Wide ouput range power supply

Armond Gauthier  Pierre­Yves Droz

I Introduction ­ specifications

II D2 dependant circuits

III Flyback

I Introduction ­ specifications

II D2 dependant circuit

III Flyback

Goal / Constraints of the project

Offline power supply.

Constraints:- cheap- wide output range

application :Power supply compatible with a wide range of devices

good point :power loss is not a big issue for offline applications, it is just limited by the heat that the box can dissipate.

I Introduction ­ specifications

II D2 dependant circuit

III Flyback

SpecificationsDC Input Voltage 260 V – 390 VMax Power 50 WOutput Current Limit 10 AOutput Voltage 2.5 V – 30 VOutput Ripple Ratio (I and V)PWM Frequency 200 kHz

±15%

0 2.5 5 7.5 10 12.5 15 17.5 20 22.5 25 27.5 300

1

2

3

4

5

6

7

8

9

10

Output range

Vout

Iout I=

D−1⋅V out

L⋅

1

f

We want to limit L to 1mH f = 200 kHz

max power

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

D (Buck)

D2 (Buck/Buck)

D/(1-D) (Flyback)

D2/(1-D) (Flyback/Buck)

D

Vou

t/Vin

I Introduction ­ specifications

II D2 dependant circuits

III Flyback

Exploring the different topologiesAssumption :Minimizing the range in which D varies will help us to reduce power dissipation.

I Introduction ­ specifications

II D2 dependant circuits

III Flyback

D2 dependant circuitsD2 D2

D−10.010 < D < 0.350.26 < D < 0.93 with N=7

0.010 < D < 0.300.36 < D < 0.77 with N=20

I Introduction ­ specifications

II D2 dependant circuits

III Flyback

D2 dependant circuitsFlyback Buck

I Introduction ­ specifications

II D2 dependant circuits

III Flyback

D2 dependant circuits ­ power dissipation

Transistor 25 W (23 W switching)Diodes 6 WTransformer 40 WInductors 2 WCapacitor neglectable

Transistor

Pdiss=

t sw⋅V ds⋅I sw⋅f

C gd⋅V ds

2⋅f

2

Transformer

Pdiss=1

2⋅Lmag I 2⋅f

Diodes

Pdiss=

V drop⋅I

Metrics of Comparison

Assume:● Cost follows power dissipation

● Package and component size● Cost follows switch stress

Why Flyback?

● Small parts count, single transistor● Cheap!

● DC isolation● DCM operation for low power application

I Introduction ­ specifications

II D2 dependant circuits

III Flyback

Switch Utilization

● Switch stress: S =

● Switch utilization: U(D) =

V∗I

P load

∑ S

Governing formula:

U(D) = 1−D∗D

Starting Methodology

● Center D variation around max U(D)● Bias voltage associated with max U(D) towards lower end of output range

I Introduction ­ specifications

II D2 dependant circuits

III Flyback

Flyback converter

Governing equations:

Vout

Vin=

n∗D

1−D

Iout

Iin=1−D

n

Transistor on Transistor off

I Introduction ­ specifications

II D2 dependant circuits

III Flyback

Implementation 1

High VMAX

:

● Look at flyback input º ● Transistor must block“stacked” voltageV

in + V

out / n

Conclusions :

● Look at “n” equation● N proportional to V

D,opt

● To decrease transistor blocking voltage,

increase VD,opt

and n.

n=V D ,opt

V i , nom.

∗1−Dopt

Dopt

I Introduction ­ specifications

II D2 dependant circuits

III Flyback

n .049(20 turns on primary side)D range .12 - .70U(D) range .305 (low D) - .251 (high D)

Transistor stress

Diode StressRequired Inductance 4.73 mH

IMAX

= 681mA, VMAX

= 990V, S = 675 W

IMAX

= 23.9A, VMAX

= 49.1V, S = 1173 W

Implementation 2

Conclusions:

● Sacrificing ?S in favor of reducing peak transistor voltage advantageous● Smaller, cheaper device with lower loss

● U(D) just a general metric, not an end in itself

How to further push down peak transistor voltage and power dissipation?

● Cannot reduce DMIN

too far º too much current stress● Try to reduce ?D...

I Introduction ­ specifications

II D2 dependant circuits

III Flyback

n .075(13 turns on primary side)D range .079 - .606U(D) range .260 (low D) - .306 (high D)

Transistor stress

Diode StressRequired Inductance 3.24 mHOverall max power dissipation 43.0 W

IMAX

= 942mA, VMAX

= 790V, S = 744 W

IMAX

= 22.56A, VMAX

= 63.8V, S = 1438 W

Multiple secondary windings

● Each winding will supply a portion of total output range● Will reduce D, but will require switching between windings

Questions:

● How to divide output range among secondary windings?● How to optimize number of secondary windings?

I Introduction ­ specifications

II D2 dependant circuits

III Flyback

Optimizing ranges● Will minimize D if:

?

● If output range divided equally, low secondary

has largest ?D

M D

n=Constant

Constant {

Advantages● ?D equal for all ranges

● U(D) held closer to optimum● Reduce peak transistor voltage

Disadvantage● Look at transformer inductance required for current ripple versus D º

I Introduction ­ specifications

II D2 dependant circuits

III Flyback

Implementation 3

* Using single diode and capacitor at output high diode stress

I Introduction ­ specifications

II D2 dependant circuits

III Flyback

Range 1: 2.5V < Vout < 5.5V n=.027Range 2: 5.5V < Vout < 12.5V n=.061Range 3: 12.5V < Vout < 30.0V n=.147D range .179 - .440U(D) range .347 (low D) - .371 (high D)

Transistor stress

Diode Stress *Required Inductance 8.14 mHOverall max power dissipation 23.8 W

IMAX

= 780mA, VMAX

= 595V, S = 464 W

IMAX

= 27.1A, VMAX

= 87.3V, S = 2366 W

0 0.25

0.5 0.75

1 1.25

1.5 1.75

2 2.25

2.5 2.75

3 3.25

3.50

1

2

3

4

5

6

7

8

9

10

11

?

M(D) / n

Pdi

ss

Switches on DC side● Advantages:

● Smaller diodes required º cheaper● Less power lost per diode

● Disadvantages:● Multiple diodes and output capacitors● Possible problem with unloaded output

Switches on transformer side● Advantages:

● Single diode and output capacitor required● Output “never” unloaded

● Disadvantages:● Large required diode● Switches must block AC voltage

I Introduction ­ specifications

II D2 dependant circuits

III Flyback