Introduction
A single-stage remoted converter, reminiscent of a bidirectional capacitor-inductor-inductor-inductor-capacitor (CLLLC), is a well-liked converter kind in power storage techniques (ESSs) to avoid wasting system prices and enhance energy density. The acquire curve of an CLLLC is flatter, nonetheless, when the switching frequency (fs) is increased than the collection resonant frequency (fr) the acquire curve will likely be undesirably flat. The parasitic capacitance of the transformer and MOSFETs would additionally considerably influence the converter acquire [1], which is able to lead the converter’s output voltage out of regulation. On this energy tip, I’ll introduce a CLLLC management algorithm and a synchronous rectifier (SR) management technique to remove this nonlinearity, utilizing a 3.6-kW prototype converter to confirm the efficiency. Determine 1 is a block diagram of a residential ESS.
Determine 1 Residential ESS block diagram with bidirectional energy issue correction (PFC)/inverter, bidirectional DC/DC converter, and most energy level monitoring (MPPT). Supply: Texas Devices
Design issues within the management stage
Determine 2 exhibits the circuit topology of the full-bridge CLLLC resonant converter with the parasitic capacitors. This topology consists of a symmetric resonant tank and full-bridge construction.
Determine 2 The circuit topology of the full-bridge CLLLC converter with parasitic capacitors. Supply: Texas Devices
Determine 3 exhibits the perfect acquire curve of the CLLLC. Just like an LLC converter, variable frequency management is a well-liked management scheme for a CLLLC resonant converter.
Determine 3 A really perfect CLLLC acquire curve that makes use of variable frequency management. Supply: Texas Devices
As talked about earlier, the acquire curve is flat when fs exceeds fr. Furthermore, with the ability degree growing, the converter must parallel extra FETs on the battery facet to deal with extra present, which signifies that the output capacitance (Coss) on the output full-bridge FETs will likely be extraordinarily massive. Contemplating the parasitic parameters of transformer interwinding capacitance and Coss, the non-monotonic acquire curve at excessive frequency is severe, which corresponds to a light-load situation, as proven in Determine 4.
Determine 4 The CLLLC acquire curve contemplating parasitic parameters such because the transformer interwinding capacitance and Coss. Supply: Texas Devices
On this case, frequency management is ineffective. Hiccup mode is a well-liked technique for addressing CLLLC resonant converter nonmonotonic options, however this technique just isn’t appropriate in battery purposes as a result of the converter must ship excessive present when the battery voltage is low. Pulse-width modulation (PWM) and phase-shift management might resolve this concern, however PWM management will make the transistors work at a hard-switching state, which decreases effectivity and limits the operational frequency. Subsequently, phase-shift management is a more sensible choice.
Management logic
Determine 5 exhibits the frequency and phase-shift mixed-control scheme diagram. The battery voltage is low throughout startup, so the converter must tender begin with low charging present to restrict the excessive present spike and lengthen the battery life. It’s a restricted impact to tender begin from a excessive frequency if the resonant inductor worth or frequency just isn’t excessive sufficient. When the battery fees to close full capability, it’s going to trickle cost with a small present and keep a continuing voltage. Each instances correspond to a light-load situation for the converter. At mild load, the output voltage tends to rise due to the parasitic capacitance and will finally exit of regulation based mostly on earlier evaluation; phase-shift management will help regulate the output voltage on this state. The controller’s calculation consequence decides whether or not the converter must enter phase-shift mode or not.
Determine 5 The management scheme in numerous cost states. Observe, the battery voltage is low throughout startup, so the converter must tender begin with low charging present to restrict the excessive present spike and lengthen the battery life. Supply: Texas Devices
Determine 6 exhibits the modulation change between frequency and section shift. When the load decreases, the frequency will improve to control the output voltage. If the calculated most frequency is increased than the setting worth, the converter will enter phase-shift modulation; then when the load will increase, the phase-shift angle will lower with the intention to regulate the output voltage. The converter will enter frequency mode once more when the phase-shift angle decreases to zero.
Determine 6 The management scheme between frequency and phase-shift modes. When the load decreases and the phase-shift angle is zero, the frequency will improve to control the output voltage (frequency mode). If the utmost frequency is increased than the setting worth, the section shift angle decreases to control output voltage (section shift mode). Supply: Texas Devices
Issues brought on by parasitic capacitance
The MOSFETs’ Coss additionally has this impact underneath phase-shift mode; the tank present will oscillate with these capacitors, as proven in Determine 7.
Determine 7 The tank present waveforms underneath phase-shift mode in open loop. Supply: Texas Devices
Determine 8 plots a acquire comparability of a CLLLC converter with and with out contemplating MOSFET Coss. In line with the determine, there will likely be fluctuation within the acquire curve. On this case, the controller could modify the phase-shift angle to the unsuitable course underneath closed-loop management, leading to a big present spike.
Determine 8 The acquire curve underneath phase-shift mode with and with out COSS. Supply: Texas Devices
Resolution for the acquire drawback
To remove the non-monotonic of acquire, using SR management as proven in Determine 9 might resolve this concern. Turning on both two higher or two decrease SR switches on the similar time in the course of the tank present oscillation interval will briefly brief the transformer’s secondary-side winding, such that Coss is not going to contain the resonant.
Determine 9 Proposed SR management scheme to remove the non-monotonic of acquire. Supply: Texas Devices
Determine 10 exhibits the take a look at consequence; there isn’t any oscillation in comparison with Determine 8. For extra detailed evaluation and take a look at outcomes, see reference [2].
Determine 10 Achieve curve underneath phase-shift mode utilizing the proposed management scheme (gray line). Supply: Texas Devices
Experimental outcomes
A prototype [3] makes use of this management scheme to confirm the efficiency. Determine 11 exhibits the soft-start waveform and Determine 12 exhibits the tank present waveforms underneath phase-shift mode with the proposed management scheme.
Determine 11 The phase-shift tender begin with 750 W of output energy. Supply: Texas Devices
Determine 12 The tank present waveforms underneath phase-shift mode with the proposed scheme. Supply: Texas Devices
Determine 13 and Determine 14 present the frequency/phase-shift modulation change take a look at. From the take a look at waveforms, the startup present is proscribed inside 28 A with 750 W of output energy. There is no such thing as a oscillation within the tank present and the converter might change the modulation easily in numerous working circumstances.
Figure 13 The phase-shift and frequency modulation change: frequency mode with a 5-A load. Supply: Texas Devices
Figure 14 The phase-shift and frequency modulation change: phase-shift mode with a 1-A load. Supply: Texas Devices
Conclusion
The proposed frequency and phase-shift mixed-control scheme limits the inrush present in the course of the startup stage and makes the acquire linear at a lightweight load situation. The converter might change between frequency modulation and section shift modulation easily. Moreover, phase-shift management additionally introduces the non-monotonic acquire concern and makes the present oscillate within the designs which have massive COSS. The proposed SR management technique will help resolve the present oscillation concern and makes the acquire monotonic.
Guangzhi Cui is a System Engineer at Texas Devices, the place he’s liable for growing energy provide design. Guangzhi earned his M.S. diploma in Electrical Engineering from Hong Kong College of Science and Expertise in 2016; and his B.S. diploma of Engineering from Hunan College in 2014.
Associated Content material
- Energy Ideas #102: CLLLC vs. DAB for EV onboard chargers
- Energy Ideas #92: Excessive-frequency resonant converter design issues, Half 2
- Energy Ideas #134: Don’t change the laborious approach; obtain ZVS with a PWM full bridge
- Energy Ideas #117: Measure your LLC resonant tank earlier than testing at full working circumstances
- Energy Ideas #97: Form an LLC-SRC acquire curve to satisfy battery charger wants
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References
- Lee, Byoung-Hee, Moon-Younger Kim, Chong-Eun Kim, Ki-Bum Park, and Gun-Woo Moon, “Evaluation of LLC Resonant Converter Contemplating Results of Parasitic Elements.” Revealed in INTELEC 2009 – thirty first Worldwide Telecommunications Vitality Convention, Incheon, Korea (South), Oct. 18-22, 2009, pp. 1-6.
- Tai, Will, Guangzhi Cui, and Sheng-Yang Yu, “Achieve Optimization Management Methodology for CLLLC Resonant Converters Below Section Shift Mode.” Revealed in PCIM Europe 2024; Worldwide Exhibition and Convention for Energy Electronics, Clever Movement, Renewable Vitality and Vitality Administration, Nürnberg, Germany, June 11-13, 2024, pp. 2513-2518.
- Cui, Guangzhi. n.d. “3.6kW Bidirectional CLLLC Resonant Converter Reference Design.” Tex as Devices reference design No. PMP41042. Accessed Nov. 6, 2024.
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