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Modeling >> Behavioral Models >> how to model a charge pump dc-dc converter
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Message started by dingforrest on Jan 1^st, 2006, 5:42pm

Title: how to model a charge pump dc-dc converter
Post by dingforrest on Jan 1^st, 2006, 5:42pm

hi. I have a quesion to ask.
I need to model a triple charge pump dc-dc converter.Can some one tell me how to model the converter and charge pump used in dc-dc converter?

Thanks.

Title: Re: how to model a charge pump dc-dc converter
Post by jbdavid on Jan 3^rd, 2006, 11:08pm

The questions I'd be asking myself are:
1. WHY am I writing the model ? what simulations am I going to use it in? what measurements do I need to make?
2. Which of the converter specifications are important in those simulations for those measurements?
3. What non-idealities am I depending on/ ?

Next I'd draw out a model schematic
Input characteristices - Transfer functions -(and its independent variables) and output characteristics.. a good reference for this is a charge-pump application note.. (ie from ADI, National, TI, Linear tech etc.. )

Then your about ready to start your implementation.. Which indep variables are block parameters and which are inputs (or simulator variables?)
Fclk, and Cout might be block parameters, but Vdd, and Vref might be input pins from external sources..
but of course its always good to start with the simplest model..

Assuming 3 stages of voltage doubling, why can't you just use Vout = pow(2,3)*Vin ?
(i'll assume there is a reason why.. ie you need to know how the output voltage varies with load, or what the output ripple is..)
or what the input current is.. or SOMETHING!.. )
the answer to that question will direct the non-idealities you need to include.

G'luck!

Title: Re: how to model a charge pump dc-dc converter
Post by jbdavid on Jan 3^rd, 2006, 11:23pm

My general model of a DC-DC converter assumes an input voltage from a thevinen eq. source and has a Thevinen eq output..
so my input current is a function of the input voltage, the transfer function (ie Vout = 8*Vin),
Pout = H*Pin, where H<1.0 and Pout = Vout*Iout..
Rout is a function of the clock rate and Capacitance ,,
so you know Vout, and Rout, the simulator can figure out Iout from your load circuit..
when you know Iout, you can calculate Pout, and thus Pin, and thus Iin ..
then your simulator can determine what your operating point is..
So
V(outi) <+ 8 * V(in);
V(out,outi) <+ I(out,outi)*Rout;// need to pick the right current direction
Pout = I(out,outi)*V(outi);
Pin = H*Pout;
I(in) <+ Pin/V(in) + Ibias; // its probably good to ensure the current (IN) also includes a fixed bias component.
Jonathan

Title: Re: how to model a charge pump dc-dc converter
Post by Jess Chen on Jan 4^th, 2006, 11:02am

I am curious about the charge pump dc-dc converter. Is it essentially a combination of diodes and capacitors configured to generate a high DC voltage when driven with an AC input? Is the application primarily RFID chips? Does the converter use any inductors or transformers?

As for the modeling question, Jonathan is quite right in asking about the purpose of the model. If you are interested in dynamics spanning thousands of clock cycles, you may want some sort of state space averaged model. If you are interested in less than 100 clock cycles, or in a model to validate a more abstract model, you probably want a detailed pulse-by-pulse model.

-Jess

Title: Re: how to model a charge pump dc-dc converter
Post by dingforrest on Jan 4^th, 2006, 6:34pm

Jbdavid & Jess Chen

The application is a TFT-LCD chip. It needs high voltage deriving from battery voltage. We have designed four step-up circuits using switch capacitor charge pump. It took too long time to simulate the dc-dc converter on transistor level. Now we want to write its behavioral model to verify the system and estimate the influence of various capacitors and load etc.

Thanks & best regards

DingForrest