ISSCC 2015 Forum

The Confluence of Resonant Switching Topologies and Wireless Charging

Sanjaya Maniktalasanjayamaniktala@yahoo.com

ISSCC 2015

Sanjaya Maniktala, Feb 26, ‘15 ISSCC Forum

The Game

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This single diagram implies that all three MI- (induction) and MR- (resonance) based WPT methods rely on the same physical principle(s)

All are based on: Faraday’s Law of Induction

Contrary to popular belief: Fundamentally, only the operating points on the resonance curve differ for MI and MR! Coupling (K) is about 20-60% for MI, and as low as 1% in MR.

MI

MI

MR

The Teams

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http://opinionator.blogs.nytimes.com/2012/04/08/the-taint-of-social-darwinism/?_r=0

A bit like our offices…

Our Strategy (in this presentation)

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We will try to develop better “resonant-mode intuition” and tear down PWM-mode intuition (which is usually very misleading).

We will progressively transition from PWM → LLC → WPT.

We will realize that MI-WPT is in essence the LLC topology (but with a variable “K”, or coupling coefficient).

And of course, the “transformer’s” Primary side is now the Transmitter (Tx), and the Secondary side is the Receiver (Rx).

We will highlight certain seeming inadequacies with the WPT-MI standards, in light of our realization/learning above.

Simple Check: Do You really understand WPT

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Standard Interview Question for designers of PWM converters: Draw the voltage and current waveforms in the switches, capacitors and magnetic elements. Over 99% answer this correctly!

Suggested Interview Question for designers of WPT systems: Draw the voltage and current waveforms in the switches, capacitors and magnetic elements. Less than 1% will answer this correctly!

1% of them get hired (competitive)!

99% of the rest will get hired !

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Topology Transitioning

PWM→LLC →WPT

Origin: The PWM Half-Bridge

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Large Cp (just DC blocking, not resonant)

Origin: The PWM Half-Bridge (alternative)

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Center-tapped Version

The LLC (resonant) Half-Bridge

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http://hifiduino.files.wordpress.com/2011/02/p1050217.jpg

Practical Implementation:

Less Coupling

The LLC (resonant) Half-Bridge (alternative)

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Synchronous rectification

Progression: An Impractical WPT System

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Still uses an optocoupler (impractical)

Progression: A Practical WPT System

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But still uses a “transformer”! No optocoupler

WPT Communication (Backscatter)

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Oh goody! Let’s get startednot Λ

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The devil is not always in the details! Don’t miss the bigger picture.

Don’t get hung up on “control error packets”, ARM processors etc., especially if you still think the “Rx

has switching losses”. It doesn’t!

Don’t Get Bogged Down Yet

Overview: LLC and WPT

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WPC

LLC

Secondary side leakage

Secondary side capacitor

Variable coupling

Fixed coupling

The secondary-side leakage branch of WPT hasn’t much effect in practice (and exists in LLC too actually), but Cs is VERY questionable as we will see (assume it is not present for now).

The LLC Transformer Model

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N=turns ratio NP/NS here

“Leakage”

K=Coupling Coefficient

In LLC: Ratio of leakage to magnetization inductance , (1-K)/K is typically 1:4 to 1:9

A WPT Transformer Model (simplified)

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N=turns ratio NP/NS hereK=Coupling Coefficient

In MI-WPT: (1-K)/K can be as much as 2:1, In MR-WPT as much as 100:1In WPC: Ratio of leakage to magnetization inductance , (1-K)/K is typically 1:1 to 2:1

In PMA standard, Cs is not present usually

Control Methodologies (PWM to Resonant)

“GAIN PROFILE”

Resonant power conversion plot:Gain versus frequency

Classical power conversion plot:Gain versus duty cycle in CCM

Since gain profile depends on load, it is more analogous to Discontinuous Conduction Mode (DCM) actually

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We correct the output by changing duty cycle We correct the output by changing frequency

LLC and PWM: Output Voltage Equations

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So in general, Resonant Circuit Gain profile is load-dependent, more akin to a PWM converter in DCM rather than in CCM.

PWM

LLC

“Inadequate” Gain Profiles in WPT

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We see all these issues in WPT,. Why not in the LLC topology?

WPT has a “Transformer” too! Scaling Applies

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In WPT the coupling is lower than in the LLC, andis unfortunately variable too (changes every timethe Rx is placed on the Tx).

“Tx” is the “Primary side” of the transformer“Rx” is the “Secondary side” of the transformer.

Therefore neither should standardpower/frequency/voltage scaling laws, basic “transformeraction” (Np/Ns) and the need for consistent resonant gainprofiles be overlooked, as discussed later.

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Intuition TransitioningPWM→LLC →WPT

"The intuitive mind is a sacred gift and the rational mind is a faithful servant. We have created a society that honors the servant and has forgotten the gift." Albert Einstein

An Intuition: Urban Legend or Jungle Myth?

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“The electrodynamic induction wireless transmission technique is near field over distances up to about one-sixth of the wavelength used. Near field energy itself is non-radiative but some radiative losses do occur. In addition there are usually resistive losses. In one of its forms, electromagnetic induction is based on the principle of using a magnetic field to generate an electric current within a conductor coil. Current flowing through the primary coil creates a magnetic field which acts on a secondary coil within this field, thereby generating current within the (coupled) secondary coil. Coupling must be tight in order to achieve high efficiency. As the distance from the primary is increased, more and more of the magnetic field misses the secondary. Evenover a relatively short range the induction method is grossly inefficient, wasting much of the transmitted energy.”

Sources: http://en.wiki2.org/wiki/Wireless_powerhttp://en.wikipedia.org/wiki/User:GaryPeterson/World_Wireless_System http://en.wikipedia.org/wiki/Wireless_power (As of Nov 2014, apparently revised by now)

This is erroneous PWM-school intuition at work; it does NOT apply to resonant topologies! Don’t instinctively equate high (low) efficiency to good (bad) coupling.

Is this true???

A Sanity Check

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Typical Flyback Efficiency About 70%. Coupling? About 99%.Typical LLC Efficiency? About 90%. Coupling? About 80%.

"LLC Resonant Converters Increase Efficiency in DC-to-DC Applications", by Alex Dumais, Microchip, Dec 7, 2011, Power Electronics magazine

So: What is typical WPT Efficiency? Its coupling is much lower than even LLC : ~ 50% in MI, down to 1% in MR. Is there any correlation with the coupling? Not necessarily!

Higher Efficiency Is Possible even with K ~ 5% (as in MR)

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Source: “The convenience of wireless charging: It’s just physics (white paper)”, by Mark Estabrook, MediaTek

MR-systems are therefore NOT necessarily more inefficient than MI-systems. That is mere FUD (fear uncertainty doubt)

The World of Classical PWM Intuition

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Answer: You may need a refresher course if you think so. Suggest Switching Power Supplies A-Z for example.

Question: Does efficiency fall as we increase the switching frequency?

Answer: Yes! Switching Losses increase with frequency (conduction losses remain steady)

Question: Does efficiency fall as we decrease the switching frequency?

In classical PWM-based systems:

The Topsy-Turvy World of Resonance

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Answer: Yes! (Have you ever seen a 50Hz resonant system?)As we lower the frequency, we allow a greater time: T/2 = 1/(2 × fsw), for the magnetization component to ramp up (see slide 30). So the peak circulating current is much higher, and we lose more energy in the parasitic resistances. Efficiency falls as frequency decreases!

Question: Does efficiency fall as we increase the switching frequency?

Answer: No! (It may even get way better!)Correctly implemented, there are virtually no switching losses in either the Primary side (Transmitter) or Secondary side (Receiver), unlike in classical PWM systems.

Question: Does efficiency fall as we decrease the switching frequency?

In resonant systems:

LLC reference

Efficiency Data from a Test System

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LOSS

EFFICIENCY

Tx Rx

5V@1A max (Audiodev/Nok9

system)

Key Currents in LLC/WPT

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From: Switching Power Supply Design and Optimization, Second Edition

Load current component riding (blue region) on magnetization current component (red line)

LLC tank

IRX_COIL

IMAG

Why Efficiency Usually Improves as Frequency Increases (in Resonant Systems)

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Load current component riding on magnetization current component

(To slide 27)

Loss Visualization in Tx & Rx

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No switching lossAn inverse “switching

loss”

Correcting our (Shaky) Path to WPT Development

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It is imperative to consciously break free from PWM-basedintuition to resonant-based intuition, if we want to do justice to resonant systems.

That is already a major hindrance in the rapid development efforts around WPT/LLC.

Corollary: A newcomer to WPT/LLC will likely learn faster than a“seasoned” or old-school “PWM-expert” with decades of(misleading) experience (like me!)

(We need to consciously start with a fresh mind)

Resonance: Intuitively Explained

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Source:

Resonance: Intuitively Explained

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Inductance and Capacitance are like opposite “genders” because oftheir opposite V-I phase angle signs. Their relative phase angle is90-(-90) = 180 deg, at resonance. Which is just reverse directioni.e. current into inductor is same as current out of capacitor.

So, by using a (small) capacitor along with a (small) inductor, we cancreate a “LC tank”, whereby energy can slosh back and forth (none isnecessarily wasted)!

In traditional PWM circuits, the input/output caps are so big (very lowresonant frequency), we don’t “see” any resonant effects, relative toour much faster switching frequencies (we don’t wait long enough).

Understanding Resonance

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The combination of L and C can be self-sustaining. This is thereforeappropriately called an “L-C tank”.

Reason? In pure reactive circuits (L and C) we can store energy, never lose energy. We lose energy only in resistances, because V andI are then in-phase.

No recycling process is perfect. So, as we slosh energy back and forthbetween the L and C, assuming we have parasitic and/or loadresistances present, we will need to constantly replenish the energylost in resistances (either useful energy in load resistance or wastedenergy in the parasitics). So we drive it continuously e.g. ½ bridge!

If we drive the tank at its resonant frequency we achieve maximum gain (e.g. in MR),otherwise lesser and lesser gain as we move away from fRES (e.g. in MI, with frequencymodulation).

Wikipedia Agrees Finally

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http://en.wikipedia.org/wiki/Electrical_reactance

“For a reactive component the sinusoidal voltage across the component is in quadrature (a phase difference) with the sinusoidal current through the component. The component alternately absorbs energy from the circuit and then returns energy to the circuit, thus a pure reactance does not dissipate power.”

Corollary: Don’t forget that any leakage inductance is nevertheless still an inductance, i.e. a reactance. So, if an inductance is “uncoupled”, it can be used constructively to return power to the input source (much as we do in ZVS phase-shifted full-bridge), or exchange it continually with a reactive element of the “opposite type” (opposite “gender” in a sense, as we do in LLC topology). And that is the phenomena of resonance.

Learning to Think “Resonant”

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Yes, leakage inductance is present in resonant topologies, far more than in classical PWM topologies. But leakage can be intelligently used to our advantage too:

Note: This is also the guiding principle behind “ZVS phase-shifted PWM-based Full-bridge”, which is a “soft-switching” (efficient version of a) PWM topology — it is a crossover topology between strict hard-switching PWM-based and resonant topologies. In fact, it is often considered a viable competitor to the LLC topology. It uses resonance between switch transitions to achieve ZVS action.

a) As part of a resonant tank (along with a resonant C), we can use it to slosh leakage energy back into the input source, rather than burn it up as in the zener clamp of a Flyback.

b) Also, in the act of sloshing the uncoupled energy back into the input rail, we can avail of natural zero-voltage switching (ZVS) action to almost eliminate (switching/transition) losses on the Primary side).

c) We also automatically get soft-switching on the Secondary side: zero current switching (ZCS). This also reduces conventional “switching losses” to near zero.

Resonant systems thus promise to be far more efficient than classical power conversion methods (steam engine analogy). Note: WPT also uses resonance, but not as effectively (yet) as say, the LLC topology.

Truth Resonates Efficiently, provided…

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Condition A: But because we don’t need to switch the FETs hard and fast, we can use cost-effective but very low-Rds FETs for example (with relatively weak FET drivers too!).

True: There is no direct relationship between poor coupling (high leakage inductance) andefficiency.Agreed: it may not be “easy” to transfer energy if the coupling is very poor — because we still need to build up a usable B-field, and that may need a much higher circulating (magnetizing) current, now that there is such a large inherent “air gap” present. But certainly: the leakage energy need not be wasted, thanks to clever use of resonant tanks.

Realization: No process is perfect, so whenever we “circulate” (high) currents, we do incur some losses, based mainly in parasitics such as Rds of FETs, DCR of coils/windings.

Condition B: To avoid switching losses we exploit ZVS by introducing a relatively large deadtime (~100ns) between switch transitions (to allow the switching node to resonate naturally up to the rail before we turn ON the FET).

Condition C: Operate on the inductive (right) side of the resonant peak (for inducing ZVS).

WPT standards make no mention of ZVS! But that is the biggest potential advantage of a resonant system.

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Two-faced! (WPT is LLC)(assume Cs bypassed for now)

missdastyle.comA makeup artist did this (look closely)!

“Shifted Resonance Peak with Load”

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Slide from presentation in Jan 2015By Steve Terry, TI

Slide from company Tech-Talk presentation on Aug 28 2014By Sanjaya Maniktala

We will make faster progress if we realize that WPT-MI is just the LLC topology (in disguise)! (BTW: the peak shifts with load and coupling!)

How Resonant Peak changes due to K

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Aug 2014By Sanjaya Maniktala

The Popularity of the LLC topology

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By using two inductors (L-L-C) and one capacitor, twoextreme-load resonant peaks are generated.…. (one of these two inductors could be discrete, or theleakage of the transformer, as in WPT)

By clever use of that unique profile, it turns out we can limit our entire operating region from zeroload to max load within those two extreme-loadresonant peaks!!!

(that is the fundamental idea behind the LLC topology and explains its popularity, over other resonant topologieswhere you may need to go to very high frequencies to regulate at light-load conditions)

The Popularity of the LLC topology

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The Popularity of the LLC topology

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This peak depends on Lp and Cp

This peak depends on Llk (i.e. K) and Cp

Slide 64

The Magic Operating Point of the LLC topology

In LLC topology, we can span the entire min to max load range at almost one (fixed) frequency: the “magic point”, bycareful correct choice of turns ratio and C/L ratio (to set theright Q)

Wireless power (WPT) also has in effect almost the sameresonant profile, but fails to exploit the above advantage because turns ratio and Q (C/L ratio) are all over the place.

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This magic operating point is akin to CCM, not DCM, since the control variable (D or f) is independent of load here!

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Gain Profile And Gain Target

Analysis

(assuming Cs bypassed at first)

What Determines Actual region of Operation in LLC or WPT?

A) It depends on the Gain profile (Q of the resonant curve)?

(“Supply”)

B) It depends on our Gain target (our output requirement)? (“Demand”)

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What Determines Actual region of Operation in Resonant Systems

Useful to think in terms of supply and demand (where they meet)

If we change the load, so will the frequency (supply will change, but demand is fixed --- for a given input-output voltage)

So we get a frequency spread, that we wish to minimize

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Load Regulation at the Moment

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Gain Target as for the A6-A combination

Load Regulation at the Moment (continued)

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Possible Load Regulation à la LLC

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Compare: Using LLC design suggestions, step load response is almost immediate

Gain Target Math

Switching Power Supplies Design and Optimization, 2e

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Gain Target Readjustment (WPT→LLC)

LLC Magic Operating Point

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Desirable target

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WPT-MI needs to look closely at the magic operating point of LLC.

For interoperability, the gain profiles (“Q”) of existing

approved Tx’s first need to be consistent: i.e. identical for the same desired power level and K.

And that’s not enough either….“Cs?”

Potential Impact of LLC in WPT

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Heuristic and Scaling Principles of Power Conversion

Which we often use without even realizing it!

They are hidden in plain sight!

Beyond Magic Points: What is Scaling?

Sample Scaling Principles

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What if we….

a) Double the load current? (keeping all else the same)…We need to halve the inductance and double the capacitance. That will keep the same loop response for example, since the LC product is unchanged (L/2 x 2C= LC) . However, now (C/L)1/2 is [2C/(L/2)] 1/2 = 2 (C/L)1/2. So power is proportional to (C/L)1/2.

b) Double the input voltage? (keeping all else the same)…We will need to double the inductance. That will keep the same current ripple ratio, in this case the same desired/optimal IAC.

c) Double the frequency? (keeping all else the same)…We need to halve the inductance and halve the capacitance. That will double to crossover frequency as expected.

Suppose we have a 500kHz 12V to 5V @2A Buck Converter, and we have (optimally) selected its L and C.

Sound familiar?

PWM: Something to Keep in Mind

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Gain equation for PWM-based Half-Bridge can be rewritten as follows:

For example if for 5V secondary winding we had 5 turns, then for 12V we will use 12 turns and so on. …if input is 24V and primary winding was say 20 turns, then if input is 12V, we will use 10 turns.

“TRANSFORMER BASED SCALING LAWS”

LLC: Something to Keep in Mind

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Gain equation for LLC-based Half-Bridge can be rewritten as follows:

…For example if for 5V secondary winding we had 5 turns, then for 12V we will use 12 turns and so on. If input is 24V and primary winding was say 20 turns, then if input is 12V, we will use 10 turns.

Essence of My Unique LLC Design Methodology

A low-frequency resonant LLC cell was randomly defined. It could be for example L1=15mH coupled inductance with L2=5mH leakage inductance (K = 15/(15+5) = 0.75), and a large resonant “Cx” of several uF. Assume the open-circuit (lower) resonant peak was thus 100Hz.

We then specified an R across this tank (the reflected output load resistor) to yield the appropriate gain profile shape (Q), to guarantee operation at the “magic point”.

For a given input DC voltage, with the specified R, we found that using Vac2/2R, we could get only 1W from this resonant tank (without distorting its shape).

Suppose we really wanted to operate at 100kHz and 5W. This is how this “cell” was translated to any power or frequency level.

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Essence of My Unique LLC Design Methodology Desired Frequency Scaling Factor is 100kHz/100Hz = 1000. So

we need to use both inductances and capacitances 1000 times smaller.

In the above process, the ratio C/L was however unchanged, so this cell is still only capable of 1W. But we want 5W max from it. So the desired Power Scaling Factor is 5/1 = 5.

So now we need to use an inductance 5 times smaller but a capacitance 5 times more.

In the process, lower freq peak is still 100kHz because the product “LC” did not change in this step! So here are our final component values: 3uH, 1uH, 5Cx(in nF)

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Wish-list for the Future of WPT

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Suppose we have Rx1 designed for 5V/1A (5W). Suppose we also have another Rx2 designed for 2.5V/2A (5W). Suppose we also have three 5W Tx’s: Tx1 working at input 5V, Tx2 with input 12V, and Tx3 with input 19V. This is what should happen, considering they have the same max output powers (under ideal assumption disregarding all losses):

a) If we place Rx1 on Tx1 or Tx2 or on Tx3, it should “never know the difference”. For all practical purposes, any Tx should produce the same operating frequency spread for any Rx load between 0A to 1A. (hopefully very little spread too, if the LLC design strategy is followed)

b) If we place Rx2 on Tx1 or Tx2 or on Tx3, it should “never know the difference” either. For all practical purposes, any Tx should produce the same operating frequency spread for any Rx load between 0A to 2A.

c) None of the Tx’s should know the difference between Rx1 and Rx2 since they have the same power output and frequency range.

That would really ensure interoperability going forward!! But this does NOT happenin WPT-MI automatically as currently defined. It does happen in an LLC designed as per the methodology in Switching Power Supply D&O 2e.

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WPT-MI needs to (re-)consider frequency, power and voltage

scaling principles, similar to my simple design methodology for LLC, also “transformer (scaling) action”, since we now realize in WPT too, we are dealing with a

transformer after all.

63

We should not complicate matters further by also allowing input voltage and/or duty cycle modulation, besides frequency

modulation…and various combinations thereof.

A plugfest can turn into a deadly bugfest ….

Creating Design Rules and Ensuring Interoperability

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Tx Coils Chart For Discussionfres needs to be very tight (e.g.100kHz).

Non-standard C values specified.

Ratio C/L must be maintained for a given max power rating to ensure same gain profile). Not so here.

There are actually no half turns conceptually (current must return to source).Allowed freq mod, PWM mod, input voltage mod too! Confusing to the Rx?

Rx should not “see” any change in Tx used, to assure interoperability.

65

Rx Test Coils Chart for discussion

fres needs to be tightened

Non-standard C values should not be specified.

Actually, there is no 100kHz secondary side resonance. As soon as power is delivered (through secondary), the resonant freq shifts (slide 44). It is at fres only in open-load condition! No need for Cs (see next slides).

Tx should not “see” any change in Rx used, to assure interoperability.Consumer Rx coils approval process too easy with no communicated guidelines on even number of turns vis-à-vis output voltage .

Entirely possible if not probable, that a future Rx will work fine with all existing Tx’s but fail to work with a Tx approved later, with turns ratios not following any obvious rule based on desired input to output voltage ratio.

Need guidelines as to max voltage rating of Cs if used. No engineer seems to know except “we measured it to be…” (but with which Tx?)

Beyond Scaling & Magic Pts: What’s the Role of Cs?

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Inclusion of Cs adds multiple resonances, involving Ls too now, and distorts this simple curve further!

E.g. You can overshoot and start regulating at two other points!

In the best case, gain is simply inadequate and the Rx will be disconnected, or enter a new state….undefined operating modes, not necessarily ZVS compatible either (phase may not be inductive)

Gain profiles being compared here

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Cs is the great unknown (beware)!

At a bare minimum, we should be able tospecify its voltage rating (no trial and error).

Cs and Ls are supposedly “tuned to fres”, as are Cp and Lp. But when Cs gets involved (non-zero load current), the resonant frequency isnot fres anymore, even without Cs (slide 39)

What does Cs really do?

Cs was likely just something added to get a little “last-minute wiggle room” to adjust gain, rather

than design the Tx/Rx turns ratio and “Q” correctly as per scaling principles.

Cs is also dissipative (up to 300mW dissipation onaccount of its ESR). It can lower Rx efficiency by 6%.

68

Any reported increase in efficiency by “tweaking Cs” is coincidental ─ on closer examination it just caused an increase in operating frequency, which lowered the “inverse switching losses” in the Tx, not in the Rx. It could be achieved and surpassed by better design of the gain profile, turns ratio etc.

Cs also costs a lot of money and real estate

So why have Cs?

What does Cs really do?

Is The Future Clear (and Rosy)?

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Seems impractical to test a future Tx with all “approved” Rx’s out there already, with

undefined design criteria and performance.

Approving a new Tx or Rxdesign “that happens to work (as of now)”,

could potentially cause any standard’s interoperability to implode sooner or

later.

We need recommended guidelines for futureRx/Tx designs? Is it already too late?

Overall Summary of WPT-MI Key Issues

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System cannot handle big, sudden load steps

(disconnects, overshoots) The rate of correction may be ~150 ms Large VRECT overshoots occur on startup/load transients Protective clamps usually required in Rx LDO/post-regulator therefore required in Rx Very wide frequency variation to regulate VRECT Dubious role of Cs Interoperability at risk

The innovative LLC design procedure introduced in Switching Power Design and Optimization 2e would have solved all of these, if an algorithm was in place to adapt todifferent couplings. See http://www.freepatentsonline.com/y2014/0369081.html

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http://www.acquisio.com/blog/acquisio-scoop/acquisio-t-shirt-contest-winner-announcement

Sanjaya’s LLC-based WPT, third proof of concept

Sent out on Oct 18, 2013 by company Email

NEXT

Testing Our Proposed Solution

Wait, that’s

not me!

LLC-WPT Experiment

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Selection of turns ratio, Cp operating frequency etc. as per Sanjaya’s LLC Mathcad design tool. No Cs used here either!

www.neatorama.com

Results Speak For Themselves

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Over 15W!Freq variation ± 8%

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This is the measured efficiency of Tx + Rx (system)

Up-s

ide

79% at 15W measured. 87% projected.

83% at 5W measured.92% projected.

Frequency variation ± 8%

Results Speak For Themselves

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Thank You!