UCC28780: UCC28780 TDM

Hi
Why I ask this question? Because during the debug, I found the TDM time is variable from the first PWMH in ABM, but the TDM is basically constant in AAM. It means in AAM the TDM could be tuned to be stable, but in ABM UCC28780 is always trying to tuned to appropriate value.
So I want to know what determines the first PWMH time in ABM.
Could you help explain it?

Also in LPM, what determines the off time of PWML for the first switching period?

The first pulse of PWMH in ABM packet is different from the next following since the first PWML is not on active clamp control and it is a DCM flyback. This can make things some different. What is your clamping capacitor value? Are you using primary resonance or secondary resonance? Did you test our EVM? It looks EVM RDM quite consistent between the first TDM and the remaining in ABM. So there should be something needed to adjust in your design such as if clamping capacitor value .

Hi,

The clamping capacitor is the same with Si ACF EVM 3pcs*0.33uF/X7T ceramic capacitor.

My design uses secondary resonance, I have no EVM board.

The figure 10 of the EVM user guide shows some difference between the first TDM and the remaining in ABM, the first TDM is smallest, and the 4/5 is longest.

I agree with you the first PWML is DCM flyback, how to understand the difference of the first PWMH you said?

Could you explain it detailly?

Thanks!

The below is from the user's guide Figure 10. It does not seem 20% difference. How did you conclude 20% difference (or 4/5, meaning?)

What is your secondary resonance capacitor values? Can you provide your schematics for review?

If you copied the clamping capacitor values from EVM, did you copy everything else? If not, there will be some modification needed. Especially, what are your transformer specs - I need to review your transformer in order to understand your design.

"I agree with you the first PWML is DCM flyback, how to understand the difference of the first PWMH you said? Could you explain it in detail?"

Please refer to page 25 and Figure 25 of the datasheet. The number of PWMH and PWML is different during each burst packet which causes some difference on TDM. But the difference does not show much different based on the EVM.

Hi
Sorry, I mean the first pulse TDM is smallest, but 4/5 means the the third and fifth pulse TDM is longest.
I want to say that UCC28780 is always trying to adjust the TDM till getting the appropriate TDM value, my understanding is right or not?
If so, there should a original(first) TDM in ABM, right?
Which factor determine this original TDM?

Hi
This design is a 65W Si ACF design with PD. The magnetic inductor is 115uH.
The schematics can't sent out.
The secondary resonance capacitor is 4pcs*33uF/25V ceramic capacitor.
The clamped capacitor is 3*33uF/250V ceramic capacitor.
The output choke and output polymer cap is the same the 45W EVM.
What do you think?

Hi
The figure 25 shows there is no difference between the first TDM and the remaining TDM, all of them are for ZVS.
So what determine the original (first) TDM?

Please refer to the datasheet, page 21, section 7.4.1 for adaptive ZVS control. The first TDM in each burst packet is determined by the previous burst packet when PWMH is in effect.

You may need to adjust RDM and RTZ at high line input (highest input voltage in your design) and at full load to make sure you get optimal ZVS. The clamping capacitor values look ok but you may need to make adjustment if the transformer turns ratio Nps is different.

I am not understanding which pulse is what you mentioned the first pulse, can you use the EVM waveform to show what you mean?

The current TDM adjustment is based on the previous TDM with criteria if ZVS is achieved or not by SWS signal. Tz is also adjusted at the same time. The target is to get ZVS.

Hi 

Thanks very much. I just want to know "The first TDM in each burst packet is determined by the previous burst packet when PWMH is in effect" you said.

"You may need to adjust RDM and RTZ at high line input (highest input voltage in your design) and at full load to make sure you get optimal ZVS. The clamping capacitor values look ok but you may need to make adjustment if the transformer turns ratio Nps is different."

1. In datasheet, it only suggest large clamping capacitor will not compromise ZCS. So it will be better if the clamping capacitor value is larger, right?

If the capacitor is larger, is there any side-effect?

2. For the optimal ZVS, what is the meaning? What is the operation condition?

3. If the RDM and RZ is setting oversize, what will be the behavior?

As mentioned before, tune up to close to optimal at high line (highest input voltage) of your design, at full load. An optimal in theory is like the show in the datasheet, page 19 and Figure 18. If RZ too high, you will see VSW ring back to positive, so you need to reduce RZ to get VSW between 0 and a few volts when PWML turns on - this helps to minimize switching losses of low side switch. Clamping capacitor values, and TDM need to adjusted based on minimizing magnetizing current at PWML turn on - this helps to minimize conduction losses of high side Switch. if TDM is too small, PWML will turn on at high Im which gives high conduction losses. If TDM too big, you may not have enough energy to achieve ZVS.

Hi,

Thanks.

I could agree with your understanding of TZ, because TZ only has 100%~140% range. But I don't agree about TDM, because there is no range limit of TDM, so the ZVS discriminator will surely adjust the TDM to reach the optimal ZVS. So I think RDM configuration is not much critical, it only helps to make ZVS discriminator find the optimal ZVS condition as quickly as possible. Do you agree?

I can't understand the sentence you say "Clamping capacitor values, and TDM need to adjusted based on minimizing magnetizing current at PWML turn on - this helps to minimize conduction losses of high side Switch."

My understanding is that the primary clamping capacitor only has effect to shape more current to secondary side, so it would be better if the value higher. Do you agree?

TDM setup helps to determine ZVS happening at what Im value along with TZ. You can refer the figure I pointed out, Figure 18. It is desired with a small Im to achieve the ZVS to minimize high side MOSFET conduction losses.

The clamping capacitance also changes the shape of both voltage and current.

The target is to achieve the ZVS at minimum conduction losses.

Hi,

I agree with you that TDM setup helps to determine ZVS happening along with TZ.

My point is that: Even if the RDM selection is not reasonable, the ZVS discriminator still could adjust the TDM to a reasonable value to realize ZVS.

Because there is no limit to TDM adjustment, the the ZVS discriminator finally could adjust the TDM to a right value.

So the RDM configuration is not important.

What's your point?