Replacing a dead flyback transformer - Page 1
Forum section: electronics / repair

author: fma
author_rank: Contributor
timestamp: September 27, 2025, 08:15:29 am
content: Hi!I bought a faulty JVC D-ILA X-35 projector for cheap. After opening it, I found that there have been some disaster on the main Power Supply!This part of the power supply is the 5V standby. There is a PFC as first stage, and even if it is not active in standby mode, this means that the input can be up to 380V (for countries using 230V as main voltage) once the power supply is fully activated.Replacing the DIP7 FET (TNY274) was easy, but I found that the little flyback transformer is deadSo, I need to find a replacement, and this is where I need your help.The chip managing the power supply is an NCP1271. In thisapplication note, they specify a transformer with a turns ratio of 30:6:5 (30 on the primary, 6 on the secondary, and 5 for the auxiliary). Here, the secondary is 19V, and I see 16V for the NCP1271 supply (datasheet says 10-20V).Could you help me calculate the correct transformer ratios for my power supply (5V output)? I admit I don't fully understand the application note... The NCP1271 is quite complex, with several operating modes (burst, PWM, etc.) depending on the load, to limit switching losses in standby mode.The topology seems standard, with a AS431 as feedback. Let me know if you need additionnal informations (components values); will check it.Thanks for your help!Best,Fred

author: fma
author_rank: Contributor
timestamp: September 27, 2025, 08:20:17 am
content: I forgot to give you the flyback transformer ref.: BKEE16-KH100029. I didn't find anything about it, but you may have ideas.BTW, this is the primary coil which failed.I was thinking to cut the transformer, to have a look inside, to help determine the turns ratio, but I don't know how to do that correctly.

author: Sorama
author_rank: Frequent Contributor
timestamp: September 27, 2025, 09:29:30 am
content: Quote from: fma on September 27, 2025, 08:15:29 amYou won’t have 380V DC when pfc is not running.Only with pfc.

author: fma
author_rank: Contributor
timestamp: September 27, 2025, 09:39:34 am
content: Yes, that's what I said: "up to 380V once the power supply is fully activated".I suppose that the main voltages (+12V and -8V) are derivated from 5V (I mean, the NCP1271 regulates the 5V, other voltages are defined relative to the 5V), so the 5V must be stable in all conditions, so the regulation should work fine with and without the PFC on.I don't know what is the allowable tolerance on the turns ratio...

author: Sorama
author_rank: Frequent Contributor
timestamp: September 27, 2025, 10:26:57 am
content: Quote from: fma on September 27, 2025, 09:39:34 amThat is not what understood from :‘ even if it is not active in standby mode, this means that the input can be up to 380V’Anyway, not important now.Are you sure there are no secondary windings for 12V and other voltages?There is no advantage having a standby PS when other voltages are derived from the 5V itself.Are you sure the transformer is dead?That happens rarely.

author: fma
author_rank: Contributor
timestamp: September 27, 2025, 10:51:45 am
content: Quote from: Sorama on September 27, 2025, 10:26:57 amI'm french, sorry for my bad englishQuoteThere is another bigger flyback transformer. I didn't checked in detail, but I guess there is something to activate it. But as there is only one controller chip, I assume it only regulates the 5V.QuoteYes, primary winding is open; other windings are also strange: auxiliary is seen as a capacitor, secondary as a very low resistor (maybe ok for this one, if there are so little turns, it is not detected as an inductance).QuoteThat's what everyone says! I suspect it didn't happen at the first place, but when someone tried to repair it, making a nice short somewhere.I tried to find a flyback transformer matching this one on Wurth site, but I can't find anything close enough. I also read on this forum thatPremier Magneticsalso makes flyback transformers, but again, I don't know what to choose!

author: Sorama
author_rank: Frequent Contributor
timestamp: September 27, 2025, 11:01:21 am
content: Did you take the transformer out prior to measure it?I still can’t believe the transformer is gone.Some regulating ic do pfc and switching too.What happens it you put 5V from a lab power supply to the switching ic ?(Or whatever ic that needs it)

author: fma
author_rank: Contributor
timestamp: September 27, 2025, 11:31:22 am
content: Yes, I removed the flyback to measure it...I can try to inject 5V, but as far as I understand, the NCP1271 won't start without the auxiliary winding, which provide voltage on Vcc (pin 6); it needs 10V-20V.The PFC is handled by a NCP1653A...

author: fma
author_rank: Contributor
timestamp: September 28, 2025, 02:45:32 pm
content: Ok, I finally unwind the transformer, to count the turns...First, as I thought, a part of the primary coil has literaly been vaporized!Here are the datas:- primary : Ø0,25mm, 121 turns (60 below secondary, 61 above the secondary)- secondary : Ø0,70mm, 11 turns- auxiliary : Ø0,25mm, 33 turnsSo, the question is now: where can I find an equivalent?

author: Slh
author_rank: Regular Contributor
timestamp: September 28, 2025, 04:18:04 pm
content: Did you measure the inductance of any of the coils before you unwound it? Or can you measure the air gap in the cores? It's a custom part.I doubt you'll be lucky enough to find one the same size off the shelf but you could always rewind it (carefully paying close attention to the primary/secondary tape).

author: fma
author_rank: Contributor
timestamp: September 28, 2025, 04:42:05 pm
content: What is the air gap?I don't plan to rewind it myself, this is too tricky from a safety point of view.The primary coil is splitted around the secondary, there are some shieldings between the coils...Even if I can't find the exact same footprint, it is not too complicated to re-route the pins outside: there is plenty of space.

author: Slh
author_rank: Regular Contributor
timestamp: September 28, 2025, 07:36:14 pm
content: The transformer in a flyback stores energy in the first half of the cycle and releases it in the second half. To prevent the core from saturating there's a gap between the two core halves which is called the air gap. Thinking about it you probably don't need to get this.If you still have the cores and they're not damaged you could put 11 turns on the core and measure the inductance of the secondary. Then the auxiliary is √3 higher and the primary is √11 higher. With all that and the value of the current sense resistor, it should be possible to figure out a rough specification.

author: fma
author_rank: Contributor
timestamp: September 28, 2025, 08:36:38 pm
content: Ok, I see. But I need to wind up half of the primary, as the secondary is winded on this half, and inductance depends on the diameter.

author: Slh
author_rank: Regular Contributor
timestamp: September 29, 2025, 06:28:07 am
content: No need to put the primary on it. It's technically true that the diameter makes a difference but the core reluctance is so low relative to air that you'll be close enough without the extra distance (within 1%)

author: fma
author_rank: Contributor
timestamp: September 29, 2025, 08:05:12 am
content: Ok, I measured 0.95µH@100kHz. I think the chip runs at 65kHz, but the LCR-meter at work does not allow this frequency. Let's say 1µH.

author: fma
author_rank: Contributor
timestamp: September 29, 2025, 05:34:01 pm
content: BTW, Rcs is 0,22Ω. Which means a max current of almost 5A!I started to dig at Yageo, but I can't find anything matching.

author: fma
author_rank: Contributor
timestamp: September 30, 2025, 04:53:18 am
content: Quote from: fma on September 29, 2025, 05:34:01 pmI checked again, and the current sens resistor is not measuring the 5V standby flyback transformer current, but the 12V one, which, I think, is not activated in stand by...

author: fma
author_rank: Contributor
timestamp: October 04, 2025, 02:01:55 pm
content: I have some news: I was totally wrong about how this power supply works!In fact, the 5V standby is not managed by the NCP1271, but totally autonomous, with the AS431 → opto → TNY274 doing an analog regulation.So, I will investigate a little more, and see if I can use another little power supply to inject this 5V standby (and probably 15V from the auxiliary winding, as I didn't yet understand what it does).BTW, I found this short thread:https://www.eevblog.com/forum/repair/jvc-x30-power-supply-sumida-iv95260-repair-help/msg3365448/#msg3365448where vk did some reverse engineering on the schematics (main voltage part). This will help.