Power Supply Not Working - Page 1
Forum section: electronics / beginners

author: lombster
author_rank: Newbie
timestamp: May 13, 2025, 10:37:42 pm
content: Hello,I'm a student designing a project. I have a PCB that I've designed, however, the power supply for it does not work. Below is a quick explanation of how it works and then the problem I am facing with it. I've also attached an annotated circuit diagram.The power supply uses the TEA2208T and 4 mosfets for active rectification of the 120VAC.From there the input of that is used by the lt3798 which does power factor correction and also modulates the flyback transformer in order to generate roughly 15vdc.The main issue I am facing is that the lt3798 is not seeing any voltage at its Vin so it's not powering on. It uses high voltage to initially power itself and then "switches" over to the auxilarly transformer winding once that gets going. The high voltage goes through two resistors. For some reason I see voltage going in at the top of the resistor chain, I see half of that voltage in the middle of the resistors, but at the end of the chain I see nothing.The other issue is minor in comparison and it involves the TEA2208T. For some reason the rectifier outputs 108VDC even though it's being fed 120VAC. I adjusted the EN/UVLO of the lt3798 to compensate, but the lt3798 still doesn't turn on.Thanks

author: WattsThat
author_rank: Frequent Contributor
timestamp: May 13, 2025, 11:37:49 pm
content: I cannot claim to understand such a complex supply but can only ask the obvious: What voltage do you expect to see at the anode of D3? There is only 500 uA being supplied through the 200K. What are the supply requirements of the LT3798?

author: lombster
author_rank: Newbie
timestamp: May 14, 2025, 12:23:53 am
content: At D3, I expect to see the output from the rectifier which is at the moment 108VDC. As for the supply requirements for the LT3798 it looks like it's about 70uA at startup.

author: Paul Price
author_rank: Super Contributor
timestamp: May 14, 2025, 02:23:14 am
content: The LT3798 is blown(very likely shorted) if voltage meas here is 0 at Vin pin.Power off, check resistance from LT3798 Vin to Gnd for a low resistance(<100 ohm).Otherwise:Check Diode D2 for short.Check Resistor R22 for open.Check C8 for capacitor value, high ESR, and for short.

author: lombster
author_rank: Newbie
timestamp: May 14, 2025, 04:20:28 am
content: Resistance is 25 ohms b/w Vin and Gnd.Diode has no short when taken off, but is shorted in circuit (that's normal???)Resistor R22 is the correct resistance and is fine.C8 is the right capacitance value and the esr is 2.75 ohm (Finrsi tweezer LCR). During continuity test it does not stop beeping, however.

author: Paul Price
author_rank: Super Contributor
timestamp: May 14, 2025, 07:43:29 am
content: If you measure 25-ohms to ground on Vin, you've aced the LT3798.Replace C8, too much ESR, probable cause of LT3798 demise.Check R23, R29 for increase in resistance out of value.If you meas D2 short when soldered back, you've got a shorted LT3798, but you still should see >20-ohms.

author: PGPG
author_rank: Frequent Contributor
timestamp: May 14, 2025, 10:53:14 am
content: Quote from: lombster on May 13, 2025, 10:37:42 pmI live in 230VAC world. I have never heard of active rectifying it. Do really 2x0.7V lose is so important that in 120VAC world active rectification is needed?In my opinion a little too long transistor being in conducting state and you get your rectified voltage being stolen back to AC supply line.

author: Paul Price
author_rank: Super Contributor
timestamp: May 14, 2025, 11:27:30 am
content: The mfg of the chip states it can yield up to 1.4% savings in power use!One transistor being on a little too long results in a loud pop and a wisp of smoke.descriptionThe TEA2208T is the first product of a new generation of active bridge rectifier controllersreplacing the traditional diode bridge.Using the TEA2208T with low-ohmic high-voltage external MOSFETs significantlyimproves the efficiency of the power converter as the typical rectifier diode-forwardconduction losses are eliminated.Efficiency can improve up to about 1.4 % at 90 V (AC)mains voltage.