L42 ECEN5817 Notes

ECEN5817, ECEE Department, University of Colorado at Boulder

DC Transformer

Ultimate switched-mode power converter:

• Minimum possible voltage and current stresses on all components

• Zero-voltage switching of all semiconductor devices

It is possible to approach the above by restricting the conversion ratio to a single value, V/Vg = const., which leads to the “DC transformer” or “DCX” or “unregulated DC-DC” concept

DCX realizations

• Any hard-switched or soft-switched converter (e.g. ZVT) operated at constant control (duty ratio or phase shift), optimized for a single conversion ratio

Si l ti t b d i

ECEN 58171

• Single-ratio converters by design

Outline:

• Introduction to DCX, dual-active-bridge DCX realization example

• Application examples

DCX derivation: basic idea

+

Vg V+–

_

ECEN 58172

ECEN5817, ECEE Department, University of Colorado at Boulder

DCX derivation: insert DC-to-AC and AC-to-DC

Q1 Q3 Q5 Q7 +

Vg V+–

v2

Q2 Q4

v4 v6

Q6 Q8

v8

_

ECEN 58173

DCX derivation: dual-active-bridge converter*

Q1

v2

Q3

v4

Q5

v6

Q7

v8

+

1:n

Vg V+–

2

Q2 Q4

4 6

Q6 Q8

8

_

ECEN 58174

* R.W.A.A. De Doncker, D.M. Divan, M.H. Kheraluwala, "A Three-phase Soft-Switched High-Power-Density DC-DC Converter for High-Power Applications," IEEE Tran. on Industry Applications, Jan/Feb 1991, Vol. 27, No. 1, pp. 63-73.

ECEN5817, ECEE Department, University of Colorado at Boulder

ZVS via magnetizing inductance

Q1

v2

Q3

v4

Q5

v6

Q7

v8

+

1:n

Vg V+–

2

Q2 Q4

4 6

Q6 Q8

8

_

ECEN 58175

State-plane analysis

ECEN 58176

ECEN5817, ECEE Department, University of Colorado at Boulder

Example

ECEN 58177

Operating waveforms: zero load

ECEN 58178

ECEN5817, ECEE Department, University of Colorado at Boulder

Same example: 1 kW load

ECEN 58179

Operating waveforms: 1 kW load

ECEN 581710

ECEN5817, ECEE Department, University of Colorado at Boulder

Effects of leakage inductance?

ECEN 581711

V = 280 V

Operating waveforms with 1% leakage inductance at 1 kW load

ECEN 581712

ECEN5817, ECEE Department, University of Colorado at Boulder

Dual-active-bridge with series inductance and phase shift between primary and secondary bridges

Q1

v2

Q3

v4

Q5

v6

Q7

v8

+

1:n

Vg V+–

2

Q2 Q4

4 6

Q6 Q8

8

_

ECEN 581713

DCX (V/nVg = 1) waveforms neglecting resonant transitions

Vg V+–

Q1

v2

Q3

Q2 Q4

v4

Q5

v6

Q7

Q6 Q8

v8

+

_

1:n

vp

vp/n

i

ECEN 581714

ir

io

ECEN5817, ECEE Department, University of Colorado at Boulder

Example

ECEN 581715

Operating waveforms at 1 kW load

ECEN 581716

Phase shift: 0.69 us

ECEN5817, ECEE Department, University of Colorado at Boulder

Details of negative-to-positive il transition at 1 kW

ECEN 581717

Details of positive-to-negative il transition at 1 kW

ECEN 581718

ECEN5817, ECEE Department, University of Colorado at Boulder

State plane analysis of ZVS condition at V/nVg = 1

ECEN 581719

State-plane analysis of ZVS condition at V/nVg = 1

ECEN 581720

ECEN5817, ECEE Department, University of Colorado at Boulder

Operation at 360 W, close to ZVS boundary

ECEN 581721

Waveforms at 360 W

ECEN 581722

Phase shift: 0.2 us

ECEN5817, ECEE Department, University of Colorado at Boulder

Details of negative-to-positive il transition: operation at 360 W

ECEN 581723

Details of positive-to-negative il transition: operation at 360 W

ECEN 581724

ECEN5817, ECEE Department, University of Colorado at Boulder

Dual active bridge DC-DC converter summary

• At V/nVg = 1 (DCX), waveforms are close to ideal if F << 1

• ZVS of all semiconductors for loads greater than a minimum

• ZVS can be extended to lighter loads by reducing magnetizing inductance

• Phase shift can be used to control the conversion ratio (non-DCX operation), but with efficiency penalties

• High step-down, or high step-up conversion ratios feasible at high efficiencies (well above 90%)

• Dual active bridge: bidirectional power flow is possible

• For standard unidirectional applications, the secondary-side bridge can be just diodes (operation is similar, but not the same)

H lf b id d h ll i ti il bl

ECEN 581725

• Half-bridge and push-pull variations are available

• Some issues: • Transformer saturation (may require a series blocking capacitor)

• Series inductance (leakage + discrete) value is very important

• Switching frequency limited (F << 1; transformer and inductor core and proximity losses)

Application example:Computing and Telecom Server Power Distribution Systems*

ECEN 581726

*Bob White, Emerging On-Board Power Architectures, IEEE APEC 2003

ECEN5817, ECEE Department, University of Colorado at Boulder

Intermediate bus architecture

ECEN 581727

*Bob White, Emerging On-Board Power Architectures, IEEE APEC 2003

Approaches to generating the 2nd-level distribution bus voltage

ECEN 581728

ECEN5817, ECEE Department, University of Colorado at Boulder

Efficiency comparison

ECEN 581729

Application example:Automotive battery power management in a fuel-cell vehicle*

ECEN 581730

*F. Krismer, J.W.Kolar, “Accurate Power Loss Model Derivation of a High-Current Dual Active Bridge Converter for an Automotive Application, IEEE Trans. On Industrial Electronics, March 2010

ECEN5817, ECEE Department, University of Colorado at Boulder

Efficiency results

ECEN 581731

Power flow control in 3-phase AC power distribution*

• Purpose: control active and reactive power flow; increasingly important function in AC power distribution systems with distributed resources

• Solution above requires bulky 50/60 Hz transformers, e.g. for a 6.6 kV, 1

ECEN 581732

* A. Inoue, H. Akagi, “A Bidirectional Isolated DC–DC Converter as a Core Circuit of the Next-Generation Medium-Voltage Power Conversion System,” IEEE Trans. on Power Elect., March 2007

q y , g ,MVA unit, each transformer weights around 4,000 kg

ECEN5817, ECEE Department, University of Colorado at Boulder

Solution based on modular DCX

• Each cell can be switched as +E, -E, or 0

ECEN 581733

• With N = 9 cells, a total 19 levels are available to synthesize high-quality sine-wave

Converter realization

ECEN 581734