A question about PWM IC's

[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.

[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

[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.

[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

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.

[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/

[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

[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

[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?

[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..

[keeney123]

Re: A question about PWM IC's

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

[Crystaleyes]

Re: A question about PWM IC's

How long is a piece of string? When you say it is reading "much" higher, can you give a value?
..

Measuring with various testers it sits between 0.3 and 0.4. The only tester with .xx digits reads 0.38Ω, and that seemed too high to me

I have various value resistors here so can mix and match if necessary.

I'm starting to see the importance of the value of this resistor now.

[keeney123]

Re: A question about PWM IC's

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

Thank you very much

Understand that this circuit you are working on is a current feed circuit. The DC voltage is constant. When the FET conducts which closes the circuit the current starts to overcome the counter Electro Motive Force of the primary winding. Once it completely overcomes the counter EMF then it is Maximum current. I believe this circuit has a greater than 50% duty cycle which makes it more difficult to control. If they were using photo optic transistor, they would have better control of the circuit. However, they claim that the optic transistor fails more often than the Mosfet. From what I have read usually the power mosfet is driven by another mosfet with-in the IC. I also read that typically the sense input has a 100K ohm resistor inside the IC. Which is going to reduce the current going in. I believe the current divider would be the 0.2 ohm as oppose to the 100K ohm internal resistor. So at MAX DC volts. A inverse ratio is set up for current. The mosfet will drop some voltage but, if we considered it as a short and 108 Vdc was at the source resistor you would have 540 ampere through the resistor and 1.08 milli ampere through the sense line. This is why they can only let the full current for a moment. If you know what wattage the source resistor is one can calculate out how much continuous current can go through the circuit. Once that current is figured then one multiplies that by 0.6 to get the actual current. Engineers always calculate a 40% factor in for protection. From the calculated 60% of maximum continuous current one can then calculate a test resistor to put in place of the source resistor and check out the winding of the circuit by placing a short across the FET.

[Crystaleyes]

Re: A question about PWM IC's

If I'm honest, I don't really follow you when you say things like, "check out the winding of the circuit by placing a short across the FET", so I'm just gonna put in a 0.12Ω 5W resistor and see if replacing the IC brings it back to life.

From what I have found elsewhere, the original Source resistor would have been 2W.

[keeney123]

Re: A question about PWM IC's

If I'm honest, I don't really follow you when you say things like, "check out the winding of the circuit by placing a short across the FET", so I'm just gonna put in a 0.12Ω 5W resistor and see if replacing the IC brings it back to life.

From what I have found elsewhere, the original Source resistor would have been 2W.

I am not telling what to do on this repair. Do as you have planned.

I am trying to explain how the circuit is working and how to figure out maximum current that could flow.

I am explaining this in the sample circuit in your data sheet with your 0.2 Ohm resistor in place of the sample source resistor.

I am explaining how to calculate how much of maximum continuous current the designer has intended to flow through the circuit.

Plus, his safety value that is calculated into his design.

I am telling you how to figure out the value of a test resistor so the winding that everything from the winding on backwards can safely be tested.

Only after all the above can be understood should you test the circuit by replacing the 0.2 ohm resistor with the calculated test resistor. Not in your present circuit but in a test circuit for understanding.

Because I do not know what the FET is I cannot try to calculate how this would be turned on.

The test resistor needs to be in place to lower the current in the circuit at the maximum allowable continuous current the designer had intended for continual use.

This current will have a 40% safety factor figured by the designer.

Then putting a short across the FET will be safe to do so the current will flow allowing the circuit to be tested out.

If you do not understand what I am explaining, then do not worry about it. Just continue on with what you do understand.

[keeney123]

Re: A question about PWM IC's

If I'm honest, I don't really follow you when you say things like, "check out the winding of the circuit by placing a short across the FET", so I'm just gonna put in a 0.12Ω 5W resistor and see if replacing the IC brings it back to life.

From what I have found elsewhere, the original Source resistor would have been 2W.

So the designer gives you a 2Watt resistor. With a 40 percent safety factor. You take the 2W X 0.60 = 1.2W This is maximum allowable power that the designer intended for that resistor.

Using Ohms Law I sq R = P , transposing this law to Isq = P/R , Then finally the sq root of I=I

Isq = 1.2W/0.2 Ohms, Isq = 6, sq root of 6 = 2.45 , I therefore equals 2.45 amps

2.45 Amps is the allowable continuous current to flow in that circuit.

The FET should be rated for this current.

If you take the total DC voltage availably 108DC and you take the maximum current allowed a safe resistor can be calculated. 108/2.45=44.08 ohms

The problem then becomes trying to work with a lower wattage resistor than the 44.08 ohms allows.

One would have to use a 3000 ohm resistor rated at 5 watts source resistor so cost of the test resistor would not be to much.

We can now measure with a meter the voltage drop across each component and the voltage drop across the supply voltage. This then will tell the resistance of each component including the supply voltage.

[keeney123]

Re: A question about PWM IC's

If I'm honest, I don't really follow you when you say things like, "check out the winding of the circuit by placing a short across the FET", so I'm just gonna put in a 0.12Ω 5W resistor and see if replacing the IC brings it back to life.

From what I have found elsewhere, the original Source resistor would have been 2W.

I would not lower the source resistor from what it is intended for. That resistor was calculated along with the other components to handle so much current. If you allow more current, then the design allows then most likely you will have fireworks and at the least stress the components for an early grave.

[R_J]

Re: A question about PWM IC's

If I'm honest, I don't really follow you when you say things like, "check out the winding of the circuit by placing a short across the FET", so I'm just gonna put in a 0.12Ω 5W resistor and see if replacing the IC brings it back to life.

From what I have found elsewhere, the original Source resistor would have been 2W.

Just a note on the source resistor, it should be a non-inductive type once you do the final repair. Also without a load on the power supply, the circuit may start but be in burst mode until a load is placed on the secondary. This is a complex, green mode ic, and the circuit operates at high frequency. The whole circuit is fairly simple as the ic does a lot of the work, I suspect and hope the ic should fix the smps.

[Crystaleyes]

Re: A question about PWM IC's

Interesting indeed.

Your input is appreciated

I now have the IC only these last few days haven't had the spare time to install it. Should be over the weekend

[keeney123]

Re: A question about PWM IC's

Interesting indeed.

Your input is appreciated

I now have the IC only these last few days haven't had the spare time to install it. Should be over the weekend

As a last thing I can point you to in understanding this circuit and others is this site.

https://training.ti.com/pulse-width-...pwm-converters

I find the free ti training course offer a good fundamental understanding.