Is this circuit a flyback circuit? - Page 1
Forum section: electronics / repair

author: Chriss
author_rank: Frequent Contributor
timestamp: August 06, 2020, 03:09:24 pm
content: Hi!I'm not sure what the hack is this circuit for ot the schematic I put as attachment here.Let's describe the circuit I draw based on the pcb I'm repairing:Please don't pay attention to the values, they are incorrect.The Q1 and Q2 are controlled by a uC.The L1 and R3 are representing a solenoid coil or relay, name it as you wish.The D1 is a TVS diode.The D2 is a fast recovery diode.I'm confused with the circuit of Q2 and D2.Are this circuit tries to be a flyback circuit which is controlled through the uC?It looks like that but I'm not sure.I never saw this kind of design.I'm also not really sure what does exactly the D1, C1 and C2 are doing?Some filtering?The D1 is a TVS diode in series with the C1 and C2 which value together is 1uF.Is this a common used design or I am missing something?Thank you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2020, 03:40:48 pm
content: Q2-D2 draws no current as shown. You've probably missed some connections or components.Also D1 only charges the capacitors once. Presumably there's a resistor to discharge that, so it's an RCD clamp snubber. Or you've drawn D1 backwards and its TVS action kicks in instead. Still doesn't make much sense though.Tim

author: Chriss
author_rank: Frequent Contributor
timestamp: August 06, 2020, 04:46:37 pm
content: Thank you and sorry for my dummy post.After you agree with the "nonsense" circuit I rechecked againand solved the "mystery" of the D2 - Q2 circuit.There are more components related to this circuit but on anotherregion of the pcb where I didn't expected to be...But I didn't found anything else about the D1 and C1 - C2.It is just as I draw it.Even no discharge R.Makes no sense at all.Thanks again and sorry, but sometime I need a brain kick...

author: magic
author_rank: Super Contributor
timestamp: August 06, 2020, 08:55:05 pm
content: Maybe one of the "capacitors" is actually a resistor if you look closer?Alternatively, the point between D1 and C1,C2 might be the 12V rail.

author: fzabkar
author_rank: Super Contributor
timestamp: August 06, 2020, 09:33:15 pm
content: Q1, D1, C1/C2, R3, L1 constitute a switchmode boost converter.Q1 switches on, and current flows through L1. When Q1 switches off, the current decays via D1 and charges C1/C2. That's the boosted output voltage. I expect that the uC should have some way of sensing the voltage on C1/C2, probably via a resistive potential divider.

author: Chriss
author_rank: Frequent Contributor
timestamp: August 06, 2020, 09:45:29 pm
content: You are right with the 12v rail.There is a 12v rail what I can't see, but after you give this idea I checked the pinout and I really found the 12v rail.Thanks mate for the idea.In this case the C1 and C2 could be a sort of noise filter or?I tried to measure resistance on the C1 and C2 but no, there is no resistance.For the C2 I'm sure it is a cap, but for the C1 90% sure.

author: fzabkar
author_rank: Super Contributor
timestamp: August 06, 2020, 11:19:57 pm
content: A boost converter steps up the voltage from 12V to something else. The voltage across C1/C2 should be greater than 12V -- that's all that I can tell from your circuit.