Probing a working SMPS based on LD7750A PWM IC - Page 1
Forum section: electronics / beginners

author: dav
author_rank: Regular Contributor
timestamp: February 14, 2017, 04:54:55 pm
content: Hi,I'm playing with this little 20V SMPS in order to better understand its operation.I took these measures with no load:V on bridge rectifier: 300Von PWM IC:Vcc: 20VV on HV pin: 157VV on COMP (feedback) pin: 1.67VV on OTP (over temperature protection) pin: 5.67Vand I can see pulses on the mosfet gate.Then I gave external DC power to the PWM IC (obviously no AC power and no load)Vcc: 20VV on HV pin: 0 (obvious)V on COMP (feedback) pin: 0V on OTP (over temperature protection) pin: 5.00Vbut I cannot see pulses on the mosfet gate.Why?I also tried to give 20V on HV pin too, but nothing changed.Maybe does the PWM IC expect a high voltage on HV pin, in order to boot?

author: dav
author_rank: Regular Contributor
timestamp: February 16, 2017, 07:04:32 pm
content: any help would be appreciated

author: dave_k
author_rank: Frequent Contributor
timestamp: February 16, 2017, 08:06:08 pm
content: Read the datasheet

author: xavier60
author_rank: Super Contributor
timestamp: February 17, 2017, 09:58:44 am
content: I would think that it is making one start attempt then OLP trips because the Comp pin goes too high. The VCC needs to be allowed to cycle to reset the OLP.

author: dav
author_rank: Regular Contributor
timestamp: February 17, 2017, 08:33:39 pm
content: So is this what's happened?No load -> no feedback (COMP pin open) -> VCOMP=6.0V (from datasheet) -> VCOMP exceeds OLP threshold (5.0V) for over 64mS -> Over Load Protection -> Gate output off and VCOMP forced to 0V.Did I well understand?What about if I put a resistor in parallel to the optocoupler output, so to drain some current from the COMP pin and force the VCOMP voltage to stay within the 5.0V?

author: xavier60
author_rank: Super Contributor
timestamp: February 17, 2017, 09:12:19 pm
content: Yes, that is what I am suspecting is happening. You should be seeing the 64ms burst of drive signal. Try loading the COMP pin.

author: dav
author_rank: Regular Contributor
timestamp: February 18, 2017, 01:43:34 pm
content: Here you are the pulsesCh1 is the VCOMP signalCh3 is the gate signalSo Ch1 goes high (6V) for 66mS and then both Gate output and VCOMP go OFF.I don't understand why adding a 220R in parallel to the optocoupler output, I still don't have pulses on gate(with 220R, neither the above boot procedure! nothing! ZERO!)VCOMP starts to 0V and so rests.

author: xavier60
author_rank: Super Contributor
timestamp: February 18, 2017, 07:26:03 pm
content: Page 11 says that the gate drive is turned off when COMP is below 2V. Like many control ICs, it has burst mode which the data sheet doesn't say much about. If COMP drops below 2V under very light load conditions, it then usually has to be allowed to rise to some higher voltage to cause gate drive to start again.

author: dav
author_rank: Regular Contributor
timestamp: February 19, 2017, 08:13:11 am
content: Yes, I read about the 2.0V threshold (ON/OFF control).What I don't understand is the boot procedure.I deduced this (based on my test):Vcc (20V) on PWM controller -> VCOMP high (value depends on the feedback)If no feedback -> VCOMP=6V (more than 64mS) -> Over load protectionI expected this too:Vcc (20V) on PWM controller -> VCOMP high (value depends on the feedback)220R feedback -> VCOMP something different to zero (or, at least, something different to zero for a very short time, then zero)This is what happens:Vcc (20V) on PWM controller -> VCOMP high (value depends on the feedback)220R feedback -> VCOMP =0 -> ON/OFF control -> gate output disabledLet's try with a higher resistor value on the feedback pin

author: xavier60
author_rank: Super Contributor
timestamp: February 19, 2017, 08:25:21 am
content: I estimate that COMP will be about 0.3V with the 220 ohm resistor. You want COMP to be a higher voltage. Use a higher value resistor.

author: dav
author_rank: Regular Contributor
timestamp: February 21, 2017, 02:59:34 pm
content: So here some of my measurements:R VCOMP GATE OUTPUT fGATE<10k <2.00V NO10k 2.00V NO11k 2.14V YES 22.0k12k 2.28V YES 22.1k15k 2.63V YES 35.5k20k 3.15V YES 64.8k30k 3.89V YES 64.9k40k 4.42V YES 65.0k