pwm vs switched capacitor dc to dc converter - Page 1
Forum section: electronics / beginners

author: alireza7
author_rank: Regular Contributor
timestamp: February 11, 2018, 08:06:53 pm
content: hias i search there are two types of dc to dc converter (i don't know if there is other types)on of them is switched capacitor and an other type is pwm.what are the differences between these two types? what are the the applications of each type ? which one is more efficient? which one's output have less noise?

author: Benta
author_rank: Super Contributor
timestamp: February 11, 2018, 08:21:52 pm
content: The two types of DC-DC converters are "switched capacitor" and "switched inductor" (which is never called that in real life), the difference being which kind of energy storage elements are used - capacitors or inductors.In practice, switched capacitor converters are only used up to a few tens of milliamperes, mostly for voltage inversion/doubling.For inductor-based converters, there are buck, boost, invert, flyback... the list goes on and on, and power levels from milliwatts to megawatts are possible.You'll need to be more specific.

author: ovnr
author_rank: Frequent Contributor
timestamp: February 11, 2018, 11:00:34 pm
content: Quote from: Benta on February 11, 2018, 08:21:52 pmGenerally. There are exceptions, like theLTC7820switched capacitor DC/DC converter, which puts out 5+ amps at >98% efficiency.

author: Benta
author_rank: Super Contributor
timestamp: February 11, 2018, 11:23:50 pm
content: Quote from: ovnr on February 11, 2018, 11:00:34 pmInteresting. I didn't know that one, thanks.

author: ArdWar
author_rank: Frequent Contributor
timestamp: February 11, 2018, 11:47:58 pm
content: Dunno if there's already some development or proprietary design, but on classic switched cap converter you usually only able to convert the input voltage to fractional value of the input (2/1, 1/2, 1/3, 2/3, 1/4, 3/4,... or inverting variant of those) and can't really regulate the output on the fly.Larger number of denominator isn't commonly used because it involves more switches, caps, and of course losses.

author: David Hess
author_rank: Super Contributor
timestamp: February 12, 2018, 05:56:57 am
content: Efficiency for a charge pump depends on minimizing the change in capacitor voltage so for instance with a regulated 2:1 buck charge pump, efficiency drops as the input voltage rises as can be seen in theLTC3251datasheet. Some regulated charge pumps dynamically switch between different ratios to keep efficiency high over a wide input and output voltage ranges.Inductor based converters can have very broad efficiency curves.

author: alireza7
author_rank: Regular Contributor
timestamp: February 12, 2018, 07:44:21 am
content: which one have better regulation and lower noise?particularly which one is more appropriate for making a negative voltage from a positive voltage for supplying negative rail of an low noise amplifier ?can you suggest me any IC?

author: David Hess
author_rank: Super Contributor
timestamp: February 13, 2018, 01:40:52 am
content: Quote from: alireza7 on February 12, 2018, 07:44:21 amIt is difficult to say and depends on the required current level. Both can benefit from LCR filtering and careful layout.QuoteThis is just going to depend on the application. What is the input and output voltage and output current range?

author: alireza7
author_rank: Regular Contributor
timestamp: February 13, 2018, 10:59:42 am
content: input voltage can be 5v or 12v and output voltage is -5v and maximum 2ma.

author: dmills
author_rank: Super Contributor
timestamp: February 13, 2018, 11:27:36 am
content: An interesting way of seeing this duality is that inductor based designs (which modulate the current thru the inductor) make sense for situations where the supply approximates a voltage source, and cap based designs (which modulate the voltage on the energy storage element) make more sense when the supply approximates a current source.the approximation here is that a voltage source has low output impedance while a current source has a very high output impedance.Modern practise has voltage sources being the normal thing, after all a sufficiently large cap makes anything low impedance, so usually an inductor based design is more natural at any sort of power level where you care.Regards, Dan.

author: David Hess
author_rank: Super Contributor
timestamp: February 13, 2018, 02:21:20 pm
content: Quote from: alireza7 on February 13, 2018, 10:59:42 am2 milliamps is well within the capability of a switched capacitor charge pump and some parts can convert 12 volts to -5 volts. Regulation is also feasible but modulation of the pumping frequency may be undesirable from a noise perspective. Charge pumping from 5 volts to -5 volts without regulation is very commonly done.You could start with the ubiquitous 7660 or theLTC1044which is the improved Linear Technology version.