I AM HAVING PROBLEM WITH MY UC3845  - Page 1
Forum section: electronics / projects

author: VAIBHAVKASTURE
author_rank: Newbie
timestamp: January 11, 2024, 09:14:55 am
content: MY MOSFET HEATING UP TREMEDOUSLY ,PWM NOT HAPENINIG CORRECTLY ,SPECIALLY FEEDBACK, PLESE TACK ALOOK TELL ME WHATS WRONG WITH

author: Andy Chee
author_rank: Super Contributor
timestamp: January 11, 2024, 10:01:43 am
content: Do you have an oscilloscope? Please post photos of the MOSFET gate waveform.

author: VAIBHAVKASTURE
author_rank: Newbie
timestamp: January 11, 2024, 10:50:26 am
content: MY SUPPLY VTG IS 35V TO 75V DC

author: VAIBHAVKASTURE
author_rank: Newbie
timestamp: January 11, 2024, 11:10:31 am
content: HERE GATE AND CURRENT WAVEFORMS

author: Andy Chee
author_rank: Super Contributor
timestamp: January 11, 2024, 11:12:48 am
content: Please post photos of the MOSFET gate waveform across gate-source.Your problem is likely because of insufficient Vgs.

author: VAIBHAVKASTURE
author_rank: Newbie
timestamp: January 11, 2024, 11:29:24 am
content: I POSTED WAVEFORM PLEASE TACK LOOK AT IT

author: Andy Chee
author_rank: Super Contributor
timestamp: January 11, 2024, 11:41:51 am
content: Please connect the probe like this and show waveform:

author: Andy Chee
author_rank: Super Contributor
timestamp: January 11, 2024, 11:53:45 am
content: Quote from: xavier60 on January 11, 2024, 11:44:59 amYep, you can use the math function CH1(red)-CH2(blue). This method is better due to better noise immunity.

author: VAIBHAVKASTURE
author_rank: Newbie
timestamp: January 12, 2024, 06:03:36 am
content: yellow ch1, red ch1-ch2, blue ch2 gate and source wavform

author: VAIBHAVKASTURE
author_rank: Newbie
timestamp: January 12, 2024, 07:17:06 am
content: i want to know why i am missing some pulses why pwm in not happening

author: VAIBHAVKASTURE
author_rank: Newbie
timestamp: January 12, 2024, 07:18:08 am
content: yes D1 is in ckt

author: VAIBHAVKASTURE
author_rank: Newbie
timestamp: January 12, 2024, 09:08:11 am
content: I HAVE TWO GROUND ONE IC AND OTHER COMMON BETWEEN INPUT AND OUT PUT

author: xavier60
author_rank: Super Contributor
timestamp: January 12, 2024, 10:30:25 pm
content: Quote from: VAIBHAVKASTURE on January 12, 2024, 09:08:11 amWhere is the isense resistor located? It's customary to filter the current sense input pin 3 with 1K and 1nF. Not saying that C5 is the total cause of the regulation loop instability.It's not clear how GND and PGND relate to each other.

author: Victorman222
author_rank: Regular Contributor
timestamp: January 13, 2024, 04:03:41 pm
content: It looks like you are trying to do a buck converter with UC3845. With typical buck configuration where the transistor is in the positive rail, UC3845 output can't turn on the transistor as it is referenced to it's ground and mosfet needs voltage between gate and source to turn on.Looking more closely to your schematic it seems as if the grounds for the ic and power are separate, but connected by things like V FEEDBACK and isense. This looks strange.If you want to do a buck with UC3845, either use a high side driver or do a low side buck (buck with positive common), but there you will need a differential amplifier that can take input of the + rail.Another reason a buck will not work is cause UC3845 is 50% duty cycle max.Check out a schematic of a battery charger i made using the low side buck concept, but it is using UC3843 that has 100% max duty cycle.

author: xavier60
author_rank: Super Contributor
timestamp: January 14, 2024, 01:37:30 am
content: The last waveform from Reply #4 is 40V peak, so the MOSFET is being driven properly by 12V pulses WRT Source.This can only mean that GND is tied to the switching node, but the schematics don't show it.The regulation instability is due to not having any loop compensation which could be tricky. May even need slope compensation.