Electronic Circuits and Projects Forum

Hi MrAl, Others,

Sorry for the long post...

I am still struggling with the idea of how the error amp works - in particular how to confidently bias it...

That being said, my impressions now on how it works are as follows:

There are two control signals one from the dead time and another from the 2 error amps. The system only recognises the highest one and uses that to set the PWM. If dead time is set to zero and also the error amps then there is a 0.7volt value as a result (OF dead time signal) and this is whats used - this provides the basis for the minimum 3% dead time.

The error amp is biased for non inverting operation. The difference between the two signals entering the (+) and (-) (with the inverting signal bigger than the non inverting signal) is amplified and forms the basis for the control signal which when applied to the saw tooth wave creates the pulses.

I am guessing the non inverting pin acts as a reference. If the non inverting signal goes below the inverting signal the difference shows up as an increased control signal.


OK my circuit as it stands...

TRACKING ERROR AMP 2.jpg

the idea here is that I am using the TL494 as a pre-regulator power supply for a linear output stage using a LT1083. In the circuit provided its assumed that mains has already been transformed down to 30 volts. (I did this to speed up the simulation - I have designed and built a transformer that should produce the voltage of ~35 V - Freq 45 kHz) Anyway its run through a filter and that forms the basis of the LT1083 input. The central idea here is that if you adjust the LT1083's voltage this will cause a difference (ultimately) between the input and the output which are set to have a specific gap - input ~2-3 volts higher than the LT1083 output at all times when in a steady state. To achieve this I set the inverting pin of the error amp to be approximately 1/12 of the output voltage and the the non inverting pin of the error amp to be approximately 1/14 the input voltage. That way when they reach an equivalent value there is a discrepancy of ~2-3 volts between the input and output...

Thats the plan!

Referring to the above circuit with the simulation traces. To make things more obvious I used a voltage controlled resistor to set two extremes for the LT1083 adjustment - PINK trace in ohms is this adjusting resistor below the LT1083. The voltage into the LT1083 is indeed higher than the output and it does track the output.. RED trace is the input and TEAL trace is the output. The BLUE and GREEN traces at the bottom are the values for the inverting and non inverting respectively.

In the sim there is a fairly slow response.. takes nearly 0.8 seconds to arrive at a new stable position..

A fair mouthful so far.... However I'd like to ask if this is a legitimate concept for tracking.... (Will it work? Whats wrong with it?) and how to properly bias the error amp - that is working out the values confidently... because there is nothing out there that describes this clearly and in depth.. the best I got was vague sub circuit with no values and no calculations etc.. short of buggering around on a work bench I cannot see a way of building this properly.

Further how does applying a gain to the difference effect the control signal.... does this mean the two values track much closer together because as the difference grows by the gain this causes the change in the control signal to be that much faster?


Sorry for the book but I guess if I describe it properly someone might take me up on this and help describe the gaps in information.

Kind Regards

Simon

Your schematic is certainly not how to use a TL494. So to explain how it is not working right is not easy.

I think you want 30 volts bucked down to drive your linear regulator. Because you are using LTSpice look for a LT buck regulator that can handle the current. Once you have the simple buck working then we can show you how to make the error amp work. Look for an example circuit on LTSpice.

What current do you want?

In this app note there are examples much like what you are trying to do.

Hi Ron... 10amps was the target... I was going to use two LT1083s in parallel.

LTC1624 Will buck 30 volts down to what you need. See the app note in post #15. A transistor or P-mosfet us used to detect voltage across the linear regulator, then drives the feed back pin (error amplifier).