Charge Pumps Vs Voltage Multipliers

Rectiiers Vs Voltage Multipliers

Hi Folks,

What are the major considerations while selecting rectifiers or Voltage Multipliers in any design?I have seen some RFID systems with Rectifers and Some other are designed with Voltage multipliers for ac-dc conversion. Is there any selection criteria?

The RF in RFID: RFID tags
What is the mechanism behind stacking(different stacking mechanism) of Dickson's charge pump shown in above url?

Please clarify me.


Thanks!

Re: Rectiiers Vs Voltage Multipliers

:razz:hi idont know about rfids but charge pumps are used to get the negative voltage also where as multipliers cant

this charge pump to make a bigger voltage from a small voltage.

The RF tag takes small voltage from the air and make it bigger with the charge pump so the ic will work.

xxtigerxx said:

this charge pump to make a bigger voltage from a small voltage.

The RF tag takes small voltage from the air and make it bigger with the charge pump so the ic will work.

Click to expand...

Can you explain the mechanism behind such type of stacking for chargepump shown in url?

To step up from AC (example, mains) then it's handy to use a voltage multiplier (capacitors and diodes). It can use conventional components.

To step up several times from a few volts DC (example, battery), it needs to be chopped into pulses. If the next stage is a voltage multiplier then you lose a bit due to diode drops. So there's an argument for using a coil-switching topology.

A voltage multiplier normally outputs a particular voltage depending on how many stages you add. A switched coil can be adjusted to a desired volt level using various adjustments.

Coil-based methods require more experimenting if you do it yourself.

BradtheRad said:

To step up from AC (example, mains) then it's handy to use a voltage multiplier (capacitors and diodes). It can use conventional components.

To step up several times from a few volts DC (example, battery), it needs to be chopped into pulses. If the next stage is a voltage multiplier then you lose a bit due to diode drops. So there's an argument for using a coil-switching topology.

A voltage multiplier normally outputs a particular voltage depending on how many stages you add. A switched coil can be adjusted to a desired volt level using various adjustments.

Coil-based methods require more experimenting if you do it yourself.

Click to expand...

Thanks BradtheRad. I got the point.
But the only bothering point to me is the cascading structure of Voltage multiplier. Why such casacading structure is chosen? I am talking about the voltage multiplier structure chosen in the url.
Some articles say that cascading structure is Chargepump and some articles in Internet mention the same cascading structure as ChargePump.
I am confused. Now, pls clarify me.
If I do google about Voltage multipliers, structure for doubling/tripling output voltage is shown in one manner.
But the schematic shown in thr url is little different. Pls help me to understand the difference.

The only difference in voltage multiplier schemes I'm aware of is in the placement of capacitors, that can be either parallel or series connected. The diodes are always series connected.

The different capacitor connection schemes aren't a big thing in my opinion. Series connection can be usually found for high voltages rectifier cascades to reduce the individual voltage rating. For signal level multiplying rectifiers, parallel circuit allows lower values, which is particularly an advantage with integrated MOS capacitors. Both circuits are more or less equivalent.

Your question title is asking for when to use multipliers. It depends on the available RF voltage/current and supply requirement of your circuit. There's always an impedance matching aspect involved. To succesfully apply voltage multipliers, the source impedance must be sufficiently low.

FvM said:

The only difference in voltage multiplier schemes I'm aware of is in the placement of capacitors, that can be either parallel or series connected. The diodes are always series connected.

The different capacitor connection schemes aren't a big thing in my opinion. Series connection can be usually found for high voltages rectifier cascades to reduce the individual voltage rating. For signal level multiplying rectifiers, parallel circuit allows lower values, which is particularly an advantage with integrated MOS capacitors. Both circuits are more or less equivalent.

Your question title is asking for when to use multipliers. It depends on the available RF voltage/current and supply requirement of your circuit. There's always an impedance matching aspect involved. To succesfully apply voltage multipliers, the source impedance must be sufficiently low.

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Thanks FvM.
As I mentioned earlier, when I came across come IEEE papers, some papers mentioned voltage multipliers and some papers mentioned rectifiers in their design. I searched in google but I didn't find any relevant answer for when and how voltage multipliers are used.
Moreover, casacading structure was a big Q mark to me. Moreover, enigmaticconsulting.com website has shown a chargepump in similar structure to Volatge multiplier design which I found in some IEEE paper.

Can we use rectifier + charge pump in our design to get our required output voltage?
Is it a good design to use voltage multipliers alone to boost the output voltage? Or a combination of voltage multiplier and Chargepump will do the job?
In which cases do we prefer to use Voltage multipliers instead of Rectifiers?
Will the output Power increases with the use of Voltage multipliers (as there is an increase in output voltage)?

Thanks!

I don't see a difference between "charge pumps" and "voltage multpliers" so far. It looks like both terms are used for the same thing.

FvM said:

I don't see a difference between "charge pumps" and "voltage multpliers" so far. It looks like both terms are used for the same thing.

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As far as I know, chargepumps are dc-dc converters and Voltage multipliers are ac-dc converters like rectifiers.
Pls correct me if I am wrong.

As far as I know, chargepumps are dc-dc converters and Voltage multipliers are ac-dc converters like rectifiers.

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Not necessarily. In your link, the voltage multiplying rectifier is e.g. called "charge pump".

The technical difference between AC/DC and DC/DC is the need of an additional generator/oscillator for DC/DC. The rectifier part is basically the same, if it's multiplying the input voltage using capacitors and either diodes or synchronous switched, you can also call it a charge pump.

your right mr shaikss both are same but chargr pumps can give negative voltage with normal voltage but where as regulators can't

Hi Shaikss,
You see the difference of generation of voltages in Charge pump and regulators. Charge pumps use current and capacitors to build voltage level, while Regulators (linear/switched) use output voltage regulation. ( what is regulation?)( this may be voltage or current controlled).
Charge pump have certain advantage:
1. simple circuitry
2. due to V-I converter, Charge pumps reduces stability problem. ( we can say Charge pumps are more controlled)

varunkant2k said:

Hi Shaikss,
You see the difference of generation of voltages in Charge pump and regulators. Charge pumps use current and capacitors to build voltage level, while Regulators (linear/switched) use output voltage regulation. ( what is regulation?)( this may be voltage or current controlled).
Charge pump have certain advantage:
1. simple circuitry
2. due to V-I converter, Charge pumps reduces stability problem. ( we can say Charge pumps are more controlled)

Click to expand...

I understood the difference between chargepump and regulator. But I was confused with Voltage multiplier and chargepump earier. FvM has clarified it.

There is a voltage multiplier (a quadrupler) in the article linked by the OP. The caption says 'Dickson charge pump'.

I have seen the same schematic called a 'half-wave parallel multiplier" at the following site. It lists a large variety of voltage multipliers.



From what I gather, 'voltage multiplier' has to do with the conventional method of AC being applied to capacitors and diodes.

'Charge-pump' seems to be a concept describing the action when charging and discharging a cap results in transferring its charge somewhere else.

Or perhaps when switching devices are used. Here's an article which shows a simple charge-pump doubler (Fig. 1). I have not seen this schematic elsewhere:

http://micrel.com/_PDF/App-Notes/an-1.pdf

It shows a switch near capacitor C1.
C1 has the action of the charge-pump.
C2 does not. It acquires a stable charge.

Perhaps the RFID article said 'charge-pump' because the author didn't want to make it seem like a conventional voltage multiplier? Or because the caption mentions mosfets as switching devices.

'Charge-pump' is a catchy name but I think it should be confined to the action on a capacitor. The capacitor may or may not be part of a voltage multiplier.

The RFID circuit has 4 capacitors but only 3 act in a charge-pump fashion. Current goes through them during both the positive and negative cycles. The fourth cap is different. It is there to acquire a stable charge, not to be in charge-pump action.

If you replace the double-throw switch in the Micrel "charge pump" by an AC voltage source, it's a standard voltage doubling cascade rectifier. It has also names like Cockcroft-Walton or Greinacher multiplier. So where's the principle difference?

Another option for different charge pump implementations is to use synchronous rectifiers (switches) instead of diodes. But the basic operation is still the same.

In my opinion, trying to relate these arbitrarily used names to circuit differences leads to nothing...

Dear Friend,
Both are same. In RFID field,we use charge pump approach in order to generate DC voltage at the output which is used in passive RFID Tags. Diodes and capacitors are used to make this circuit. Input RF amplitude is generally low. So, we need multiple stages of voltage doubler.
In RFID, Rectifier block is combination of voltage multipliers, limiter and regulator (As you have seen in IEEE Papers).

---------- Post added at 22:11 ---------- Previous post was at 22:10 ----------

Dear Friend,
Both are same. In RFID field,we use charge pump approach in order to generate DC voltage at the output which is used in passive RFID Tags. Diodes and capacitors are used to make this circuit. Input RF amplitude is generally low. So, we need multiple stages of voltage doubler.
In RFID, Rectifier block is combination of voltage multipliers, limiter and regulator (As you have seen in IEEE Papers).

Charge pump is a kind of broad term, You can use that method to develop voltage level or to follow the input voltage.

FvM said:

If you replace the double-throw switch in the Micrel "charge pump" by an AC voltage source, it's a standard voltage doubling cascade rectifier. It has also names like Cockcroft-Walton or Greinacher multiplier. So where's the principle difference?

Another option for different charge pump implementations is to use synchronous rectifiers (switches) instead of diodes. But the basic operation is still the same.

In my opinion, trying to relate these arbitrarily used names to circuit differences leads to nothing...

Click to expand...

Nevertheless aren't a few of us like the OP, trying to zero in on the distinguishing characteristics? There is a distinction. I for one have a collection of voltage multiplier circuits. Each one needs a descriptive name (as well as doubler or tripler or quadrupler, etc.).

There's Greinacher and Cockcroft-Walton and Villard-cascade and parallel and series-parallel and half-wave and full-wave. Throw in a few unconventional designs as well.

I have not come across many single-switch charge pumps. And what is a charge pump? Here's the opening line at wikipedia:

"A charge pump is a kind of DC to DC converter that uses capacitors as energy storage elements to create either a higher or lower voltage power source."

So a charge pump is not the same as a conventional voltage doubler.

The Micrel doubler is different from our garden-variety voltage doubler. Notice that the DC supply rails stay connected at all times. All the action derives from just one pulse train.

The Micrel datasheet is also unusual because it shows us what they put inside the black box. Countless manufacturers just slap on the caption 'charge pump'. I have a hunch they want to keep their working circuit a secret. Hence it's common to see articles about charge pumps as they operate in theory, with 'switches' rather than a working circuit.

Which brings us back to the RFID multiplier. Is it a voltage multiplier or a charge pump? Is the author calling it something other than the conventional term? For whatever reason? Because it seems to me that it's more of a voltage multiplier than a charge pump.

I guess you know, that term definitions can't be right or wrong. They are suitable or not, commonly used or unusual. Although I agree, that the term charge pump is often used in conjunction with capacitive voltage converters, there's no fucntional difference to the voltage doubling rectifiers.

So a charge pump is not the same as a conventional voltage doubler.

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You can't conclude a real difference from just the definition in a Wikipedia article.

Please consider, that the original question was not about a suitable name for the circuit rather about decision criteria for different circuits. But these circuits aren't different (whatever you call them).

If you ask, is there a reason to use a different drive frequency for the RFID voltage doubler than the carrier frequency received by the antenna, my answer is: No, as far as I know.