UCC2808A-1: half-bridge forward converter power-up issue when dry

Hi,

Your attached circuit does not seem show UCC2808A or UCC3808.Please include either into your circuit.

If your CS is not from the real current sensing, then you need to adjust its magnitude so proper driving pulses can be obtained if you see the driving pulses become too small.

The schematic of the supply is in the file "WEFA339 H4 - FLAT.pdf" from my previous post, the UCC3808 (now UCC2808A) is U12 (top right).
The magnitude on the CS ramp is [0 - 400mV].

Regards

How do you control the minimum width of the pulses from the amplitude of the ramp? Is there an easy relationship?
Up to now I only controlled the maximum width of the pulses when breaking 500mV.  

Q40 (J177) is no longer used as the UCC3808-1 has been replaced by an UCC2808A-1 including an internal FET clearing the capacitor connected to the CS pin. With the UCC2808A-1, the ramp on CS is in range [0 - 400mV]. I don't think the problem is to do with the ramp anymore, even if certainly influence the gain.

R17, C24 is just the oscillator ramp set to 200kHz, I have no reason to change the switching frequency. The ramp I had problems with is the one generated on the CS pin by R135, R138, C69 but I think it is now in range [0 - 400mV]. I know the duty cycle gets unstable without ramp generated on the CS pin but I don't know which results I could expect reducing slightly the amplitude of the ramp. I know it should not excess 500mV otherwise I would not be able to get the maximum duty cycle.

Thanks for your reply, that is difficult for me to capture at the same time, COMP, CS, OUTA and OUTB as I only have 2 insulated probes and as they are not shielded they interfere with normal operation of the product.

Is that OK to measure COMP and OUTA and assume CS is always [0 - 400mV] ?

Below is a scope capture I did with a battery scope of the signal on CS with the UCC2808A-1, but this area of the circuit is very sensitive.

I noticed the compensation set on U12 (C68, C88, R130) could help solving my problem. I removed C88 an my unit started. But I am not confident in modifying the feedback on a product which has been used for years. Could you give me some guidance on that? 

Hello,

Sorry, my mistake, I removed C89 to reduce the time constant. Concerning the feedback, for me the UCC3808-1 and UCC2808A-1 had the same characteristics and I did not think a change in the feedback was required. I did bode plots of the unit using the UCC2808A-1 and I did not find instability issue: at 1A: phase margin 83°, gain margin ~33dB

My problem is at power-up when the unit is running dry. I did not success doing bode plot when running dry, I have been told that is because the unit is not in its linear region of operation and bode plots are not relevant in this condition. The problem at power-up is the voltage on the 35.8V rail on which the feedback is connected gets well above the set point which result in reducing the duty cycle to a level not sufficient to maintain the supply of the UCC2808A-1. I have been able to solve this issue by 2 ways: removing C89 or increasing C21 from 22uF to 100uF.

Hello,

Below are some scope captures just before the UCC2808A-1 stops switching at power-up without load.

OUTA of U12 just before VCC of U12 becomes too low to operate:

The pulsing rate of OUTA is ~100kHz, the pulsing width is 340ns (seems stable) amplitude ~8.4V. 
CH1: 35.8V rail, CH2: OUTA U12, CH4: VCC U12, CH3: "output com +"
     

COMP of U12 just before VCC of U12 becomes too low to operate:

The amplitude on COMP is ~640mV, it seems they are cycles for which no pulse is generated on the COMP pin as if a command was skipped (that does the same removing the 20MHz limit on CH2).
CH1: 35.8V rail, CH2: COMP U12, CH4: VCC U12, CH3: "output com +"
             

Hello,

1) The bode plot I sent you was in steady state. I was thinking a problem in steady state could also be the cause of my problem powering-up the unit when dry, but I did not find problems in steady state.

2) I only have 2 isolated probes and I cannot record COMP, CS, OUTA and OUTB on the same capture especially if I want to trigger when the problem occurs (the PWM chip stops switching). The COMP pin is very sensitive and I did not manage to record it with other channels without disturbing the unit. My best result was measuring it with a battery scope. The signal on CS is a [0 - 400mV] ramp at the same rate as the oscillator.

My previous scope captures show COMP and OUTA captured on the same event (when the switching stops) even if that is not the same capture.

3) The circuit has not changed, the PCB is the same, the only difference is now U12 is an UCC2808A-1 and Q40 is not fitted.

4) You mentioned COMP is too low, what do you mean by this, what should I expect, what can I try.
Could you give me some guidance to adjust the RC network on COMP?
Note: the supply of the emitting diode of OC2 is the 26.5V rail which also collapses when duty cycle reduces too much.

5) I don't think there is a major problem in normal operation. The problem I have is at power-up without any load.
The voltage on the 35.8V rail gets very high at start-up especially after the PFC has reached its nominal voltage. U12 seems to have no control of the 35.8V when the PFC voltage raises to ~400V.
Once the PFC voltage is established, the duty cycle to get back to 35.8V gets so low that is not sufficient to maintain VCC and U12 switching. 

Hello,

I have done some measurements disconnecting the earth of the scope to have COMP, CS and OUTA on the same screen capture.

I did not manage to trigger when the switching stops, it seems that is when the chip starts up. The duty cycle increases progressively from a small value to the maximum value (I assume that is because there is no 26.5V for the feedback).

Sorry, I forgot:

CH1: CS, CH2 COMP, CH3 OUTA, CH4 VCC U12

Hello,

If the driving of the output is consistent with levels on COMP and CS, I come back to the initial problem and the best way to fix it.

1) At power-up without load, the 35.8V rail can get well above the set point: ~40V with a set point set to 35.8V.
I have identified 2 possibles causes to this:
a) the 26.5V rail used for the feedback is turned ON  ~25ms after the power-up.
b) the PFC voltage is raising to ~400V and the 35.8V rail raises at the same rate.

2) When the feedback operates, the duty cycle is too low to maintain the supply of U12 in range VCC. VCC and the 26.5V rail drop up to the point the UCC2808A-1 stops switching.

Below are my main attempts at fixing the problem:

- enabling the 26.5V rail earlier: no improvement. Enabling the 26.5V rail after the PFC voltage is set-up: no improvement.

- reducing the time constant on the COMP by removing C89: that allows the unit to start, but the UCC2808A-1 powers-up 2 times before to get to normal operation.

- increasing C21 to 100uF, this is the capacitor supplying the 15V regulator of the UCC2808A-1: that seems fixing the problem, the UCC2808A-1 gets into normal operation after the 1st power-up.

I think the best way to maintain VCC during the power-up sequence is probably to avoid the duty cycle becomes too low due to a too high level of the 35.8V rail.

Do you have any suggestion?

How can I change the time constant on the COMP pin of U12? 

Hello,

I did some empirical testing to evaluate how the compensation network of the UCC2808A-1 could help fixing my power-up issue when running dry.
It seems using ~8k2 for R130 and keeping 470nF for C68 and C89 fixes the problem. My plan is to get the bode plots covering the load range [1A - 8A] to check if if gain and phase margin are still acceptable. If the compensation needs modifying, I think limiting the changes to the TL431 compensation network if possible.

Do you have a better method for changing the compensation network of the UCC2808A-1?

Is there a difference between the UCC3808-1 and the UCC2808A-1 which could explain why the effect of the compensation seems different?

Below are the scope captures I did:
CH1: 35.8V rail, CH2: VCC U12, CH3: "output com +"
R130 = 1K2                                                                               R130 = 4K7
  

R130 = 8K2                                                                                R130 = 11K (noisy under load duty cycle probably unstable)
    

Any advice welcome.

Note: I am now on vacation and will restart on this problem from the 26/09/22. 

Hello,

The only change on the variant using the UCC2808A is Q40 is not fitted as not required anymore to discharge of C69. The signal on CS remains generated by R135, R138, C69, the amplitude of the ramp on CS is approximately [0 - 400mV]. On the variant using a J177 for Q40, the amplitude of the ramp is in the same range for samples operating normally but can be a lot higher [150mV - 600mV] on samples getting problems. This seems due to variation in the cut-off voltage of the J177, that is the reason why I though using the UCC2808A with an internal discharge on CS was a safer option. The problem is that some units have instability at power-up with no load. I did not find in the datasheet differences between the UCC2808A and the UCC3808 which could explain that. 

changes from UCC3808-1 to UCC2808A-1.pdf

Let me know if that is more understandable in the attached pdf file: left original (UCC3808-1, Q40), right modified (UCC2808A-1, Q40 not fitted). 

Hi,

I will review your circuit differences and will get back to you in a day or two.

Hi,

I had a quick review, and would need more review. But it looks Q40 is removed. To find out if the issue is due to the difference with and without Q40, can you please put Q40 back and everything back to the previous, then test to find if the circuit can work like the before? This will help to find a clue how to make debugging.

While you do such a check, I will continue reviewing your circuit. If I have new suggestions I will send to you. But you can start this check to put Q40 back and keep the same as before with UCC3808 to see if your circuit can work without the issue.

Hello,

That is difficult for me to perform the test you requested. The unit which has problem is an unit which already had problem with the default fit (Q40 J177, U12 UCC3808-1). 

The initial reason for which changes were performed on these PSU is a lot of them failed to start with the default fit when manufacturing them with J177 from Calogic instead of Fairchild and U12 UCC3808-1. The identified cause was the ramp on CS being out of range: not starting from 0V and getting greater than 500mV. This seemed to be due to a low cut-off voltage of the J177. Note: the cut-off voltage of the J177 was still in range.

The first change which has been implemented was to slow down the raising of the ramp on CS by increasing R135 and R138 to 220K: this solved the issue on most of the faulty units but some of them were still faulty.

The 2nd change was to use an UCC2808A-1 for U12, removing Q40 and keeping 180K for R135 and R138: this solved the problem on most of the remaining faulty units but one unit was still not starting without load.  Increasing R130 on the compensation network of U12 allowed it to start. Note: R130 is 4K7 on the schematic but the actual default value is 2K2, the faulty unit did start properly if using higher values: 4K7, 8K2, 11K. 

Attached is a test I did which is the closest to what you requested: I had 2 units a 2022 unit, sample 65 with the new fit UCC2808A-1 without Q40 which failed to start and an healthy 2013 unit fitted with the UCC3808-1 and a J177 Fairchild. I tried swapping the switching options and it appears the problem follows the UCC2808A-1.UCC3808-1 J177 vs UCC2808A-1.pdf

Hi,

Did you try ucc2808 and J177 in the same board of ucc3808 and J177? What is the test result?

Hi,

I had a full review of this thread. I noticed:

"increasing C21 to 100uF, this is the capacitor supplying the 15V regulator of the UCC2808A-1: that seems fixing the problem, the UCC2808A-1 gets into normal operation after the 1st power-up."

It looks your previous C21 is 10uF. While you changed it to 100uF, the startup became ok. I suspect two things. One is your current used 10uF for UCC2808 has larger leakage current than the capacitor used for UCC3808. Second is UCC2808 VDD startup current is at higher end while UCC3808 at lower end based on the specs. So you may need to increase your capacitance for VDD to store energy to deal with them. You may try a new UCC2808 to see if 10uF can make startup ok if by a chance this new IC can have smaller startup current. Is it possible you can change C21 to 22uF, or 33uF to see if the startup can be ok?  

Thanks for your reply, unfortunately, I am not able to conduct tests on these units at the moment (hopefully by the end of the week).

Could you confirm the UCC2808A-1 has a larger leakage current than the UCC3808-1?

Concerning your last reply discussing the value of C21:

All units fitted with the UCC2808A-1 were units which did not start-up with the default fit: UCC3808-1 and the J177.
They did not start either with the initial mod slowing down the ramp on CS which fixed the problem on most of other faulty units (R135, 138 220K instead of 180K).

I have tried different values for C21, 22uF, 44uF and 100uF. Even if the start-up was satisfactory with 100uF, I was not happy with this option as to me that is just hiding a problem: the feedback on the 35.8V starts to operate when the voltage is well above the set point resulting to an ON/OFF phenomenon on the duty cycle. As there is no load on the 35.8V rail, the time required to get back to the set point is slightly long which causes VCC of the UCC2808A-1 to drop below its operating voltage. It seems that increasing the resistor on the compensation network of the UCC2808A-1 results in less abrupt changes of the duty cycle allowing VCC to remain in the operating range during all the time required for the 35.8V rail to drop back to the set point (R130 increased from 2K2 to 4K7 the value on the schematic is wrong). That is where I am at the moment. My initial plan was to increase R130 to fix the power-up issue and to check if gain and phase margins are still acceptable for load in the range [1A - 8A].

Hello,

I have recorded the bode plots on an healthy unit using the following variants:
1) Default fit: U12 UCC3808-1, Q40 J177, R130 2K2
2) U12 UCC2808A-1, Q40 not fitted, R130 2K2 (configuration to avoid using J177)
3) U12 UCC2808A-1, Q40 not fitted, R130 5K1 (configuration which allowed a faulty unit to power-up)

The below table contains my results:

From these results:

1) It seems there is no noticeable difference between the UCC3808-1 and UCC2808A-1 when using the same compensation network.

2) The phase margin is not sufficient and increasing R130 from 2K2 to 5K1 brings it even worse.

3) Some changes are probably required in the feedback.
   
Could you give me some guidance on how I could better adjust the feedback?

There are 2 compensations, one local to U12 (R130, C69, C89) and one local to the TL431 (U13).

Could you tell me how I could determine poles and zeroes of the compensation of U12 U12 (R130, C68, C89) ?
I did not find help in the datasheet.

Do you have any method I could use to adjust the compensation networks on U12 and U13? 

Thanks for you help.

Regards

Gilles

Hello,

Thanks for your reply.

I would like to get the power for the feedback LED not from the 26.5V rail, but directly from the 35.8V rail in order to get the feedback active asap.

As the TL431 is specified to 36V I cannot directly swap the 26.5V and the 35.8V. Is there a standard arrangement for using the TL431 with a voltage above the specified range?  

Thanks for your help.

Thanks for your reply.
I have another question, is there a way to make sure the ton on the UCC2808A-1 never goes below below a certain width even if the feedback is full ? 

Thanks for your reply,

1) could you give me an example on how I could make sure ton is never going to be below a certain value?

2) do you have a practical method to set poles and zeroes on the compensators?
You said our product uses 2 type 3. Is that not 2 type 2?

3) I have tried to get the feedback LED supplied from the 35.8V rail, that solves the problem of powering-up when dry but a similar problem happens when changing the load from 8A to 0A. How could I bring the feedback faster and increase the phase margin?

I think our product uses 2 "type 2" compensators in series, one directly connected to the PWM chip and the other connected to the TL431 as in the below sketch:

If I am right, the compensation (s1/E) should be like the below chart (amplitude only):

Is it common to have 2 "type 2" in series ?

I don't have any accurate model of the transfer function which could help to determine what should be the compensation to get the expected gain and phase margin.

What I noticed when I did the bode plots (including the corrector) is the phase margin could be quite small at ~1kHz, 3kHz.
I don't know if I should add a phase boost in this frequency range and if yes, on which compensator.
I am looking at a practical method reducing the number of test to tune up to parameters of the compensators when the  transfer function is unknown.