Hi, everyone,
I'm facing some issue when fixing SMPS, I'm appreciate if you have any opinion or advice![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Brief:
A forward converter using pulse transformer to drive high-side MOSFET will short through with low-side MOSFET when applying 0-5A@1kHz dynamic load, and the power supply can deliver maximum 12V, 10A.
Continue load is okay, load drop is also good, only happened when applying heavy load.
After some test and try, I found adding few ohm R (10-47) and reduce C (to 0.1uF) at primary side of pulse transformer can help.
I'm wondering how to calculate the coupling capacitor, or actually my direction was wrong, its not capacitor's issue.
The story:
I have a forward converter using NCP1562A as main controller, the schematic is in attatchment.
Sorry for the covered part, but its only for the start up voltage tuning and some protection circuit, not important for this issue.
schematic
The main issue is when testing dynamic load, the high-side MOSFET and low-side MOSFET will short through and causing the OCP protection althought the MOSFETs are not damaged.
The failure waveform seems like this, channel information is inside the picture.
The blue signal (U3 OUT2) somehow drop a little bit (the third pulse count from left),
so the pulse transformer secondary also raise a little bit and turn-on the high-side MOSFET then cause short through.
After the failure, next PWM comes in and cause main transformer (T2) saturate.
![1713922406344.png 1713922406344.png](https://forum.allaboutcircuits.com/data/attachments/308/308223-d9cdf9c4a4112b14f750b7b5dde49c51.jpg)
Note:
The blue signal should be invert because the pulse transformer is inverse, and all the probe are keep ground path as small as possable.
I had test following item to find the source of fault:
1. Check is secondary self-excited circuit
I measure secondary SR MOSFET Q17 Ids to check is the fault waveform/energy transmit to another side, and it seems not.
And low-side driving signal acts correct.
![1713923625500.png 1713923625500.png](https://forum.allaboutcircuits.com/data/attachments/308/308225-234c98f0e55dfff3c07de7dc6ed9a0df.jpg)
Note:
The blue signal sudden raise is where the short through happened, main transformer secondary current seems okay.
2. Check high-side driving current
Because NCP1562A has different sink/source capability at two output, so I guess maybe this cause some issue.
It seems the IC should sink current but actually, its push some current and drive the gate to high.
But I'm not sure is the voltage cause the current high or the current it self cause the transformer high.
![1713924686019.png 1713924686019.png](https://forum.allaboutcircuits.com/data/attachments/308/308226-85d090cc404c78b54a546c7f4ab2c380.jpg)
3. So I try to digging in the gate drive circuit and its behavior, I add 20ohm resistance and change coupling capaciance to 0.1uF.
Compare to 0.47uF (light color waveform), it makes huge difference in gate driving current, also fix the short through issue.
I also measure the voltage of coupling capacitance but I'm not sure is there any information can be found here.
![1713928470469.png 1713928470469.png](https://forum.allaboutcircuits.com/data/attachments/308/308229-defc0f2a0e4b47cc37d3ae109e148d62.jpg)
For now, I found a reference document for coupling capacitance calculation here:
Fundamentals of MOSFET and IGBT Gate Driver Circuits (Replaces SLUP169) (Rev. A) (ti.com)
I'm trying to verify is the root cause is the coupling capacitor.
The question is:
1. Is this phenomenon cause by wrong valus of capacitor or maybe its caused by pulse transformer saturation?
2. How do you think about the analysis above, is there anything I missed?
Thanks for you reading here, its a long artical.
Have a nice day!
I'm facing some issue when fixing SMPS, I'm appreciate if you have any opinion or advice
Brief:
A forward converter using pulse transformer to drive high-side MOSFET will short through with low-side MOSFET when applying 0-5A@1kHz dynamic load, and the power supply can deliver maximum 12V, 10A.
Continue load is okay, load drop is also good, only happened when applying heavy load.
After some test and try, I found adding few ohm R (10-47) and reduce C (to 0.1uF) at primary side of pulse transformer can help.
I'm wondering how to calculate the coupling capacitor, or actually my direction was wrong, its not capacitor's issue.
The story:
I have a forward converter using NCP1562A as main controller, the schematic is in attatchment.
Sorry for the covered part, but its only for the start up voltage tuning and some protection circuit, not important for this issue.
schematic
The main issue is when testing dynamic load, the high-side MOSFET and low-side MOSFET will short through and causing the OCP protection althought the MOSFETs are not damaged.
The failure waveform seems like this, channel information is inside the picture.
The blue signal (U3 OUT2) somehow drop a little bit (the third pulse count from left),
so the pulse transformer secondary also raise a little bit and turn-on the high-side MOSFET then cause short through.
After the failure, next PWM comes in and cause main transformer (T2) saturate.
![1713922406344.png 1713922406344.png](https://forum.allaboutcircuits.com/data/attachments/308/308223-d9cdf9c4a4112b14f750b7b5dde49c51.jpg)
Note:
The blue signal should be invert because the pulse transformer is inverse, and all the probe are keep ground path as small as possable.
I had test following item to find the source of fault:
1. Check is secondary self-excited circuit
I measure secondary SR MOSFET Q17 Ids to check is the fault waveform/energy transmit to another side, and it seems not.
And low-side driving signal acts correct.
![1713923625500.png 1713923625500.png](https://forum.allaboutcircuits.com/data/attachments/308/308225-234c98f0e55dfff3c07de7dc6ed9a0df.jpg)
Note:
The blue signal sudden raise is where the short through happened, main transformer secondary current seems okay.
2. Check high-side driving current
Because NCP1562A has different sink/source capability at two output, so I guess maybe this cause some issue.
It seems the IC should sink current but actually, its push some current and drive the gate to high.
But I'm not sure is the voltage cause the current high or the current it self cause the transformer high.
![1713924686019.png 1713924686019.png](https://forum.allaboutcircuits.com/data/attachments/308/308226-85d090cc404c78b54a546c7f4ab2c380.jpg)
3. So I try to digging in the gate drive circuit and its behavior, I add 20ohm resistance and change coupling capaciance to 0.1uF.
Compare to 0.47uF (light color waveform), it makes huge difference in gate driving current, also fix the short through issue.
I also measure the voltage of coupling capacitance but I'm not sure is there any information can be found here.
![1713928470469.png 1713928470469.png](https://forum.allaboutcircuits.com/data/attachments/308/308229-defc0f2a0e4b47cc37d3ae109e148d62.jpg)
For now, I found a reference document for coupling capacitance calculation here:
Fundamentals of MOSFET and IGBT Gate Driver Circuits (Replaces SLUP169) (Rev. A) (ti.com)
I'm trying to verify is the root cause is the coupling capacitor.
The question is:
1. Is this phenomenon cause by wrong valus of capacitor or maybe its caused by pulse transformer saturation?
2. How do you think about the analysis above, is there anything I missed?
Thanks for you reading here, its a long artical.
Have a nice day!
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