A question about PWM IC's

[keeney123]

Re: A question about PWM IC's

You must have a smps around , try bypassing the fet with a diode like you suggest.
The difference between dc resistance and ac inductance is what you are overlooking. The DC resistance of the primary is likely about 2~10Ω, apply (using N.A. line input values) 165vdc, what would be the current?

So your telling me that the peak to peak voltage on the AC line is 486 Volts? We do not know what the resistance of the primary is. We do not know what the value of the FET is. We do not know what the value of the bottom resistor is. If you give me these valves I can figure out the current in the circuit at 165 Volt DC I can tell you whether the FET can handle that current. If it can not handle that current the circuit needs a FET capable of doing it. If none go up that far then the circuit needs to be re-design to accommodate a lower current. In this way one does not blow up the circuit when something goes bad.

[keeney123]

Re: A question about PWM IC's

You must have a smps around , try bypassing the fet with a diode like you suggest.
The difference between dc resistance and ac inductance is what you are overlooking. The DC resistance of the primary is likely about 2~10Ω, apply (using N.A. line input values) 165vdc, what would be the current?

No actually I have none of that. I do not like working on these things. Instead I enjoy doing art. AC inductance you will have your apparent power, real power. Real power is resistive power. Apparent is power is figured by Xc. Xl and resistance. It is figured in a sinusoidal wave frequency. Real power is actually what is going through the circuit. However, we have pulsating DC. So when the circuit gets turned on it is full on not a gradual sign wave this will affect the circuit differently. The moment the voltage is applied the transformer it will see all voltage. The current will lag behind by 90 degrees in a sinusoidal waveform. With pulsating DC with a varying duty cycle I would not begin to try and figure that one. I am sure there are equations for this an long explanation. I do not wish to go into that as I have no desire to be an electrical/electronic engineer. Still the current will see the resistance of the coil and the resistance of the FET and the resistance of the bottom resistor. The real current will be resistance and this will give the real power. Phase shifting the Voltage as oppose to Current effects only the apparent power. The apparent power will always be greater than the real power.

[R_J]

Re: A question about PWM IC's

The cap with bridge rectifier should not be at a greater voltage that what the designer had intended whether it is DC or a pulsating DC square wave. So when voltage is applied to its max at either DC or pulsating DC one have the same current going through the primary. If the designer did not allow for a steady state to happen then I would he under design the circuit. Remember I am going by the sample circuit as yet I have not seen anything else posted in this thread.

You must have a smps around , try bypassing the fet with a diode like you suggest.
The difference between dc resistance and ac inductance is what you are overlooking. The DC resistance of the primary is likely about 2~10Ω, apply (using N.A. line input values) 165vdc, what would be the current?

[keeney123]

Re: A question about PWM IC's

This would result in the resistor (0.12Ω) blowing open. The fet must be driven by high frequency drive signal, Bypassing the fet with a diode? this would cause excessive current through the primary and at a minimum blow the main fuse. You do not want the transformer in a dc circuit, it must be ac, and in this case it is designed for 20~60khz

In the OP other thread, the problem was the mosfet shorted causing the source resistor to go open, when this happens the sense input to the ic which is connected to the mosfet source via R16 is subject to the full dc voltage, 165~350vdc, the ic can never handle this.

There is not AC going through the transformer but, a pulsating DC waveform with a varying duty cycle. This is what the bridge rectifier is putting out DC.
The cap with bridge rectifier should not be at a greater voltage than what the designer had intended whether it is DC or a pulsating DC square wave. So when voltage is applied to its max at either DC or pulsating DC one has the same current going through the primary. If the designer did not allow for a steady state to happen then I would say he under design the circuit. Remember I am going by the sample circuit as yet I have not seen anything else posted in this thread. Looking at the other thread you referred too I see no schematic in that thread either.
Also, understand that the DC voltage has a primary winding in it and a resistor under the FET. I do not know what the FET value is but, when it conducts it will have a very low resistance. Perhaps the bottom resistor is not correct and allowing to much current to flow. Perhaps there needed to be another resistor in the FET circuit to make a voltage divider for the sense. Or it may need a resistor above the FET to lower the current. I do not know as I do not have the valves and schematic to the real circuit.

[R_J]

Re: A question about PWM IC's

Yes your right the ripple will not appear without a load because the capacitor needs a path to discharge and recharge.
One could first bypass the FET with a diode and put the transformer into play. The resistor on the bottom of the FET should protect the current draw. If this was OK then one could bias the FET on to see if that works.

This would result in the resistor (0.12Ω) blowing open. The fet must be driven by high frequency drive signal, Bypassing the fet with a diode? this would cause excessive current through the primary and at a minimum blow the main fuse. You do not want the transformer in a dc circuit, it must be ac, and in this case it is designed for 20~60khz

In the OP other thread, the problem was the mosfet shorted causing the source resistor to go open, when this happens the sense input to the ic which is connected to the mosfet source via R16 is subject to the full dc voltage, 165~350vdc, the ic can never handle this.

[keeney123]

Re: A question about PWM IC's

Is this the same power supply that had the blown source resistor? https://www.badcaps.net/forum/showthread.php?t=105220

If it is, then that was why the ic was damaged internally.

You won't measure any "ripple" on the main capacitors in the state is in now. The main filter caps will charge and without any load, they will not discharge, so you may want to discharge them with a large resistor. at least check the voltage across them before working on the circuit to make sure they are discharged.
Once the ic is installed, the main dc voltage will not change that much, likely just be slightly lower dc value.

I am thinking the path for the discharge and recharge are though the diode bridge as the AC voltage becomes less the cap voltage becomes less because the cap has access to ground through the bridge diodes. The cap will become a supply voltage when the bridge voltage becomes less on the down part of each half wave of the AC voltage. So it would be worth it for someone to set up a bridge rectifier an capacitor to see with a scope if there is a discharge. It has been a while since I did any of this work and would say not to take anyone's work for it but, to actually try it in a circuit.

One could first bypass the FET with a diode and put the transformer into play. The resistor on the bottom of the FET should protect the current draw. If this was OK then one could bias the FET on to see if that works.

[keeney123]

Re: A question about PWM IC's

The IC is due to arrive on Wednesday, and as for the circuit, it has a standard single piece, four pin bridge rectifier, which is testing fine with or without the IC connected, as are the two filter caps.
I also have one of those multi-component testers and the primary all seems fine (MOV, NTC, FET, caps, diodes, resistors etc.) except that the voltage across the transformer primary is just a constant DC. No frequency, and the DMM I have is good enough to detect that.

I'm eager to see how changing the IC will affect things.

That is what you are looking for a constant DC with "no" IC in place. It is so much easier with a Oscilloscope. So with the DC voltage on the transformer it tell you that the bridge circuit and everything that comes before is working. One can actually figure out what DC voltage will be on the Primary. One would take the peak to peak voltage and divide it in half then by 0.668. I am going on a 40 year old memory on the .0.668 I believe that to be very close. That would be called average power which is the DC equivalent of a sinusoidal waveform of 60 Hz better know as 60 cycles per second.

[R_J]

Re: A question about PWM IC's

Is this the same power supply that had the blown source resistor? https://www.badcaps.net/forum/showthread.php?t=105220

If it is, then that was why the ic was damaged internally.

You won't measure any "ripple" on the main capacitors in the state is in now. The main filter caps will charge and without any load, they will not discharge, so you may want to discharge them with a large resistor. at least check the voltage across them before working on the circuit to make sure they are discharged.
Once the ic is installed, the main dc voltage will not change that much, likely just be slightly lower dc value.

[Crystaleyes]

Re: A question about PWM IC's

Did your replace the IC? I am going by your data sheets typical circuit. Is this what you have? The bridge rectifier is the 4 diodes group diamond shape. If the resistor leg is lifted to Vin and you measure the capacitor going to the bridge rectifier at capacitor pin1 you should have a DC voltage. If no DC voltage on the capacitor check pins 1 to pins3 of the bridge rectifier for you AC input Voltage. If you have the AC Voltage then check the diodes of the rectifier.

The IC is due to arrive on Wednesday, and as for the circuit, it has a standard single piece, four pin bridge rectifier, which is testing fine with or without the IC connected, as are the two filter caps.
I also have one of those multi-component testers and the primary all seems fine (MOV, NTC, FET, caps, diodes, resistors etc.) except that the voltage across the transformer primary is just a constant DC. No frequency, and the DMM I have is good enough to detect that.

I'm eager to see how changing the IC will affect things.

[keeney123]

Re: A question about PWM IC's

Here is the front end of your sample circuit. This is the first half cycle of what happens with the AC voltage. I showed the current flow of what happens in the diodes of the bridge rectifier. When the bottom half of the cycle of AC voltage happens, the top voltage goes negative with respect to ground. Then it will proceed through the other two diodes of the bridge rectifier. That then in one cycle. In the US this happens 60 times a second. In England I believe it is 50 times a sec. The diodes are just one-way switches. The line on the diode forms a letter K which stands of Cathode. A cathode needs a negative voltage to work. So that means the back end of the blackened arrow is a Anode. A anode will need a positive voltage to work. Each silicon diode drops 0.7 Volts DC in order to forward bias the diode so current will flow.

[keeney123]

Re: A question about PWM IC's

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?

Did your replace the IC? I am going by your data sheets typical circuit. Is this what you have? The bridge rectifier is the 4 diodes group diamond shape. If the resistor leg is lifted to Vin and you measure the capacitor going to the bridge rectifier at capacitor pin1 you should have a DC voltage. If no DC voltage on the capacitor check pins 1 to pins3 of the bridge rectifier for you AC input Voltage. If you have the AC Voltage then check the diodes of the rectifier.

Again, I am going by the typical circuit in your data sheet. If you are measuring with a DVDM then on the AC scale you might pick up the ripple which should be a sawtooth wave form as AC Voltage. This sawtooth waveform rides on top of the DC which is normal without the IC connected. When the IC is connected an working you should have a pulsating DC square wave. Depending on the frequency and the meter you are using you may or may not be able to read it with that meter.

[Crystaleyes]

Re: A question about PWM IC's

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?

[keeney123]

Re: A question about PWM IC's

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.

[keeney123]

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.

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.

[Crystaleyes]

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.

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.

[R_J]

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.

[Crystaleyes]

Re: A question about PWM IC's

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.

[Crystaleyes]

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,

[R_J]

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.

[keeney123]

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.