Load Control Circuit: Reduce Wasted Power in Design

[PMASwork]

Hi, all.

(please see the attachment in the next post)

I'm re-using an old power converter in a design, which unfortunately requires a rather large minimum load to properly regulate. I can't alter this converter; I have to use it as-is. But, I can monitor the output, and add circuitry as needed to a new card that is being conencted to the old converter's output.

I'd like to utilize a circuit that will provide this wasted minimum load, but will also reduce this wasted load as the useful current (to the rest of the system) increases. That is, I'm intetrested in wasting as little power as possible, given my design constraints.

I thought about using a small resistor in series with the useful load, and monitoring the voltage across this resistor (call it Vx). I would want to effectively increase the resistance of the wasted load as Vx increases. This would drive down the wasted power as the useful power increases.

At first, I thought of using an op amp as a differential amplifier (amplifying Vx), and connecting the output of the op amp to the gate of an n-channel FET (having the FET operate in its linear region, effectively as a resistor). But, for a constant Vds, an increase in Vx would decrease the Rds of the FET, not increase it.

I can't hlep but think that I'm close here, but I'm missing some key tweak that would make this work.

Any ideas?

Thanks.

 

[PMASwork]

Crude drawing:

 

[berkeman]

Hi, all.

(please see the attachment in the next post)

I'm re-using an old power converter in a design, which unfortunately requires a rather large minimum load to properly regulate. I can't alter this converter; I have to use it as-is. But, I can monitor the output, and add circuitry as needed to a new card that is being conencted to the old converter's output.

I'd like to utilize a circuit that will provide this wasted minimum load, but will also reduce this wasted load as the useful current (to the rest of the system) increases. That is, I'm intetrested in wasting as little power as possible, given my design constraints.

I thought about using a small resistor in series with the useful load, and monitoring the voltage across this resistor (call it Vx). I would want to effectively increase the resistance of the wasted load as Vx increases. This would drive down the wasted power as the useful power increases.

At first, I thought of using an op amp as a differential amplifier (amplifying Vx), and connecting the output of the op amp to the gate of an n-channel FET (having the FET operate in its linear region, effectively as a resistor). But, for a constant Vds, an increase in Vx would decrease the Rds of the FET, not increase it.

I can't hlep but think that I'm close here, but I'm missing some key tweak that would make this work.

Any ideas?

Thanks.

You will have to be careful to avoid stability issues when you try to adjust your ballast load current versus the useful application current. Depending on how your useful load varies (how often, how fast, and by how much), it may be pretty hard to have your ancillary circuit work stably with the converter.

Can you make the wasted/ballast current do something useful, like fun a fan and/or light a light? Maybe run a nice lighted sign with your company name on it or something... Or think of some other semi-useful feature or ancillary task that can burn this minimum load current all the time...

To illustrate the bandwidth/stability issues, consider when the useful load is heavy, and your ballast current is low. All of a sudden, the useful current drops to zero, and your ballast current has to respond quickly enough so that the power converter output does not overvoltage and blow the application circuitry. But having such a fast response time means that the ballast circuit is operating within the bandwidth of the feedback circuit of the power converter, which could cause stability issues with the two control circuits "talking" to each other, and also leading to an oscillation that causes an overvoltage transient and blown application circuitry.

 

[PMASwork]

That is a very good point indeed; the bandwidth of any control circuit I add can affect the frequency response converter itself. I could perhaps design a filter to mitigate this, but I really don't want to add extra components if I don't have to.

Unfortunately, there's nothing useful to be done with this excess power in this particular application. Thermal is a huge issue.

Part of the problem is that the specification for several of the system loads is 0A minimum. Experience tells me that this is not really the case; I'm trying to press folks to find out what the minimum actually is. That will help me here.

Thanks for your feedback.

 

[berkeman]

That is a very good point indeed; the bandwidth of any control circuit I add can affect the frequency response converter itself. I could perhaps design a filter to mitigate this, but I really don't want to add extra components if I don't have to.

Unfortunately, there's nothing useful to be done with this excess power in this particular application. Thermal is a huge issue.

Part of the problem is that the specification for several of the system loads is 0A minimum. Experience tells me that this is not really the case; I'm trying to press folks to find out what the minimum actually is. That will help me here.

Thanks for your feedback.

What about the fan approach? Especially if the thermal situation is an issue...? Or add a display that shows system status or something...

 

[vk6kro]

If there isn't enough load, presumably the voltage from the power converter rises.

So, if you add a shunt regulator, this will pull the voltage down when needed by drawing extra current.

There are lots of shunt regulators here:
http://www.google.com.au/search?hl=...=1&q=shunt+regulator&aq=f&aqi=g1g-m2&aql=&oq=

 

[skeptic2]

Berkeman has some good suggestions especially the fan idea. Anything you can use to take some of the heat away from the transistor is good. Even a power resistor attached to the drain would help.

What is the maximum current the power converter supplies? Perhaps you could represent the maximum current with a fixed voltage say Vmax. Then as you monitor the current in the load, you could subtract the voltage representing the current through the load (Vload) from Vmax. The remainder should be the current through Rwasted. A good way to monitor Rwasted would be with a resistor between the source and ground.

Vmax is a reference voltage adjusted with a pot.
Vload is the voltage developed across a resistor in series with the load (opamp1).
Vload is subtracted from Vmax with an opamp (opamp2).
The output from opamp2 goes to the noninverting input of opamp3.
The inverting input of opamp3 comes from the top of the source resistor, part of Rwasted.
Unless some of the opamps have gain, the source resistor should have the same resistance as the resistor in series with Rload.