A question about PWM IC's

[R_J]

Re: A question about PWM IC's

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

[PeteS in CA]

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.

[Crystaleyes]

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.

[sam_sam_sam]

Re: A question about PWM IC's

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

[Crystaleyes]

Re: A question about PWM IC's

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?

[Crystaleyes]

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........

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

[sam_sam_sam]

Re: A question about PWM IC's

Lot of power on pins need a PWM signal for it to work correctly

[Crystaleyes]

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...

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

[keeney123]

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.

[R_J]

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...

[Crystaleyes]

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?