
An scheme that simplifies part of the primary side of the converter is shown:
My questions are going to be listed so that you guys can answer easily
a) Should I take in the numerator the Overcurrent threshold value (red) or the Maximum Input value (green)?
b) What is the effect of the Gain parameter shown in the datasheets? Should I include it in the calculations?
c) Is it better to include the magnetization current value in the calculations?
Additionally, I am going to share a capture of the behavior of the measured current and the output voltage of the converter. The converter squeals (high-pitch noise) once the controller is activated. I am afraid that there are subharmonic oscillations (correct me please if I am wrong), which means that there is not enough slope compensation.
d) Am I wrong with my last comment?
As always, thank you for your help.
Best regards,
David.
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