How to Select Bidirectional DC/DC Converter?

It is not entirely clear what you are trying to do.  Are you trying yo supply a constant 24 V output from a 5 to 35 V source?  Or charge a capacitor bank up to 35 V from a 34 V source?  Perhaps a block diagram may help.

Hi John

If you imagine a DC-DC converter, the left side is connected to an ultracap, so it's voltage will vary from approx. 5V - 35V depending on it's state of charge.

The right side of the DC-DC converter is connected to a 24V DC bus, making it relatively constant.

So the DC-DC converter should be bidirectional to allow both discharging of the ultracap to DC bus, as well as charging of ultracap from DC bus. Therefore, the converter must be controllable so I can control it's direction and magnitude.

I think your best bet is two independent buck boost converters.  One for 24 V input to 5 V to 35 V output for charging charging.  The other 5 V to 35 V input to 24 V output.  You can independently control which is operating based on your monitoring processor.

Hi John

So which category on the TI products page should I find the required buck-boost converters?

Is it Power Modules -> DC/DC Switching Regulator -> Controller (External Switch) -> Buck/Boost Inverting Controller?

Also, the 'Controller (Integrated Switch)' and 'Charge Pump' categories don't seem to have the 24V output required.

Can you suggest a model based on my specs in the original post? Thanks!

Your specification is not complete.

converter 1 = 5 to 35 V input, 24 V output at ??A

converter 2 = 24 V input, 5 to 35  V variable output at ??A (or is this just a constant current charger?

converter 1 = 5 to 35 V input, 24 V output max at 3.5A

converter 2 = 24 V input, 5 to 35  V variable output. Max input at 3.5A. So assuming 100% efficiency, the output would vary from 2.4A to 16A.

The 24V, 3.5A side specification is fixed. For the 5 to 35V side, the range can be reduced if no such converter is available.

If the 24 V input max current is 3.5 A, does it have to draw that?  You could limit the current on the output side to some other range.  That would make that solution easier, or make the maximum output less than 24 V.  Then you could use a simple buck converter.

Ok, let's say I modify my specs

Left side: 5-17V variation

Right side: constant 24V DC bus, max current at 3.5A (this end is connected to a inverter/motor, and may reach 3.5A depending on load)

Is there a single bidirectional DC/DC converter which can do the above application, i.e. buck in one direction, boost in the other? Thanks

No, you will need two converters still, buck from 24 to whatever from 5 to 17 (is this supposed to be constant current charging?) and boost from 5-17 input to 24 V output at 3.5 A max.  You will have to provide ythe logic to swtch between the converters.

Ok thanks. No, it wouldn't be constant current charging . The current input at the 24V DC bus may be fluctuating, so the output current (ultracap end) would also fluctuate.

So what would be the suggested converters for the above application, considering they need to be controllable (i.e. a microprocessor would do the PWM signals)?

So what voltage do you want to buck the 24 V input down to?  How much current?

The buck converter would be 24V input to 5-17V output. The input current would be max 3.5A. So output current would be approx max 16A assuming 100% efficiency.

The boost converter would be exactly opposite, with 5-17V input to 24V output. The output current would be max 3.5A.

The 5-17V range can be narrowed to say 10-17V if no such converter exists for the original specs.

You need to specify the output voltage of the buck. Typically it will be a fixed output set by appropriate feed back resistors.  It can be adjustable, but what will be used to adjust the output?

Is it possible for a microprocessor to control the PWM signals for the switches? That would adjust the output.

That would essentially be designing your own controller.  The usual method to adjust output is to change the lower resistor in the feedback path.  You could do that with a digital potentiometer or by switching resistors in and out of the FB path using digitally controlled FETs or switches.

Ok, that seems like a good alternative.

Can a microprocessor control a digital potentiometer?

Otherwise, I could use a microprocessor to control the FETs to switch the resistors in/out.

You might want to look at some on line resources to help you choose.  Here is an example:

http://www.analog.com/static/imported-files/overviews/Choosing_the_Correct_Digipot.pdf

I2C is a popular option.  We have some converters that operate directly from I2C control.

Thanks, I will take a look at that article.

So now, back to the original question. Assuming, I use a digipot or FET switches to control the output of the DC/DC converters, what would be a suggested converter or which product category should I look for it in?

What's the difference between 'integrated switch' and 'external switch' converters?

In addition to the previous post, I have another question.

If I use constant current charging for the ultracap, what would be different in the set-up? Can the constant current output be adjusted using resistors as well? (i.e. output current can be varied)

In general, there are 3 levels of integration.  Modules have all the external components integrated, they are plug and play. Some do require you to supply appropriate input and output capacitors.  Easy to use.  Integrated solutions mean the switching FETs are on the same package as the IC.  They may be integrated on the same die or sometimes multichip separate FET die mounted in the same mold compound.  Not as easy to use as a module, but many of the power supply functions are integrated and you don't have to design them. Controllers use external FETs.  They are the most flexible, but usually targeted towards the advanced designer.  You probably want to stick with integrated solutions, but your requirements may drive you towards controllers anyway.  

Usually, the output voltage is fed back to the controller.  You are regulating the voltage.  For constant current application, you have a series current sense resistor with the load.  The voltage generated is fed back to the converter.  The converter attempts to keep the current constant.  The output voltage will be whatever as long as it is in the range (for buck converter) below VIN.  When you decide on your topologies, I can try to help you select the simplest option.

Thanks for your explanation. So for the buck converter, these are my requirements.

Input: 24V constant, max 3.5A

Output: 5V to 16.2V range. Requires adjustable constant current output from 5.2A to 16.8A (assuming 100% converter efficiency)

This means I can vary the output current required, i.e. 6A for 1st minute, 7A for 2nd minute, etc.

Ideally, a microprocessor would control the switching directly, but as you said in previous posts, this would be equivalent to building your own DC/DC converter. So preferably something simpler, like an integrated or module solution. A digipot or FET with resistors to control the current output would be alright.

So what would be a suitable buck converter for this situation?

For the boost converter, let me think about the requirements again and I'll post again later.

So this is the requirement for the boost converter.

Input: 5V to 16.2V range. Requires adjustable constant current input from 5.2A to 16.8A

Output: 24V constant, max 3.5A.

In summary, this means I'm using  constant current charge (adjustable) and discharge for the ultracap.

*bump*

Please help!

*bump^2*

If no such specifications are available, then you could give me the closest one. The DC-link side is fixed at 24V, so I can only vary the required specifications on the supercap side.

I would suggest you use Webench to search for the best converters.  There is still one thing I am not clear on though.  You are basing your buck current requirement on the maximum 3.5 A available from the 24V rail.  Actually, the load side will determine the output current, not the input side.  for a fixed load impedance, you would then be drawing maximum current at the higher output voltage rather than the lowest output voltage as you suggest.  But I suppose you can still use that criteria as your Webench inputs if that is really what you want.

Ok, I will check out Webench,

To answer your question, the buck converter would be used for charging the ultracap from the 24V DC-link. The charging will occur when there is regenerative braking from the motor connected to the DC-link, and the maximum current generated would be 3.5A. So the regenerative current may fluctuate.

So I would want an adjustable constant current ultracap charging. Let's say we want to charge the ultracap at a constant 1A. So naturally as the ultracap is being charged, its voltage would increase (the 5V - 16.2V specification as mentioned earlier). This means that the current input to the converter would decrease as the ultracap is being charged. In the next moment, I may want to increase the ultracap charging to 2A to combat the decreasing current input to the converter. That's why the current output or input of the converter must be adjustable.

The rest of the current not going to the ultracap would be absorbed by the battery, which is directly connected to the 24V DC-link.