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

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• Single-ratio converters by design

Outline:

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

• Application examples

DCX derivation: basic idea

+

Vg V+–

_

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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

_

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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

_

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* 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

_

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State-plane analysis

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ECEN5817, ECEE Department, University of Colorado at Boulder

Example

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Operating waveforms: zero load

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ECEN5817, ECEE Department, University of Colorado at Boulder

Same example: 1 kW load

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Operating waveforms: 1 kW load

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ECEN5817, ECEE Department, University of Colorado at Boulder

Effects of leakage inductance?

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V = 280 V

Operating waveforms with 1% leakage inductance at 1 kW load

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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

_

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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

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ir

io

ECEN5817, ECEE Department, University of Colorado at Boulder

Example

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Operating waveforms at 1 kW load

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Phase shift: 0.69 us

ECEN5817, ECEE Department, University of Colorado at Boulder

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

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

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ECEN5817, ECEE Department, University of Colorado at Boulder

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

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

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ECEN5817, ECEE Department, University of Colorado at Boulder

Operation at 360 W, close to ZVS boundary

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Waveforms at 360 W

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Phase shift: 0.2 us

ECEN5817, ECEE Department, University of Colorado at Boulder

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

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

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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)

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• 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*

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

ECEN5817, ECEE Department, University of Colorado at Boulder

Intermediate bus architecture

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

Approaches to generating the 2nd-level distribution bus voltage

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ECEN5817, ECEE Department, University of Colorado at Boulder

Efficiency comparison

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

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*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

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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

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* 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

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

Converter realization

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