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How do I use webbench to design a variable output supply

Jeff Sylvester55
Prodigy 70 points
Other Parts Discussed in Thread: TPS40060

I am using webbench to design a power supply and I am evaluating simple switcher and other switcher parts.  there are about 20 of them and so it is a toss up of price v.s. board area v.s. cost, etc..  my question is all the designs have you type in a fixed load.   in my case 5 A, and I want output of 12V to 0V.   effectively.   We are designing a heater control and instead of doing PWM with a transistor on 12V output to vary heat,  we want to do varying voltage.  12V down to 0v.   My question is i see on some of the designs it is not recommended that output goes less then 5 v.   so how do I know which switcher chip is compatible with this kind of design.  (other than just using a liner regulator which will probably get too hot?)

for the record power in is 24VDC,  and output is wanted to be 12V @ 4A,  (this is full on 100% power into 3 Ohm load for 48Watts)  down to effectively 0 or perhaps low voltage of 3V @ 1A (3Watts) 

On most of the switcher products, can you just add a variable resistor into the feedback pin to effectively make all the regulators Adjustable.   If so, where is the gotcha point or failure point where I hit the device limits? (for example instability, or poor regulation, large noise output, or internal comparators and voltage references cant be overcome. )

  • 0
    TI__Guru**** 191445 points

    Let me make a few general comments.  Yes you can use a variable resistor in the FB path to vary the output.  There will be some maximum voltage, usually limited by maximum duty cycle or minimum off time constraints.  The minimum voltage will be limited by the internal reference, usually less than 1 V.  So your output voltage can vary between Vref and about 85% of Vin (maybe higher).  For your case, you only need 12 V max from a 24 V source so that should not be a problem.

    Another factor is the control method.  For voltage mode control, the lower feed back resistor that you will vary is not inside the ac control loop.  You can vary the output voltage over a wide range without affecting loop.  Changing the output voltage will change the inductor ripple current though, so you will need to take that into account for your design.  For example the inductor ripple current can vary 775% from Vout = 0.8 to Vout  = 12 V.

    For current mode control, the resistor divider is included in the ac transfer function.  So you will need to design your loop compensation to accommodate both extremes.  The inductor ac ripple current is also affected as above.

    Over that wide range I would probably recommend a voltage mode control device.  I might look at TPS40060 to start with.  After you pick a specific IC, you might want to post a separate topic in the power and lighting tools forum to ask how to vary the output voltage in webench (I think you will find it is not easy,  you will probably have to create separate designs for each case you want to check).