DC to DC boost controller LM3478 behaviour - Page 1
Forum section: electronics / beginners

author: higginsdj
author_rank: Contributor
timestamp: May 07, 2025, 01:14:44 am
content: I am trying to fix an oldish electronic target LCD monitor. The monitor is powered from a 12v Battery (so 12-14v DC). There is no schematic available. The board generates 3 supplies - 3.3v, 5v and 35v (for the LCD backlight). The 35v is generated via an LM3478 boost controller and a Mosfet. The Mosfet is not short but was cooking (120 deg C using a thermal camera) before it got to 5v on the supply limited at 500mA (circuit fused at 1.1A).I removed the Mosfet and tested it - no short and in a component tester still registered as an N Channel Mosfet. With the Mosfet removed, the board came on at full voltage and 80mA and the 3.3v and 5v supplies worked. I also tested the drive pin on the LM3478 and it showed no pulses but was always high (7v). I assume that means the Mosfet was always on and this is why it cooked.I am assuming that the drive pin should have been producing pulses even with the Mosfet removed so it has failed - would that be a correct assumption? I have a new LM3478 and Mosfet on order. I assume the Mosfet needs replacing since it would have easily exceeded the 150 degrees C junction temp limit even though it tested as a Mosfet.CheersDavid

author: julian1
author_rank: Frequent Contributor
timestamp: May 07, 2025, 04:18:13 am
content: A useful check could be the current sense resistor, that should be found somewhere close by the fet.The lm3478 is a current mode controller, so it turns the fet on with the internal oscillator clk, and off based on the current sensed at the resistor, as it ramps through the inductor.During normal operation, the off set-point will depend on the loop regulation.There is also a hard override turn-off at 324mV according to the datasheet - to handle the output going short circuit, at startup/ or in a fault condition.It seems more likely that it is the lm3478 or fet would have failed.But if replacing these doesn't fix it, perhaps check the sense-resistor is OK, and that the dampening cap across it isn't shorted.Otherwise the controller wont get the necessary feedback signal for fet turn-off.

author: higginsdj
author_rank: Contributor
timestamp: May 07, 2025, 06:23:38 am
content: I've checked all resistors, caps, diodes and coils in the small 35v circuit and they all appear fine (the reason I was asking previously about SMD Resistor values).

author: mikerj
author_rank: Super Contributor
timestamp: May 07, 2025, 01:34:29 pm
content: With the MOSFET removed it's unlikely the LM3478 would be working as intended since it wouldn't be getting any current feedback. Was the MOSFET still getting hot with the backlight disconnected from the output of the boost converter?

author: higginsdj
author_rank: Contributor
timestamp: May 08, 2025, 01:12:30 am
content: Quote from: mikerj on May 07, 2025, 01:34:29 pmYes

author: higginsdj
author_rank: Contributor
timestamp: May 08, 2025, 01:16:32 am
content: Here's a schematic I drew from what I could see/probe on the board if it helps. (Note C5 is a 1nF not 100uf cap)

author: higginsdj
author_rank: Contributor
timestamp: May 09, 2025, 12:42:42 am
content: OK so I replaced the mosfet and the controller but I still have the same behaviour.B U TI gave the supply voltage unlimited current and now all is well - no overheating et al and the LCD screen now comes on. Perhaps I should have done a similar test BEFORE I started replacing parts......