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Increasing flyback output power

Posted by: wystewart on

I have an insteon hub that uses a lnk306 in an isolated flyback. I've changed the internal computer board to a raspberry pi 2 which needs more power than it seems the flyback can provide. It does seem to work when the cover is off. When I put the cover on, it does seem to trip off for brief period causing the raspberry pi to reboot. I presume it is overheating or hitting current limit. I've determined the schematic, attached, the problem is that I don't know anything about the transformer to use pi expert to check the design. Anything that I could measure to determine if the transformer is the problem? I do notice that the clamp circuit gets very hot. In fact even with the normal designed load the PCB is discolored at the resistors. They are 0603 so I am pretty sure they are under sized. So if there is a away to make this work with the new load I would change the clamp resistors to 1206 and also the input resistor from 270 ohms to 8.2 per the app notes. Any suggestions to make this work reliably? Would swapping out the lnk306 to lnk354 or lnk364 help? As these operated at a higher frequency and have the same pinout. It is not really practical to make significant changes to the design. Thanks for your help.

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Schematic 144.93 KB

Comments

Submitted by PI- MarcusJoaquin on 04/14/2021

Hi Wystewart,

I review the schematic the outputs are 5V/10mA and 21.5V/120mA.
What version of Raspberry Pi you are using?
Correct me if I'm wrong supply for Raspberry Pi is 5V with 4.63V as the lowest.
With a lower Raspberry Pi version, the typical minimum bare-board current consumption is 100mA. Suggested PSU current capacity is 1.2A.

I suggest to check first the raspberry pi using a DC source instead of the flyback power supply design with LNK306. Monitor the initial current drawn and the normal operation.

Also check the LNK306 flyback power supply, disconnect the output from any load and connect the output to e-load and see what is the maximum output load capability.

Thanks!

Submitted by wystewart on 04/14/2021

Its a raspberry pi 2b and draws .355A nominally and about 0.542A peakfrom 5v. The 0.12A and 0.15A peak shown on the schematic on the 21.5v are the actual measured numbers. There a buck dcdc on the board that converts from 21.5v to 5v. The 5v supply shown on the schematic only powers an opto coupler that is for zero crossing detection of the ac input.

I don't think the problem is the max load capability it easily powers the raspberry pi open frame but it does get hot according to thermal camera. Its when I put the cover on that there's an issue. So I suspect temperature. I have no info on the transformer so it is possible it is saturating. Hoping that changing to the lnk354 or 364 might help with the higher switching frequency but it is unclear from the datasheets what differentiates the 306, 354 and 364 in terms of output power capability. Its not clear me why the datashhet says the 364 has a higher output power capability to the 354. The 306 is difficult to compare since the datasheet only gives output current for a non-isolated buck.

Submitted by PI- MarcusJoaquin on 04/14/2021

What type of core is used, primary inductance, number of primary and secondary turns?

Submitted by wystewart on 04/15/2021

Thats the problem I didn't design the transformer, its a insteon hub I purchased to modify. Its a custom transformer with no markings on the core. It has yt-23799 marked on it but that does seem to be an off the shelf transformer part number. Because of this I can't really use pi expert to analyze the design.

Any sort of measurements I can take in circuit to assess how the transformer is doing?

Submitted by PI- MarcusJoaquin on 04/15/2021

1. What is your input voltage? Try both input line 120Vac and 265Vac and see if this occur on both lines.
1. Do the thermal scan without the cover. All areas must be thermal scanned to see if there are components that are already heating up without cover in particular the transformer.
2. If you can measure the primary current that would be better while running it with raspi 2 as output load. Measure the actual load at the output of the power supply when raspi 2 is connected. While running without cover you can use a heat gun and heats-up the transformer gradually while monitoring the primary current of the transformer.
3. When the power supply is covered how long before the power supply shutdown? Does it return to operation or recover after the shutdown and the power supply cools off? You can also put a thermocouple on the transformer, identify the hot spot on the transformer usually at the winding. Also put thermocouple on other critical parts such as the IC and output diode. Let the unit covered the powered up and see if there's a temperature rise at the transformer.

Possibly transformer saturates as the load increase and it already trigger the peak current limit of the IC. Once peak current limit of the IC is triggered the primary MOSFET will be turned off.

If you look at DER-58, though it uses LNK304 a lower current rating versus LNK306 it can actually deliver 3W.

LNK354 and LNK364 are also a good replacement but transformer will be needed to redesign. Both are capable for higher power.

Submitted by wystewart on 04/15/2021

Great thanks I'll try to take some temperature measurements. Regarding 3, basically with the cover on the recovery is immediate, the raspberry pi just reboots. I would think if it was thermal shutdown it would be off longer since it has to cool to 75c. So maybe it is over current. I have a different model insteon hub also with exactly the same transformer and same schematic configuration but it instead uses the lnk354 and the input series resistor is 27 ohms instead. So would it be worth trying the lnk354 instead of the lnk306?

Submitted by PI- MarcusJoaquin on 04/15/2021

Yes, it is worth trying LNK354.
Maybe test the power supply separately with the output connected to e-load and gradually increase the load up to the desired level or equivalent to the max load requirement of Raspi 2.
Monitor the current at the primary and test at low and high line.

Thanks!