DC/DC Converter Question

[Speakerguy]

I am having an argument with a coworker.

You have a 3V input voltage. You have two DC/DC converters, one step up to 5.5V and another step up to 5V.

You connect a load across 5.5V and 5V of 1 ohm. 500ma of current flows through the load from 5.5V to 5V.

When the 5V regulator sinks this 500ma, where does it go? to ground or to Vin?

Thanks!

 

[Speakerguy]

Follow-up: same question, but bucking 3V in down to 2.5 and 2V, connecting load across 2.5 and 2V.

 

[Hero999]

In reality the DC to DC converter probably won't sink the current, the 5V converter will shut down and read 5.5V. This is true unless it has a load on it drawing more than 500mA of course, in which case it'll go through the load and read 5V.

 

[Blatman Bond]

Most boost converter won't sink the current. But synchronous buck converter do it well.

 

[Speakerguy]

Will it sink it to ground or Vin from the high voltage? Vin is two series AA batteries if it matters. Thanks,

Mark

 

[Hero999]

No, your converter converts 3V to 5V so it's a boost converter and as such it doesn't sink the current at all, the output will just sit at the highest output power suply's level.

 

[mneary]

You cannot buck two AA cells down to 2.5 volts. At least not for long. A "1.5V" alkaline cell supplying a moderate current is 1.2 volts after only 10% of its life. At about 50% it's about 1.1 volts.

A NiMH or NiCD is 1.2 volts for most of its charge.

 

[Blatman Bond]

There are two question.
For boost converter, the lower (5V) DC-DC converter will open (almost no current).
For buck converter, synchronous buck converter sink it to the ground.

 

[Hero999]

I think you're both missing the point. The DC-DC converters in question steps up 3V to 5V and 5.5V respectively therefore they must be boost converters which will not sink current.

 

[Bob Scott]

I think you're both missing the point. The DC-DC converters in question steps up 3V to 5V and 5.5V respectively therefore they must be boost converters which will not sink current.

Speakerguy's situation is hypothetical. I bet in reality that both supplies would normally each have a load resistance to ground. If the 5V supply has an external load greater than 500mA, then you can add the 1 ohm resistor between 5.5 and 5V and will then reduce the current demand of the 5V supply by 500mA.

Of course in reality, I can't think of a reason that a piece of equipment would need those separate 5.5 and 5V voltages available and referenced to the same ground.

As a real example, I did personally did design a circuit in which a 12V computer fan was wired between +12V and +5V supplies inside a computer. Thus the fan ran slower on the difference voltage of 7V. It works because the normal load on the +5V line required a lot more current than the fan. It actually slightly relieves the current load of the +5V supply.