coulomb
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Re: Troubleshooting and repair for On-board Charger (OBC), DC-DC Converter

Sat Sep 12, 2020 10:19 pm

I find it quite odd that the feedback is applied to pin 2, and not pin 1. I mean, what is the error amplifier doing now? It's not connected to anything, except for its own feedback R and C. Maybe "FB" on pin 2 is deliberately ambiguous; it may refer to feedback for the error amplifier itself, or feedback for the whole PWM control system, if you happen to want the feedback to work in the opposite direction.

In the example schematic from the manufacturer, feedback is applied to pin 1, where too high an output will pull pin 2 low.

In the OBC circuit, too high an output will turn on the TL431, which will turn on the opto, which will pull pin 2 low directly, regardless of what happens with pin 1 and the error amplifier. The datasheet doesn't seem to suggest this as normal operation. If the error amplifier output isn't designed to be shorted low (e.g. open drain with a pull-up resistor), then it seems to me that they are using this part outside its design parameters.

I'm also wondering if I got the purpose of pin 6, "CS", wrong. Perhaps it's part of the "soft start" circuit; a time constant of around 500 ms makes sense for that too. The "C" would be for Capacitor, and the "S" for Soft Start. It's so hard to follow the internal diagram when you don't know whether inputs to a box inhibit with positive or inhibit with negative. Similarly, the UVLO (under-voltage lock-out) block outputs: are then alternatively positive? If so, which one is positive normally? Often you can guess, but it's possible to make a plausible theory that is completely wrong.

Edit: since this is the start of a new page, I'll duplicate the reference schematic from the datasheet here:

Image

zetafunction
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Re: Troubleshooting and repair for On-board Charger (OBC), DC-DC Converter

Sun Sep 13, 2020 5:14 am

Thank you so much for your effort and great details about IC702! I will try out your suggestions today and add further information to this post.

One thing that is strange is that shorting pin 1 and pin 2 with a 4.7k resistor actually makes IC702 switching again. Something has to go wrong when powering the circuit on. I just tried using another power supply and powering up both pins at different times but it seems like my test setup is right.

After powering the boards off and on again the same issue accurs but after shorting pin 1 and pin 2 with a resistor it starts working again. This is completely repeatable and I tried it for many times now.

kiev
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Re: Troubleshooting and repair for On-board Charger (OBC), DC-DC Converter

Sun Sep 13, 2020 5:20 am

@ coulomb
That is a very nebulous chip. i had the same thoughts that maybe it wasn't being used properly, but couldn't find any application notes or user guide for their regulator chips.

At first i was thinking FB was feedback, but now i'm thinking it means "Fly Back" and somehow provides information from the pulsating voltage on the secondary side.

They seem to show an input bias on the PWM, SCP and LO (lock out, or latch output?), but don't show a value--are we to assume it is the same 0.52V as used on the error amp?

@ zeta
Thanks for the update on the pin1-2 shorting test. How did you determine to use 4.7k--did higher values not work, or was that all you had handy?

Now i'm wondering if it will start if you just short pin 2 to pin 3, or pin 2 to GND (somewhere convenient to touch)?

@ all
Is it the voltage applied to pin 1 that causes the start, or the pulling down of pin 2?
[edit] found the connection of pin 1 to pin 3 later.
Last edited by kiev on Sun Sep 13, 2020 10:21 am, edited 2 times in total.
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coulomb
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Re: Troubleshooting and repair for On-board Charger (OBC), DC-DC Converter

Sun Sep 13, 2020 5:46 am

kiev wrote: That is a very nebulous chip.

Indeed! And it's in a really weird circuit.

I've just realised that with the large C and small R (relative to the reference circuit) in series, there is nothing connected to pin 1 in the steady state. It's free to float anywhere! I guess there is a high value pull-down resistor to ground.

Or is there perhaps some other connection to pin 1 (perhaps through vias under the chip) that haven't been discovered yet? I'm suspicious about the opto second from the short right-angle in the slot bridged by many optos on the top side. [ Edit: Oops! I've been sucked in again by those test points that look like vias with solder in them. But of course, there is no reason for vias to have any solder in them. But I still feel that there could be some other connection to pin 1. ]

Is it the voltage applied to pin 1 that causes the start, or the pulling down of pin 2?

I think it is the voltage applied to pin 1 that causes the start. But all that does is pull pin 2 low, so pulling pin 2 low some other way would do the same thing, it seems to me.

@zetafunction, when you connect pins 1 and 2 via a 4k7 resistor, does the whole circuit work? In other words, do you then see reasonable voltages at the transformer outputs after rectifiers? I guess it would be unregulated, so best not to do this for too long.

Does it keep going after you take away the resistor? I can imagine that with the output going above and below the reference voltage, pin 1 would get pulses that might keep it running. I wonder if a 1 MΩ resistor (as in the reference circuit) would be enough? It seems crazy not to have any DC bias to the error amplifier input.

zetafunction
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Re: Troubleshooting and repair for On-board Charger (OBC), DC-DC Converter

Sun Sep 13, 2020 6:49 am

The 4.7k resistor works but something like 10k seems to be to high. So the 4.7k resistor is the highest value this reliably is working. When I short pin 1 and 2 absolutely nothing happens, the board starts working after removing the resistor and stays in this state.

At the time the board was magically working again, I could turn off the power and after turning the power back on it was still working. Accidentely touching the chip does not explain this behavior, so the board could really fix it self back then.

@kiev
Are pin 1 and 3 not both connected to GND? I have a resistance of 0.34 Ohm between these pins. Shorting pins 2 and 3 also results in the same behavior.

@coulomb
I am pretty sure the board is working. The voltages are looking fine to me and the current measurements match a known working board exactly.

kiev
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Re: Troubleshooting and repair for On-board Charger (OBC), DC-DC Converter

Sun Sep 13, 2020 7:02 am

did it start when pin 2 was shorted or pulled low using a resistor to ground? Or only started when the resistor connection was made from pin1 to 2?

[edit] i just measured my board and pin1 appears connected directly to pin 3 ground, i measured .1-.2 Ohms. Sorry i never checked for this before, i never expected that an INPUT line would be shorted to ground.
Last edited by kiev on Sun Sep 13, 2020 7:16 am, edited 1 time in total.
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zetafunction
Posts: 35
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Re: Troubleshooting and repair for On-board Charger (OBC), DC-DC Converter

Sun Sep 13, 2020 7:14 am

I get 0.016 Ohms between pin 1 and 3 (4 wire measurement) and I think I even traced it out a few days ago, they are both connected to GND. So I can only connect the resistor between pin 2 and GND and it starts after removing the resistor.

I also just replaced C844 with a new one from a good brand, but it did not affect the problem at all.

kiev
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Re: Troubleshooting and repair for On-board Charger (OBC), DC-DC Converter

Sun Sep 13, 2020 7:20 am

so it still has the issue of not starting until you pull down pin2? It won't start up without help after power down?

What do FB and CS look like when it's running?
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zetafunction
Posts: 35
Joined: Fri Jul 31, 2020 4:26 pm

Re: Troubleshooting and repair for On-board Charger (OBC), DC-DC Converter

Sun Sep 13, 2020 7:47 am

C843 and C844 are now both new but the problem is still the same.

Feedback when it is running:

Image
Image

CS is at 1V but got some noise on it, I tried to capture it with AC coupling:

Image

kiev
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Re: Troubleshooting and repair for On-board Charger (OBC), DC-DC Converter

Sun Sep 13, 2020 8:03 am

So FB is running at around 0.54 V or a little bit higher than the Ref. of 0.52 V. i see a 75kHz pattern to the signal, likely that is the oscillator/PWM frequency set by the RC on pin 7; then the ringing is due to the recovery overshoot after the opto triggers. The first edge ringing is ~ 8MHz, the second larger one is ~ 17 MHz.

And CS is operating at the setting of the voltage divider. The ringing ~18MHz.

What could be causing the latch up to prevent starting? The TL431 and optocoupler would seem to be okay for it to operate.

something may be shorting or pulling down the supply during startup and causes either LO or SCP to latch, then it won't start normally, then with a little help, it runs?

[edit] i can see the trace under the chip from 1-3 with a strong light and magnifier.
Last edited by kiev on Sun Sep 13, 2020 8:50 am, edited 1 time in total.
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