A question about PWM IC's

[keeney123]

Re: A question about PWM IC's

I love it! How long is a piece of string?

[R_J]

Re: A question about PWM IC's

Hi again.

The source resistor is supposed to be 0.12Ω but is reading much higher, so can I ask you how far either side of that value would be safe to go if I can't find the correct value in the shops here?

How long is a piece of string? When you say it is reading "much" higher, can you give a value?
Remember to take your meter lead resistance into consideration. In the lowest resistance scale, short your probes together and note the value, this lead resistance will add to the value when you read the resistor, so you need to subtract the lead resistance from the measured value to get the resistors value. The source resistor is critical in that it sets the operating current of the circuit. The voltage developed across the source resistor is fed back to the ic and the ic modifies it's drive signal to compensate for load etc..

[Crystaleyes]

Re: A question about PWM IC's

[QUOTE=R_J;1125693
I would check the mosfet and source resistor, if they are ok I would replace the ic[/QUOTE]

Hi again.

The source resistor is supposed to be 0.12Ω but is reading much higher, so can I ask you how far either side of that value would be safe to go if I can't find the correct value in the shops here?

[Crystaleyes]

Re: A question about PWM IC's

Here is a video of what is happening in a coil with DC applied and pulsating DC. When the current charges the coil of the primary of the transformer it builds a magnetic field around the coil. This magnetic field has an opposite voltage when the field collapses going through a diode and resistor to equalize/discharge the current at a certain time rate. When the FET conducts again the Electromotive Force of the coil will oppose the current flow again until no Electromotive Force is left. This then charges up the Magnetic Force of the coil and then discharges. On the secondary of the transformer when the primary discharges the Magnetic force then the Secondary will feel this force in an opposite polarity. The secondary will also have a polarity opposite of the charging current of the primary. Many times, the secondary circuit has on diode in it, and they use only half the cycle. I suppose one could put a bridge rectifier in the secondary and use the full cycle.

https://www.youtube.com/watch?v=LVNxrN4jgvs

Nice little video. Clarifies nicely how inductors help to smooth out a pulsing circuit.

Thank you very much

[keeney123]

Re: A question about PWM IC's

Here is a video of what is happening in a coil with DC applied and pulsating DC. When the current charges the coil of the primary of the transformer it builds a magnetic field around the coil. This magnetic field has an opposite voltage when the field collapses going through a diode and resistor to equalize/discharge the current at a certain time rate. When the FET conducts again the Electromotive Force of the coil will oppose the current flow again until no Electromotive Force is left. This then charges up the Magnetic Force of the coil and then discharges. On the secondary of the transformer when the primary discharges the Magnetic force then the Secondary will feel this force in an opposite polarity. The secondary will also have a polarity opposite of the charging current of the primary. Many times, the secondary circuit has on diode in it, and they use only half the cycle. I suppose one could put a bridge rectifier in the secondary and use the full cycle.

https://www.youtube.com/watch?v=LVNxrN4jgvs

[keeney123]

Re: A question about PWM IC's

Originally, I qualified in electrical engineering many moons ago, and was working building the relay-logic control panels for industrial air conditioning systems, so have a reasonable amount of experience working with circuit tracing and power calculation.

I have actually signed up for the RSD Academy course but it is not so inspiring going back over the very basics of Ohm's Law and the like, however I shall persevere in order to arrive at the stuff which I have not previously studied.

Well done. This may help with the understanding of what is happening.

https://en.wikipedia.org/wiki/Buck%E...oost_converter


https://www.watelectronics.com/flyba...esign-working/

[Crystaleyes]

Re: A question about PWM IC's

If you have greater desire of the actual theory behind circuit I would suggest that you lean toward becoming an engineer. In this case it would be best to learn this in a school.

Originally, I qualified in electrical engineering many moons ago, and was working building the relay-logic control panels for industrial air conditioning systems, so have a reasonable amount of experience working with circuit tracing and power calculation.

I have actually signed up for the RSD Academy course but it is not so inspiring going back over the very basics of Ohm's Law and the like, however I shall persevere in order to arrive at the stuff which I have not previously studied.

[Crystaleyes]

Re: A question about PWM IC's

There is only one primary winding, that is the one connected between the main DC voltage and the mosfet drain. The other winding on the primary side is the feedback winding, it will supply the ic' vcc voltage (via a diode and cap) to keep the ic working after startup.
I would not disconnect the mosfet gate to check the ic output. The ic needs to have the feedback vcc voltage as well as the source voltage to operate otherwise it will go into protection.
If you are going to check the ic output you need an ISOLATED scope, so the scope must be battery operated, If the scope is line powered you need an isolation transformer to connect the power supply to. If you don't do this, as soon as you connect the scope ground you will blow the power supply main fuse and usually the bridge rectifier as well.

Excellent info. Thank you very much.

I had been wondering about the second winding - Nice one!

There is actually a battery powered Tektronix scope here which I bought for parts (as the plastic case had been smashed - I assumed the pcb's had been damaged, but no) and have been working my way through repairing it. Just a short somewhere on the +95v rail (down to +78v), but I shall value the info you gave above.

[Crystaleyes]

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

.

Hi,

yes, it is.

[keeney123]

Re: A question about PWM IC's

Do you have a bridge rectifier and a electrolytic cap of around 10~100µf with a voltage rating of 200vdc. connect them up and check the voltage. without the filter cap the raw dc output will be around 108vdc

So that makes sense if the input is 120VAC RMS the Average voltage would be a little lower 108VDC

[keeney123]

Re: A question about PWM IC's

I give up!

Hold on don't give up. If one measures with a volt meter in the DC volt setting from the ground of the primary to the output of the rectifier what does that read? You said you read 165 DC. Is that measured from earth ground? If the Ground of the primary is risen or lowered by some means it will blow the circuit when the ground of the scope is attached because it shorts the voltage to ground. It might also explain why there is 165 VDC on the output of the rectifier. If one is looking from earth ground to bridge rectifier output it will seem to be more because of the rectifier ground going up. So in this case one can simply measure from the two grounds with a volt meter and see if there is a voltage difference.

[R_J]

Re: A question about PWM IC's

How is the ground of the primary winding circuit being risen above earth ground?

I give up!

[R_J]

Re: A question about PWM IC's

You simply can not get more power out of something that you put into it.125VAC bridge rectifier will never put out 165VDC. I do not see a voltage multiplier on the input. We have to ask is the 120VAC a RMS voltage or a Peak to Peak voltage. RMS voltage is figured from Peak to Peak voltage by divided the Peak to Peak voltage in half and then multiplying by 0.707. Average voltage which is the DC equivalent can be figured by dividing the Peak to Peak voltage in half and then multiplying by 0.678. And through algebra one can find the correlation between the two.

Do you have a bridge rectifier and a electrolytic cap of around 10~100µf with a voltage rating of 200vdc. connect them up and check the voltage. without the filter cap the raw dc output will be around 108vdc

[keeney123]

Re: A question about PWM IC's

There is only one primary winding, that is the one connected between the main DC voltage and the mosfet drain. The other winding on the primary side is the feedback winding, it will supply the ic' vcc voltage (via a diode and cap) to keep the ic working after startup.
I would not disconnect the mosfet gate to check the ic output. The ic needs to have the feedback vcc voltage as well as the source voltage to operate otherwise it will go into protection.
If you are going to check the ic output you need an ISOLATED scope, so the scope must be battery operated, If the scope is line powered you need an isolation transformer to connect the power supply to. If you don't do this, as soon as you connect the scope ground you will blow the power supply main fuse and usually the bridge rectifier as well.

How is the ground of the primary winding circuit being risen above earth ground? Earth ground is usually connected to the neutral line at the fuse box and it is the ground of a plug.

[keeney123]

Re: A question about PWM IC's

How do you come up with this? The line voltage in N.A. is 120vac, with a full wave bridge and a filter capacitor the DC voltage across the filter capacitor is approximately 165VDC. The primary DC resistance was just a guess, I went and checked one I have and it measured about 1Ω.

You simply can not get more power out of something that you put into it.125VAC bridge rectifier will never put out 165VDC. I do not see a voltage multiplier on the input. We have to ask is the 120VAC a RMS voltage or a Peak to Peak voltage. RMS voltage is figured from Peak to Peak voltage by divided the Peak to Peak voltage in half and then multiplying by 0.707. Average voltage which is the DC equivalent can be figured by dividing the Peak to Peak voltage in half and then multiplying by 0.678. And through algebra one can find the correlation between the two.

[R_J]

Re: A question about PWM IC's

Hi fellas.

That was an informative few posts. Thanks.

As for the values, the original source resistor is unknown however looking at almost identical boards with the exact same output values, then I believe it to be 0.12Ω. The transformer has two primaries which both measure super low 0.1 ish and the original fet was a 12 60 of some sort (don't remember right now).

Given the question around the source resistor, would it not be worth first switching it on with the gate disconnected, in order to check that the chip output is fluctuating correctly?

There is only one primary winding, that is the one connected between the main DC voltage and the mosfet drain. The other winding on the primary side is the feedback winding, it will supply the ic' vcc voltage (via a diode and cap) to keep the ic working after startup.
I would not disconnect the mosfet gate to check the ic output. The ic needs to have the feedback vcc voltage as well as the source voltage to operate otherwise it will go into protection.
If you are going to check the ic output you need an ISOLATED scope, so the scope must be battery operated, If the scope is line powered you need an isolation transformer to connect the power supply to. If you don't do this, as soon as you connect the scope ground you will blow the power supply main fuse and usually the bridge rectifier as well.

[keeney123]

Re: A question about PWM IC's

Hi fellas.

That was an informative few posts. Thanks.

As for the values, the original source resistor is unknown however looking at almost identical boards with the exact same output values, then I believe it to be 0.12Ω. The transformer has two primaries which both measure super low 0.1 ish and the original fet was a 12 60 of some sort (don't remember right now).

Given the question around the source resistor, would it not be worth first switching it on with the gate disconnected, in order to check that the chip output is fluctuating correctly?

I you lift up one leg of the resistor connected to the FET which I believe you said was the source leg and measure it then you will know it's values. The reason for the two primaries is explained in your data sheet. When working on a circuit it is important to know exactly what you are working on. To know the schematic and the components and how they interact with one another. This is a key to successfully repairing electronic so they do not come in your door more than once. I trust R_J's experience and I believe he will finish up with you in working on this circuit. If you have greater desire of the actual theory behind circuit I would suggest that you lean toward becoming an engineer. In this case it would be best to learn this in a school.

[R_J]

Re: A question about PWM IC's

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.

How do you come up with this? The line voltage in N.A. is 120vac, with a full wave bridge and a filter capacitor the DC voltage across the filter capacitor is approximately 165VDC. The primary DC resistance was just a guess, I went and checked one I have and it measured about 1Ω.

[keeney123]

Re: A question about PWM IC's

Hi fellas.

That was an informative few posts. Thanks.

As for the values, the original source resistor is unknown however looking at almost identical boards with the exact same output values, then I believe it to be 0.12Ω. The transformer has two primaries which both measure super low 0.1 ish and the original fet was a 12 60 of some sort (don't remember right now).

Given the question around the source resistor, would it not be worth first switching it on with the gate disconnected, in order to check that the chip output is fluctuating correctly?

I would go by what R_J is telling you about the FET and DC voltage. I read through the chips information and they do all sorts of things to protect from over current and overvoltage. R_J is an active technician doing repairs and should know how these things work. My intention is not to take that away from him. My intention is to provide you with what basic theory of electronics is. With that I still believe that the mosfet in the primary winding circuit needs to handle the full DC current running through it. This is a condition that the designers are trying to avoid with all the over voltage and current protection built with-in the chip. However, we know that in your case this did not work. When Engineers I have worked with have come across such a problem they go back to the circuit and find the cause and then make the circuit more robust so the same condition is not done again.

[Crystaleyes]

Re: A question about PWM IC's

Hi fellas.

That was an informative few posts. Thanks.

As for the values, the original source resistor is unknown however looking at almost identical boards with the exact same output values, then I believe it to be 0.12Ω. The transformer has two primaries which both measure super low 0.1 ish and the original fet was a 12 60 of some sort (don't remember right now).

Given the question around the source resistor, would it not be worth first switching it on with the gate disconnected, in order to check that the chip output is fluctuating correctly?