DC Transformer. DCX derivation: basic idea

Transcription

1 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/V g = 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 Single-ratio converters by design Outline: Introduction to DCX, dual-active-bridge DCX realization example Application examples 1 DCX derivation: basic idea V g V _ 2

2 DCX derivation: insert DC-to-AC and AC-to-DC Q 1 Q 3 Q 5 Q 7 v 2 v 4 v 6 v 8 V g V Q 2 Q 4 Q 6 Q 8 _ 3 DCX derivation: dual-active-bridge converter* Q 1 Q 3 Q 5 Q 7 v 2 v 4 1:n v 6 v 8 V g V Q 2 Q 4 Q 6 Q 8 _ * 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

3 ZVS via magnetizing inductance Q 1 Q 3 Q 5 Q 7 v 2 v 4 1:n v 6 v 8 V g V Q 2 Q 4 Q 6 Q 8 _ 5 State-plane analysis 6

4 Example 7 Operating waveforms: zero load 8

5 Same example: 1 kw load 9 Operating waveforms: 1 kw load 10

6 Effects of leakage inductance? 11 Operating waveforms with 1% leakage inductance at 1 kw load V = 280 V 12

7 Dual-active-bridge with series inductance and phase shift between primary and secondary bridges Q 1 Q 3 Q 5 Q 7 v 2 v 4 1:n v 6 v 8 V g V Q 2 Q 4 Q 6 Q 8 _ 13 DCX (V/nV g = 1) waveforms neglecting resonant transitions Vg Q 1 v 2 Q 3 v 4 1:n Q 5 v 6 Q 7 v 8 V Q 2 Q 4 Q 6 Q 8 _ v p v p /n i r i o 14

8 Example 15 Operating waveforms at 1 kw load Phase shift: 0.69 us 16

9 Details of negative-to-positive i l transition at 1 kw 17 Details of positive-to-negative i l transition at 1 kw 18

10 State plane analysis of ZVS condition at V/nV g = 1 19 State-plane analysis of ZVS condition at V/nV g = 1 20

11 Operation at 360 W, close to ZVS boundary 21 Waveforms at 360 W Phase shift: 0.2 us 22

12 Details of negative-to-positive i l transition: operation at 360 W 23 Details of positive-to-negative i l transition: operation at 360 W 24

13 Dual active bridge DC-DC converter summary At V/nV g = 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) Half-bridge bid 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) 25 Application example: Computing and Telecom Server Power Distribution Systems* *Bob White, Emerging On-Board Power Architectures, IEEE APEC

14 Intermediate bus architecture *Bob White, Emerging On-Board Power Architectures, IEEE APEC Approaches to generating the 2 nd -level distribution bus voltage 28

15 Efficiency comparison 29 Application example: Automotive battery power management in a fuel-cell vehicle* *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

16 Efficiency results 31 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 MVA unit, each transformer weights around 4,000 kg * 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

17 Solution based on modular DCX Each cell can be switched as E, -E, or 0 With N = 9 cells, a total 19 levels are available to synthesize high-quality sine-wave 33 Converter realization 34