Flyback LED driver with unusual error amplifer connection is poor?

[treez]

Hello,
We have got a contractor to do a 100W flyback led driver for us, and we believe that he has not done a very good job. The circuit works but we believe that its performance is compromised in the way that he has done it.

The circuit schematic and LTspice simulation are attached.

He has used an external current sense error amplifier with a 0 to 5V supply, and strangly he has used ground as the reference voltage into this error amplifier.

His current sense signal is then going below ground as the current builds up, and he has biased this signal up so that the led driver is in regulation when the inverting and non-inverting inputs of the error amplifier are both at 0V.

The problem is, the opamp’s negative supply is the 0V rail, and when the input to the inverting input of the error amplifier goes below zero, then the opamp does not “know” what this voltage is, because it is supplied from a 0 to 5V supply.

So what I am saying is that the opamp will not respond as strongly as it might to the feedback signal going too high negatively. The feedback circuit in this setup means that the opamp will not be as able to develop sufficient over-drive in order to regulate against higher-than-wanted LED currents.

Do you agree?

 

[D.A.(Tony)Stewart]

There is nothing wrong with using Bottom rail for CM & DM reference; that is what it is designed to work with on this chip, but there may be CMRR penalties for noise below ground.

The use of ground on both sides of transformer and then use an optoisolator is wrong.

The bigger problem could be your lack of requirement specs for all PSU designs. Ingress, egress, safety leakage, hipot, environment, efficiency etc etc etc

 

[treez]

yes but when the input voltage on the inv pin goes below ground, the opamp is not biased to be able to deal properly with that...its dynamic performace will surely be affected?....(also the ground thing is just to get the simulation going)

 

[FvM]

As long as one of the OP inputs doesn't swing below the negative rail, the basic OP function will be maintained. In so far there's no problem with this circuit. By working of overall feedback, the OP input voltage stays at negative rail. Input bias current is increased at low common mode voltage, causing an additional offset. This may a be problem in precision amplifiers, but not for the present circuit.

 

[E-design]

Feedback compensation values look good in my simulation results, but R9 seems a bit high in value for my liking.

 

[treez]

As long as one of the OP inputs doesn't swing below the negative rail, the basic OP function will be maintained. In so far there's no problem with this circuit. By working of overall feedback, the OP input voltage stays at negative rail

The opamp inout on the inv pin can swing below the negative rail.

The point is, how does the opamp know that the inv input has gone below the non-inv input if the non-inv is clamped to the negative supply rail?..it cannot.....this is surely not a proper setup for a regulating circuit?

I know the circuit works....but the dynamics are surely affected to the point where the transfer function calculation for this opamp will not be true, and a proper gain phase margin in the overall smps feedback loop cannot accurately be worked out.?

 

[FvM]

You considerations about negative input swing are essentially wrong.

As long as the feedback loop is active, the OP has zero input differential voltage and the inverting input stays a negative rail. A negative input voltage can occur while the OP is in saturation. As in any other regular OP circuit, the OP will recover from saturation by working of the feedback loop, pulling the input differential voltage back to zero. It's however important that the OP doesn't show phase reversal while the common range is exceeded, which apparently doesn't happen with this OP.