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    A question about PWM IC's

    Hi all.

    Do PWM chips usually start triggering with only the +ve and -ve connected or do they need other signals before they start working?

    I am particularly asking this for the following chip

    https://cdn.badcaps-static.com/pdfs/...2d6a23c015.pdf

    Is the VDD needed or should the Vin and Gnd be enough?
    Attached Files

    #2
    Re: A question about PWM IC's

    It usually needs the other signals like CS and feedback, otherwise it will likely shut down.

    Why do you ask...

    Comment


      #3
      Re: A question about PWM IC's

      On page 3 of your attached pfd files it explains what each pin is used for. Then if you go to page 4 it gives you an internal block diagram of the chip. Inside the block diagram are abbreviations, for each part of the block. On page 5 is the electrical characteristics of the block with the abbreviations and definitions. There is a big "and" gate in the middle which means all signals have to be high in order to get a high out otherwise they are low. The output of the "and" gate goes to the "reset pin" of a flip flop. The other pin of the flip flop is the "set pin" that is connected to the internal oscillator. A flip flop circuit does just that it flips the output Q and Qnot high and low. So, whatever state is high on Q or Qnot it goes low when the reset pin is pulsed. The data sheet has a simple clear to set on these pins which means this is mono state flip flop so the Qnot is high with a high going pulse on set. Then when the reset is triggered most likely a high going pulse then Qnot goes low. Anyway, the data sheet can be broken down for a very good understanding of what the chip is going to need to work.
      Last edited by keeney123; 04-13-2022, 08:42 PM.

      Comment


        #4
        Re: A question about PWM IC's

        Originally posted by R_J View Post
        It usually needs the other signals like CS and feedback, otherwise it will likely shut down.

        Why do you ask...
        Hi R_J.

        I'm learning about SMPS's and am needing a deeper understanding of the control circuitry.

        I have a small lighting PSU which has one of these IC's and it has ~5v on the Vin pin yet is not 'chopping'.
        Therefore there is nothing happening on the primary or secondary of the transformer, and therefore no feedback signal, nor current flow for the FB and CS pins to detect.

        That is why I asked

        Comment


          #5
          Re: A question about PWM IC's

          Lot of power on pins need a PWM signal for it to work correctly
          9 PC LCD Monitor
          6 LCD Flat Screen TV
          30 Desk Top Switching Power Supply
          10 Battery Charger Switching Power Supply for Power Tool
          6 18v Lithium Battery Power Boards for Tool Battery Packs
          1 XBox 360 Switching Power Supply and M Board
          25 Servo Drives 220/460 3 Phase
          6 De-soldering Station Switching Power Supply 1 Power Supply
          1 Dell Mother Board
          15 Computer Power Supply
          1 HP Printer Supply & Control Board * lighting finished it *


          These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

          1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
          2 Hem Saw Computer Stack Board

          All of these had CAPs POOF
          All of the mosfet that are taken out by bad caps

          Comment


            #6
            Re: A question about PWM IC's

            Originally posted by keeney123 View Post
            On page 3 of your attached pfd files it explains what each pin is used for. Then if you go to page 4........
            Obviously I had been looking at all the pages, trying to understand how the chip worked, but reading again, it appears that that the VDD is needed too, so I'll check that out when I get indoors.

            Thanks

            Comment


              #7
              Re: A question about PWM IC's

              Originally posted by sam_sam_sam View Post
              Lot of power on pins need a PWM signal for it to work correctly
              But there must be a 'basic' state in which the PWM chip turns on, no?

              Comment


                #8
                Re: A question about PWM IC's

                Originally posted by Crystaleyes View Post
                But there must be a 'basic' state in which the PWM chip turns on, no?
                This something like a micro controller of some type or on some devices you can just not use it or you have either make it high or low depending on the device data sheet
                9 PC LCD Monitor
                6 LCD Flat Screen TV
                30 Desk Top Switching Power Supply
                10 Battery Charger Switching Power Supply for Power Tool
                6 18v Lithium Battery Power Boards for Tool Battery Packs
                1 XBox 360 Switching Power Supply and M Board
                25 Servo Drives 220/460 3 Phase
                6 De-soldering Station Switching Power Supply 1 Power Supply
                1 Dell Mother Board
                15 Computer Power Supply
                1 HP Printer Supply & Control Board * lighting finished it *


                These two repairs where found with a ESR meter...> Temp at 50*F then at 90*F the ESR reading more than 10%

                1 Over Head Crane Current Sensing Board ( VFD Failure Five Years Later )
                2 Hem Saw Computer Stack Board

                All of these had CAPs POOF
                All of the mosfet that are taken out by bad caps

                Comment


                  #9
                  Re: A question about PWM IC's

                  So let me see if I am understanding this correctly...

                  Voltage applied between Vin, VDD and Gnd sends the chip 'high' on the output (Trig) which turns on the FET, allowing current to flow, which signals the 'SENSE' pin, causing the chip to go 'low', whilst at the same time, a signal is detected across the output by the 431 which in turn varies the opto-coupler resistance to signal the 'FB' pin, ultimately affecting the 'TRIG' signal frequency.

                  The chip then goes high again due to the Vin, VDD and Gnd and the process repeats at many KHz

                  Forgetting any other pin for now, is it essentially something like that?


                  EDIT

                  I'm a bit confused...

                  Looking at the datasheet and the 'Typical Application', the VDD pin connects to an electrolytic, a resistor, a diode, an inductor, and then straight to Gnd.

                  On the 'Terminal Assignments' however, it has VDD as 'DC Power Supply pin.' I had assumed that had meant 5v or so, but clearly not.

                  Therefore this chip should need 5v in the Vin, compared to 0v on the Gnd and VDD.


                  That is what I am measuring anyway.
                  Last edited by Crystaleyes; 04-14-2022, 09:00 AM.

                  Comment


                    #10
                    Re: A question about PWM IC's

                    Start-up current comes from "VIN" (also, line voltage sense). After it has started, house-keeping power for the IC comes from "VDD".

                    It is a current mode regulator, so it needs the output voltage sense signal at "FB" and the sensed primary current at "SENSE" to regulate. "RI" sets the switch frequency, and "RT" is for over-temperature protection.

                    The block diagram shows flyback topology, in which the "transformer" is actually a coupled inductor, in which the "primary" functions as the output inductor.
                    PeteS in CA

                    Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
                    ****************************
                    To kill personal responsibility, initiative or success, punish it by taxing it. To encourage irresponsibility, improvidence, dependence and failure, reward it by subsidizing it.
                    ****************************

                    Comment


                      #11
                      Re: A question about PWM IC's

                      Originally posted by Crystaleyes View Post
                      Hi R_J.

                      I'm learning about SMPS's and am needing a deeper understanding of the control circuitry.

                      I have a small lighting PSU which has one of these IC's and it has ~5v on the Vin pin yet is not 'chopping'.
                      Therefore there is nothing happening on the primary or secondary of the transformer, and therefore no feedback signal, nor current flow for the FB and CS pins to detect.

                      That is why I asked
                      I suspect your ic is damaged, The Vin should be higher than 5v, I suspect it should be around 20v at least to provide around 15 volts on the vdd line for startup. The datasheet doesn't show it but there is likely an internal startup circuit that takes the Vin and provides about 15v to the vdd line to charges the startup cap and start the ic, once it starts the vdd voltage is supplied by a feedback winding from the transformer
                      I would check the mosfet and source resistor, if they are ok I would replace the ic
                      Last edited by R_J; 04-14-2022, 05:09 PM.

                      Comment


                        #12
                        Re: A question about PWM IC's

                        Vin is a current sensing device that is passing through and external resistor. This then develops a voltage on the internal operational amplifier. The same internal amplifier has the input from the sense current that develops a voltage across another external resistor which is called the leading-edge blanking pulse. This internal operational amplifier is labeled OC. Which is listed in the electrical characteristics as I_V_DD Operation called operational current. This is the current regulator which then controls the amount of voltage on the top pin of the external primary winding of the transformer.

                        The other internal Operational Amplifier in for the modulation of the Pulse Width.

                        When one considers start up then they have to consider all these factors. One has to have current sensing to regulate. Internal oscillation with sense current input trigger for slope compensation, (Ri) for reference voltage. The Gate voltage is an output of the IC. RT is an output current in the electrical characteristics listed as for temperature protection which give minimum and maximum voltage which will shut down the IC.

                        As a technician I would first look at the outputs of the chip to see if it is working. I would then look at VDD and see if there is a DC supply. Then look at Vin on both sides of the external resistor. If the primary of the transformer has the supplied voltage on it but is not pulsing, then there could be a problem with Gate, the sense, or any other pin of the IC. Most likely the Vin will be good however one can check this by disconnecting the Vin External resistor.
                        Last edited by keeney123; 04-14-2022, 08:40 PM.

                        Comment


                          #13
                          Re: A question about PWM IC's

                          If you disconnect the Vin resistor the ic will not work at all, where do you think the ic gets it's voltage to operate? I would first make sure the Vin resistor is not open, If there is 165vdc on one side and 5v on the other either the resistor is close to open, or the ic is bad.

                          Comment


                            #14
                            Re: A question about PWM IC's

                            There are two 470K resistors in series before the Vin, which both measure very close to 470KΩ, so the Vin resistor is NOT open.

                            The voltage here (Brazil) is 116v which means that after the bridge, and at the first resistor, we have 168v, followed by 82v inbetween, and then the 5.4v which is on the Vin pin.

                            When connected up to a 230V supply, the Vin voltage only went up to 6.3v

                            After having individually tested all the other components, it really only leaves the controller.

                            I'll order one online and we'll see,

                            Comment


                              #15
                              Re: A question about PWM IC's

                              Originally posted by R_J View Post
                              I suspect your ic is damaged,
                              I would check the mosfet and source resistor, if they are ok I would replace the ic
                              The FET and Source resistor are both fine so I'll order an IC and let you all know.


                              Even though some of these fascinating devices can be quite complexed, the pieces are definitely starting to fall into place regarding a basic enough comprehension of SMPS design to feel comfortable performing some repairs. My goal is to do this professionally, so just have to keep practicing.

                              The chopper and feedback circuits are finally becoming clear, so just wanted to say a big thank you for all the input everyone has given.

                              It is truly appreciated.

                              Comment


                                #16
                                Re: A question about PWM IC's

                                I would order a couple just in case, Sometimes a voltage spike comes down the a/c line and since the ic Vin is connected directly via the two resistors, the ic gets damaged, Hopefully that's all that it needs.
                                Without seeing the board, I would also replace the Vdd electrolytic capacitor, these have a tendency to get weak over time, and testing it usually does not work, I would just replace it.

                                Comment


                                  #17
                                  Re: A question about PWM IC's

                                  Originally posted by R_J View Post
                                  I would order a couple just in case, Sometimes a voltage spike comes down the a/c line and since the ic Vin is connected directly via the two resistors, the ic gets damaged, Hopefully that's all that it needs.
                                  Without seeing the board, I would also replace the Vdd electrolytic capacitor, these have a tendency to get weak over time, and testing it usually does not work, I would just replace it.
                                  Thanks for the heads-up.

                                  I shall follow your suggestions.

                                  It'll be switched-on through a voltage stabiliser and dimbulb, so hopefully that'll protect it enough.

                                  Pretty sure that the original damage came from a power surge of some kind. I'm in Rio at the moment and power surges/cuts/failures are very common.

                                  Comment


                                    #18
                                    Re: A question about PWM IC's

                                    Originally posted by R_J View Post
                                    If you disconnect the Vin resistor the ic will not work at all, where do you think the ic gets it's voltage to operate? I would first make sure the Vin resistor is not open, If there is 165vdc on one side and 5v on the other either the resistor is close to open, or the ic is bad.
                                    The lifting of the resistor is to illuminate the IC and concentrate on the other parts of the circuit. While the IC is being powered it can affect the other parts of the circuit. Such as a shorted IC or faulty internal components. I would then be looking for a rippled DC level on the top of the transformer. There would be a little ripple because the regulating circuit in the IC would not be working. However, it would not have a lot of ripple. That might indicate a bad capacitor on the output of the Bridge rectifier. If I received a pulsating DC I would suspect a bad diode in the bridge rectifier. If I received the DC ripple at the appropriate voltage I then would believe the bridge rectifier and capacitor was alright. With the FET turned off I would be then looking for this voltage on the top leg of the FET. One would have to know the voltage level the bridge rectifier is putting out.

                                    In the process of troubleshooting a problem one has to eliminate portions of the circuit that may interfere with other portions. If You read my #12 post again you will see a process of troubleshooting. At the point where the V in is wrong one has to eliminate the chip.
                                    Last edited by keeney123; 04-16-2022, 01:18 AM.

                                    Comment


                                      #19
                                      Re: A question about PWM IC's

                                      Originally posted by Crystaleyes View Post
                                      The FET and Source resistor are both fine so I'll order an IC and let you all know.


                                      Even though some of these fascinating devices can be quite complexed, the pieces are definitely starting to fall into place regarding a basic enough comprehension of SMPS design to feel comfortable performing some repairs. My goal is to do this professionally, so just have to keep practicing.

                                      The chopper and feedback circuits are finally becoming clear, so just wanted to say a big thank you for all the input everyone has given.

                                      It is truly appreciated.
                                      If you have a desire to learn and study, you will become a very good technician. I was an electronic technician for 15 years. I went to two schools for this. One was Pinellas Vocational Technical Institute. The other one was Springfield Technical Community College. I really did not desire to become an electrical/electronic technician. It was something my father wanted me to do. I am at heart a drawing & painting artist. You need not go to school to learn all this stuff. Some people learn better by doing. When problems come up they learn. Good Luck to You.

                                      Comment


                                        #20
                                        Re: A question about PWM IC's

                                        Originally posted by keeney123 View Post
                                        The lifting of the resistor is to illuminate the IC and concentrate on the other parts of the circuit. While the IC is being powered it can affect the other parts of the circuit. Such as a shorted IC or faulty internal components. I would then be looking for a rippled DC level on the top of the transformer. There would be a little ripple because the regulating circuit in the IC would not be working. However, it would not have a lot of ripple. That might indicate a bad capacitor on the output of the Bridge rectifier. If I received a pulsating DC I would suspect a bad diode in the bridge rectifier. If I received the DC ripple at the appropriate voltage I then would believe the bridge rectifier and capacitor was alright. With the FET turned off I would be then looking for this voltage on the top leg of the FET. One would have to know the voltage level the bridge rectifier is putting out.

                                        In the process of troubleshooting a problem one has to eliminate portions of the circuit that may interfere with other portions. If You read my #12 post again you will see a process of troubleshooting. At the point where the V in is wrong one has to eliminate the chip.
                                        Hi.
                                        Excuse the delay.

                                        I lifted a leg of the resistor and had a poke around measuring in both DC and AC for any ripple.

                                        There's no working scope here so the testing was done with the DMM and revealed nothing on the DC although did appear to show some AC ripple when measuring across the filter caps and transformer primary.

                                        It shouldn't be the caps as they are new replacements

                                        If I'm honest, I'm not really sure what signs I should be looking for other than low voltages, but perhaps someone can enlighten me?

                                        Comment

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