Billings, chapters2.17,

“The DC-to-DC Transformer Concept,”and 2.20,

“DC-to-DC Switching Regulators”

John GriffinEE 136 ProjectDecember 2003

Professor Zhou/SJSU

Outline• Chap. 2.17

– Obsolete DC-to-DC transformer design

– Modern transistor-switched DC-to-DC transformerChap. 2.20

• Chapter 2.20– Switch mode families– Cuk converter– Ripple regulator

• Relevant URLs

• Chap. 2.17– Obsolete mechanically switched

DC-to-DC transformer design– Modern transistor-switched DC-

to-DC transformerChap. 2.20

Obsolete mechanically-switched DC-to-DC

transformer

• Chap. 2.17– Obsolete mechanically switched

DC-to-DC transformer design– Modern transistor-switched DC-

to-DC transformerChap. 2.20

Modern DC-to-DC transformer: self-oscillating, square wave,

push-pull converter with bi-phase rectification

– Advantages: simplicity, low cost, size– Disadvantages: inefficient; frequency

varies depending on load & input voltage

DC transformer• Frequency can

change with input voltage and load changes, but with screening the electromagnetic interference (EMI) that results can be screened out

• Circuit does not provide regulation, so normally a regulator circuit is added after this circuit’s output

Square wave transformer circuit analysis: half-cycle 1

Square wave transformer circuit analysis: transition

Square wave transformer circuit analysis: half-cycle 2

Trying to simulate DC transformer…

• Simplest representation works fine…

Trying to simulate DC transformer…

…partially built circuit works fine; output voltage trace was like that of simpler version…

…but for the full circuit Simplorer would not

cooperate• Every try led to error, ‘out of memory’,

etc., such as pictured here…

…and here.

• Varying choices of component sizes could not overcome simulator hanging at its first attempt at transformer polarity change

• Chap. 2.20– Switch mode families– Cuk converter– Ripple regulator

Buck, boost, b-b, Cuk, w / current signals

Which type of regulator to choose?

For good performance without needing detailed design: “For most applications, for up to about 100 W, go with the CCM [continuous current] flyback. Over 100 W, line-powered and isolated, use either the isolated Cuk topology or a half-bridge, controlled as a PFC [power factor corrector].”

-- Dennis Feucht, power elect. Designer (article is in list of URLs)

• Chap. 2.20– Switch mode families– Cuk converter– Ripple regulator

History of Cuk converter• Further work at Caltech,

especially by Cuk in his PhD Thesis, produced a fourth member of the basic dc-to-dc switching regulators which has been described as an optimum topology because of its symmetrical structure and non-pulsating input and output currents. The new optimum topology dc-to-dc switching regulator is now commonly known as the Cuk converter after its inventor.

Cuk converter operation

"The Cuk converter is obtained by using the duality principle on the circuit of a buck – boost converter. Similar to the buck – boost, the Cuk provides a negative-polarity regulated output voltage with respect to the common terminal of the input voltage. The most important feature of this topology is the fact that a capacitor, instead of an inductor, is used as the primary means of storing and transferring energy from input to the output. This causes energy transfer to occur during both ON and OFF gated switch intervals. Operation is performed at almost zero ripple current at both the input and output of the circuit. These characteristics make the Cuk converter the closest to an ideal DC power supply of any topology.”

-- "2001 Future Energy Challenge Final Report“, on

inverter design project at UTEP (article in URL list)

Some Cuk converter applications

• Magnetic Recording Head Bias• Digital camera CCD bias• LCD bias• GaAs FET bias• Positive to negative conversion

(Source: National Semiconductor LM2611A/B 1.4MHz Cuk Converter data sheet)

• Chap. 2.20– Switch mode families– Cuk converter– Ripple regulator

Ripple regulator purpose, action

• Strictly limits output voltage ripple to “hysteresis” of op amp comparator

• Tends to be used with buck converter more than with other types

• Hysteresis ramp-up and ramp-down voltage are defined by inductor, output capacitor, supply voltage, load current

Fig. 2.20.5: Ripple Regulator in a buck converter (basic buck converter is below)

Ripple regulator output voltage: magnitude and frequency can be varied by choice of components.

Often around 40 mV peak-to-peak.

Hysteretic control of ripple by regulator

• From Texas Instruments design guide for synchronous buck converters: “Although some may see the variable switching frequency of hysteretic control as a drawback, it is usually not a problem. Input voltage, output capacitor ESR, capacitor ESL, and output inductance have the most influence on switching frequency. If the input voltage is regulated within a relatively narrow range, and stable filter components are used, the frequency variation is small. Also, since the output voltage ripple is well controlled, this variation is rarely a problem for overall system operation. In addition to excellent output-voltage regulation and user-adjustable output-voltage ripple, the controller also provides user-adjustable soft-start and overload protection, fixed overvoltage protection, and a logic level enable input.”

Ripple regulator simulation

Ideal and simulated ripple reg. output voltage

• Ideal:

• Simulated: as component values vary, ripple size varies; flat peaks of wave indicate saturation of ripple regulator op amp

Relevant URLs / Billings 2.17 & 2.20• Ripple regulator application (re. 2.17)

– http://focus.ti.com/lit/ug/slvu007/slvu007.pdf• Case study of using a DC-to-DC transformer (re.

2.17)– http://www.reed-electronics.com/ednmag/

archives/1995/031695/06df4.htm• Choosing the right power converter article (re.

2.20)– http://www.analogzone.com/col_0106.htm

• History of swtiched-mode power supplies (re. 2.20)– http://www.steve-w.dircon.co.uk/fleadh/mphil/

history.htm• Lots of Cuk information (re. 2.20)

– http://www.energychallenge.org/2001Reports/UTEP.pdf

• Cuk converter applet (re. 2.20)– http://www.ece.umr.edu/links/

power_electronics/CukConverter.html• LM 2611 datasheet (Cuk converter IC) (re. 2.20)

– http://www.national.com/pf/LM/LM2611.html